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J. Arthur Thomson
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Title: The Outline of Science, Vol. 1 (of 4)
A Plain Story Simply Told
Author: J. Arthur Thomson
Release Date: January 22, 2007 [EBook #20417]
Language: English
*** START OF THIS PROJECT GUTENBERG EBOOK OUTLINE OF SCIENCE ***
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Distributed Proofreading Team at http://www.pgdp.net
[Illustration: THE GREAT SCARLET SOLAR PROMINENCES, WHICH ARE SUCH A
NOTABLE FEATURE OF THE SOLAR PHENOMENA, ARE IMMENSE OUTBURSTS OF FLAMING
HYDROGEN RISING SOMETIMES TO A HEIGHT OF 500,000 MILES]
THE
OUTLINE OF SCIENCE
A PLAIN STORY SIMPLY TOLD
EDITED BY
J. ARTHUR THOMSON
REGIUS PROFESSOR OF NATURAL HISTORY IN THE
UNIVERSITY OF ABERDEEN
WITH OVER 800 ILLUSTRATIONS
OF WHICH ABOUT 40 ARE IN COLOUR
IN FOUR VOLUMES
G. P. PUTNAM'S SONS
NEW YORK AND LONDON
The Knickerbocker press
Copyright, 1922
by
G. P. Putnam's Sons
_First Printing April, 1922
Second Printing April, 1922
Third Printing April, 1922
Fourth Printing April, 1922
Fifth Printing June, 1922
Sixth Printing June, 1922
Seventh Printing June, 1922
Eighth Printing June, 1922
Ninth Printing August, 1922
Tenth Printing September, 1922
Eleventh Printing Sept., 1922
Twelfth Printing, May, 1924_
Made in the United States of America
INTRODUCTORY NOTE
By Professor J. Arthur Thomson
Was it not the great philosopher and mathematician Leibnitz who said
that the more knowledge advances the more it becomes possible to
condense it into little books? Now this "Outline of Science" is
certainly not a little book, and yet it illustrates part of the meaning
of Leibnitz's wise saying. For here within reasonable compass there is a
library of little books--an outline of many sciences.
It will be profitable to the student in proportion to the discrimination
with which it is used. For it is not in the least meant to be of the
nature of an Encyclopædia, giving condensed and comprehensive articles
with a big full stop at the end of each. Nor is it a collection of
"primers," beginning at the very beginning of each subject and working
methodically onwards. That is not the idea.
What then is the aim of this book? It is to give the intelligent
student-citizen, otherwise called "the man in the street," a bunch of
intellectual keys by which to open doors which have been hitherto shut
to him, partly because he got no glimpse of the treasures behind the
doors, and partly because the portals were made forbidding by an
unnecessary display of technicalities. Laying aside conventional modes
of treatment and seeking rather to open up the subject as one might on a
walk with a friend, the work offers the student what might be called
informal introductions to the various departments of knowledge. To put
it in another way, the articles are meant to be clues which the reader
may follow till he has left his starting point very far behind. Perhaps
when he has gone far on his own he will not be ungrateful to the simple
book of "instructions to travellers" which this "Outline of Science" is
intended to be. The simple "bibliographies" appended to the various
articles will be enough to indicate "first books." Each article is meant
to be an invitation to an intellectual adventure, and the short lists of
books are merely finger-posts for the beginning of the journey.
We confess to being greatly encouraged by the reception that has been
given to the English serial issue of "The Outline of Science." It has
been very hearty--we might almost say enthusiastic. For we agree with
Professor John Dewey, that "the future of our civilisation depends upon
the widening spread and deepening hold of the scientific habit of mind."
And we hope that this is what "The Outline of Science" makes for.
Information is all to the good; interesting information is better still;
but best of all is the education of the scientific habit of mind.
Another modern philosopher, Professor L. T. Hobhouse, has declared that
the evolutionist's mundane goal is "the mastery by the human mind of the
conditions, internal as well as external, of its life and growth." Under
the influence of this conviction "The Outline of Science" has been
written. For life is not for science, but science for life. And even
more than science, to our way of thinking, is the individual development
of the scientific way of looking at things. Science is our legacy; we
must use it if it is to be our very own.
CONTENTS
INTRODUCTION 3
I. THE ROMANCE OF THE HEAVENS 7
The scale of the universe--The solar system--Regions of
the sun--The surface of the sun--Measuring the speed of
light--Is the sun dying?--The planets--Venus--Is there
life on Mars?--Jupiter and Saturn--The moon--The
mountains of the moon--Meteors and comets--Millions of
meteorites--A great comet--The stellar universe--The
evolution of stars--The age of stars--The nebular
theory--Spiral nebulæ--The birth and death of
stars--The shape of our universe--Astronomical
instruments.
II. THE STORY OF EVOLUTION 53
The beginning of the earth--Making a home for life--The
first living creatures--The first plants--The first
animals--Beginnings of bodies--Evolution of
sex--Beginning of natural death--Procession of life
through the ages--Evolution of land animals--The flying
dragons--The first known bird--Evidences of
evolution--Factors in evolution.
III. ADAPTATIONS TO ENVIRONMENT 113
The shore of the sea--The open sea--The deep sea--The
fresh waters--The dry land--The air.
IV. THE STRUGGLE FOR EXISTENCE 135
Animal and bird mimicry and disguise--Other kinds of
elusiveness.
V. THE ASCENT OF MAN 153
Anatomical proof of man's relationship with a Simian
stock--Physiological proof--Embryological proof--Man's
pedigree--Man's arboreal apprenticeship--Tentative
men--Primitive men--Races of mankind--Steps in human
evolution--Factors in human progress.
VI. EVOLUTION GOING ON 183
Evolutionary prospect for man--The fountain of change;
variability--Evolution of plants--Romance of
wheat--Changes in animal life--Story of the
salmon--Forming new habits--Experiments in locomotion;
new devices.
VII. THE DAWN OF MIND 205
A caution in regard to instinct--A useful law--Senses of
fishes--The mind of a minnow--The mind and senses of
amphibians--The reptilian mind--Mind in
birds--Intelligence co-operating with instinct--The
mind of the mammal--Instinctive aptitudes--Power of
association--Why is there not more intelligence?--The
mind of monkeys--Activity for activity's
sake--Imitation--The mind of man--Body and mind.
VIII. FOUNDATIONS OF THE UNIVERSE 243
The world of atoms--The energy of atoms--The discovery of
X-rays--The discovery of radium--The discovery of the
electron--The electron theory--The structure of the
atom--The new view of matter--Other new views--The
nature of electricity--Electric current--The
dynamo--Magnetism--Ether and waves--Light--What the
blue "sky" means--Light without heat--Forms of
energy--What heat is--Substitutes for coal--Dissipation
of energy--What a uniform temperature would
mean--Matter, ether, and Einstein--The tides--Origin of
the moon--The earth slowing down--The day becoming
longer.
ILLUSTRATIONS
FACING
PAGE
THE GREAT SCARLET SOLAR PROMINENCES, WHICH ARE SUCH A
NOTABLE FEATURE OF THE SOLAR PHENOMENA, ARE IMMENSE
OUTBURSTS OF FLAMING HYDROGEN RISING SOMETIMES TO A
HEIGHT OF 500,000 MILES
_Coloured Frontispiece_
LAPLACE 10
PROFESSOR J. C. ADAMS 10
Photo: Royal Astronomical Society.
PROFESSOR EDDINGTON OF CAMBRIDGE UNIVERSITY 10
Photo: Elliot & Fry, Ltd.
THE PLANETS, SHOWING THEIR RELATIVE DISTANCES AND
DIMENSIONS 11
THE MILKY WAY 14
Photo: Harvard College Observatory.
THE MOON ENTERING THE SHADOW CAST BY THE EARTH 14
THE GREAT NEBULA IN ANDROMEDA, MESSIER 31 15
From a photograph taken at the Yerkes Observatory.
DIAGRAM SHOWING THE MAIN LAYERS OF THE SUN 18
SOLAR PROMINENCES SEEN AT TOTAL SOLAR ECLIPSE, MAY 29,
1919. TAKEN AT SOBRAL, BRAZIL 18
Photo: Royal Observatory, Greenwich.
THE VISIBLE SURFACE OF THE SUN 19
Photo: Mount Wilson Observatory.
THE SUN PHOTOGRAPHED IN THE LIGHT OF GLOWING HYDROGEN 19
Photo: Mount Wilson Observatory.
THE AURORA BOREALIS (_Coloured Illustration_) 20
Reproduced from _The Forces of Nature_ (Messrs. Macmillan)
THE GREAT SUN-SPOT OF JULY 17, 1905 22
Yerkes Observatory.
SOLAR PROMINENCES 22
From photographs taken at the Yerkes Observatory.
MARS, OCTOBER 5, 1909 23
Photo: Mount Wilson Observatory.
JUPITER 23
SATURN, NOVEMBER 19, 1911 23
Photo: Professor E. E. Barnard, Yerkes Observatory.
THE SPECTROSCOPE, AN INSTRUMENT FOR ANALYSING LIGHT; IT
PROVIDES MEANS FOR IDENTIFYING SUBSTANCES (_Coloured
Illustration_) 24
THE MOON 28
MARS 29
Drawings by Professor Percival Lowell.
THE MOON, AT NINE AND THREE QUARTER DAYS 29
A MAP OF THE CHIEF PLAINS AND CRATERS OF THE MOON 32
A DIAGRAM OF A STREAM OF METEORS SHOWING THE EARTH PASSING
THROUGH THEM 32
COMET, SEPTEMBER 29, 1908 33
Photo: Royal Observatory, Greenwich.
COMET, OCTOBER 3, 1908 33
Photo: Royal Observatory, Greenwich.
TYPICAL SPECTRA 36
Photo: Harvard College Observatory.
A NEBULAR REGION SOUTH OF ZETA ORIONIS 37
Photo: Mount Wilson Observatory.
STAR CLUSTER IN HERCULES 37
Photo: Astrophysical Observatory, Victoria, British Columbia.
THE GREAT NEBULA IN ORION 40
Photo: Yerkes Observatory.
GIANT SPIRAL NEBULA, MARCH 23, 1914 41
Photo: Lick Observatory.
A SPIRAL NEBULA SEEN EDGE-ON 44
Photo: Mount Wilson Observatory.
100-INCH TELESCOPE, MOUNT WILSON 45
Photo: H. J. Shepstone.
THE YERKES 40-INCH REFRACTOR 48
THE DOUBLE-SLIDE PLATE-HOLDER ON YERKES 40-INCH REFRACTING
TELESCOPE 49
Photo: H. J. Shepstone.
MODERN DIRECT-READING SPECTROSCOPE 49
By A. Hilger, Ltd.
CHARLES DARWIN 56
Photo: Rischgitz Collection.
LORD KELVIN 56
Photo: Rischgitz Collection.
A GIANT SPIRAL NEBULA 57
Photo: Lick Observatory.
METEORITE WHICH FELL NEAR SCARBOROUGH AND IS NOW TO BE SEEN
IN THE NATURAL HISTORY MUSEUM 57
Photo: Natural History Museum.
A LIMESTONE CANYON 60
Reproduced from the Smithsonian Report, 1915.
GEOLOGICAL TREE OF ANIMALS 61
DIAGRAM OF AMOEBA 61
A PIECE OF A REEF-BUILDING CORAL, BUILT UP BY A LARGE
COLONY OF SMALL SEA-ANEMONE-LIKE POLYPS, EACH OF WHICH
FORMS FROM THE SALTS OF THE SEA A SKELETON OR SHELL OF
LIME 64
From the Smithsonian Report, 1917.
A GROUP OF CHALK-FORMING ANIMALS, OR FORAMINIFERA, EACH
ABOUT THE SIZE OF A VERY SMALL PIN'S HEAD 65
Photo: J. J. Ward, F.E.S.
A COMMON FORAMINIFER (POLYSTOMELLA) SHOWING THE SHELL IN
THE CENTRE AND THE OUTFLOWING NETWORK OF LIVING MATTER,
ALONG WHICH GRANULES ARE CONTINUALLY TRAVELLING, AND BY
WHICH FOOD PARTICLES ARE ENTANGLED AND DRAWN IN 65
Reproduced by permission of the Natural History Museum
(after Max Schultze).
A PLANT-LIKE ANIMAL, OR ZOOPHYTE, CALLED OBELIA 68
Photo: J. J. Ward, F.E.S.
TRYPANOSOMA GAMBIENSE 69
Reproduced by permission of _The Quart. Journ. Mic. Sci._
VOLVOX 69
PROTEROSPONGIA 69
GREEN HYDRA 72
Photo: J. J. Ward, F.E.S.
DIAGRAM ILLUSTRATING THE BEGINNING OF INDIVIDUAL LIFE 72
EARTHWORM 72
Photo: J. J. Ward, F.E.S.
GLASS MODEL OF A SEA-ANEMONE 72
Reproduced from the Smithsonian Report, 1917.
THIS DRAWING SHOWS THE EVOLUTION OF THE BRAIN FROM FISH TO
MAN 73
OKAPI AND GIRAFFE (_Coloured Illustration_) 74
DIAGRAM OF A SIMPLE REFLEX ARC IN A BACKBONELESS ANIMAL
LIKE AN EARTHWORM 76
THE YUCCA MOTH 76
Photo: British Museum (Natural History).
INCLINED PLANE OF ANIMAL BEHAVIOUR 76
VENUS' FLY-TRAP 77
Photo: J. J. Ward, F.E.S.
A SPIDER SUNNING HER EGGS 77
Reproduced by permission from _The Wonders of Instinct_ by
J. H. Fabre.
THE HOATZIN INHABITS BRITISH GUIANA 82
PERIPATUS 83
Photograph, from the British Museum (Natural History), of a
drawing by Mr. E. Wilson.
ROCK KANGAROO CARRYING ITS YOUNG IN A POUCH 83
Photo: W. S. Berridge, F.Z.S.
PROFESSOR THOMAS HENRY HUXLEY (1825-95) 86
Photo: Rischgitz.
BARON CUVIER, 1769-1832 86
AN ILLUSTRATION SHOWING VARIOUS METHODS OF FLYING AND
SWOOPING 87
ANIMALS OF THE CAMBRIAN PERIOD 90
From Knipe's _Nebula to Man_.
A TRILOBITE 90
Photo: J. J. Ward, F.E.S.
THE GAMBIAN MUD-FISH, PROTOPTERUS 91
Photo: British Museum (Natural History).
THE ARCHÆOPTERYX 91
After William Leche of Stockholm.
WING OF A BIRD, SHOWING THE ARRANGEMENT OF THE FEATHERS 91
PICTORIAL REPRESENTATION OF STRATA OF THE EARTH'S CRUST,
WITH SUGGESTIONS OF CHARACTERISTIC FOSSILS (_Coloured
Illustration_) 92
FOSSIL OF A PTERODACTYL OR EXTINCT FLYING DRAGON 94
Photo: British Museum (Natural History).
PARIASAURUS: AN EXTINCT VEGETARIAN TRIASSIC REPTILE 94
From Knipe's _Nebula to Man_.
TRICERATOPS: A HUGE EXTINCT REPTILE 95
From Knipe's _Nebula to Man_.
THE DUCKMOLE OR DUCK-BILLED PLATYPUS OF AUSTRALIA 95
Photo: _Daily Mail_.
SKELETON OF AN EXTINCT FLIGHTLESS TOOTHED BIRD, HESPERORNIS 100
After Marsh.
SIX STAGES IN THE EVOLUTION OF THE HORSE, SHOWING GRADUAL
INCREASE IN SIZE 101
After Lull and Matthew.
DIAGRAM SHOWING SEVEN STAGES IN THE EVOLUTION OF THE
FORE-LIMBS AND HIND-LIMBS OF THE ANCESTORS OF THE MODERN
HORSE, BEGINNING WITH THE EARLIEST KNOWN PREDECESSORS OF
THE HORSE AND CULMINATING WITH THE HORSE OF TO-DAY 104
After Marsh and Lull.
WHAT IS MEANT BY HOMOLOGY? ESSENTIAL SIMILARITY OF
ARCHITECTURE, THOUGH THE APPEARANCES MAY BE VERY
DIFFERENT 105
AN EIGHT-ARMED CUTTLEFISH OR OCTOPUS ATTACKING A SMALL CRAB 116
A COMMON STARFISH, WHICH HAS LOST THREE ARMS AND IS
REGROWING THEM 116
After Professor W. C. McIntosh.
THE PAPER NAUTILUS (ARGONAUTA), AN ANIMAL OF THE OPEN SEA 117
Photo: J. J. Ward, F.E.S.
A PHOTOGRAPH SHOWING A STARFISH (_Asterias Forreri_) WHICH
HAS CAPTURED A LARGE FISH 117
TEN-ARMED CUTTLEFISH OR SQUID IN THE ACT OF CAPTURING A FISH 118
GREENLAND WHALE 118
MINUTE TRANSPARENT EARLY STAGE OF A SEA-CUCUMBER 119
AN INTRICATE COLONY OF OPEN-SEA ANIMALS (_Physophora
Hydrostatica_) RELATED TO THE PORTUGUESE MAN-OF-WAR 119
Photo: British Museum (Natural History).
A SCENE IN THE GREAT DEPTHS 119
SEA-HORSE IN SARGASSO WEED 120
LARGE MARINE LAMPREYS (_Petromyzon Marinus_) 120
THE DEEP-SEA FISH _Chiasmodon Niger_ 120
DEEP-SEA FISHES 120
FLINTY SKELETON OF VENUS' FLOWER BASKET (_Euplectella_), A
JAPANESE DEEP-SEA SPONGE 121
EGG DEPOSITORY OF _Semotilus Atromaculatus_ 121
THE BITTERLING (_Rhodeus Amarus_) 124
WOOLLY OPOSSUM CARRYING HER FAMILY 124
Photo: W. S. Berridge.
SURINAM TOAD (_Pipa Americana_) WITH YOUNG ONES HATCHING
OUT OF LITTLE POCKETS ON HER BACK 125
STORM PETREL OR MOTHER CAREY'S CHICKEN (_Procellaria
Pelagica_) 125
ALBATROSS: A CHARACTERISTIC PELAGIC BIRD OF THE SOUTHERN
SEA 128
THE PRAYING MANTIS (_Mantis Religiosa_) 138
PROTECTIVE COLORATION: A WINTER SCENE IN NORTH SCANDINAVIA 138
THE VARIABLE MONITOR (_Varanus_) 139
Photo: A. A. White.
BANDED KRAIT: A VERY POISONOUS SNAKE WITH ALTERNATING
YELLOW AND DARK BANDS 140
Photo: W. S. Berridge, F.Z.S.
THE WARTY CHAMELEON 140
Photos: W. S. Berridge, F.Z.S.
SEASONAL COLOUR-CHANGE: SUMMER SCENE IN NORTH SCANDINAVIA 141
PROTECTIVE RESEMBLANCE 142
Photo: J. J. Ward, F.E.S.
WHEN ONLY A FEW DAYS OLD, YOUNG BITTERN BEGIN TO STRIKE THE
SAME ATTITUDE AS THEIR PARENTS, THRUSTING THEIR BILLS
UPWARDS AND DRAWING THEIR BODIES UP SO THAT THEY RESEMBLE
A BUNCH OF REEDS 143
PROTECTIVE COLORATION OR CAMOUFLAGING, GIVING ANIMALS A
GARMENT OF INVISIBILITY (_Coloured Illustration_) 144
ANOTHER EXAMPLE OF PROTECTIVE COLORATION (_Coloured
Illustration_) 144
DEAD-LEAF BUTTERFLY (_Kallima Inachis_) FROM INDIA 146
PROTECTIVE RESEMBLANCE BETWEEN A SMALL SPIDER (_to the
left_) AND AN ANT (_to the right_) 146
THE WASP BEETLE, WHICH, WHEN MOVING AMONGST THE BRANCHES,
GIVES A WASP-LIKE IMPRESSION 147
Photo: J. J. Ward, F.E.S.
HERMIT-CRAB WITH PARTNER SEA-ANEMONES 147
CUCKOO-SPIT 147
Photo: G. P. Duffus.
CHIMPANZEE, SITTING 156
Photo: New York Zoological Park.
CHIMPANZEE, ILLUSTRATING WALKING POWERS 156
Photo: New York Zoological Park.
SURFACE VIEW OF THE BRAINS OF MAN AND CHIMPANZEE 157
SIDE-VIEW OF CHIMPANZEE'S HEAD 157
Photo: New York Zoological Park.
PROFILE VIEW OF HEAD OF PITHECANTHROPUS, THE JAVA APE-MAN,
RECONSTRUCTED FROM THE SKULL-CAP 157
After a model by J. H. McGregor.
THE FLIPPER OF A WHALE AND THE HAND OF A MAN 157
THE GORILLA, INHABITING THE FOREST TRACT OF THE GABOON IN
AFRICA (_Coloured Illustration_) 158
"DARWIN'S POINT" ON HUMAN EAR 160
PROFESSOR SIR ARTHUR KEITH, M.D., LL.D., F.R.S. 161
Photo: J. Russell & Sons.
SKELETONS OF THE GIBBON, ORANG, CHIMPANZEE, GORILLA, MAN 161
After T. H. Huxley (by permission of Messrs. Macmillan).
SIDE-VIEW OF SKULL OF MAN AND GORILLA 164
THE SKULL AND BRAIN-CASE OF PITHECANTHROPUS, THE JAVA
APE-MAN, AS RESTORED BY J. H. MCGREGOR FROM THE SCANTY
REMAINS 164
SUGGESTED GENEALOGICAL TREE OF MAN AND ANTHROPOID APES 165
THE GIBBON IS LOWER THAN THE OTHER APES AS REGARDS ITS
SKULL AND DENTITION, BUT IT IS HIGHLY SPECIALIZED IN THE
ADAPTATION OF ITS LIMBS TO ARBOREAL LIFE 166
Photo: New York Zoological Park.
THE ORANG HAS A HIGH ROUNDED SKULL AND A LONG FACE 166
Photo: New York Zoological Park.
COMPARISONS OF THE SKELETONS OF HORSE AND MAN 167
Photo: British Museum (Natural History).
A RECONSTRUCTION OF THE JAVA MAN (_Coloured Illustration_) 168
PROFILE VIEW OF THE HEAD OF PITHECANTHROPUS, THE JAVA
APE-MAN--AN EARLY OFFSHOOT FROM THE MAIN LINE OF MAN'S
ASCENT 170
After a model by J. H. McGregor.
PILTDOWN SKULL 170
From the reconstruction by J. H. McGregor.
SAND-PIT AT MAUER, NEAR HEIDELBERG: DISCOVERY SITE OF THE
JAW OF HEIDELBERG MAN 171
Reproduced by permission from Osborn's
_Men of the Old Stone Age_.
PAINTINGS ON THE ROOF OF THE ALTAMIRA CAVE IN NORTHERN
SPAIN, SHOWING A BISON AND A GALLOPING BOAR (_Coloured
Illustration_) 172
PILTDOWN MAN, PRECEDING NEANDERTHAL MAN, PERHAPS 100,000 TO
150,000 YEARS AGO 174
After the restoration modelled by J. H. McGregor.
THE NEANDERTHAL MAN OF LA CHAPELLE-AUX-SAINTS 175
After the restoration modelled by J. H. McGregor.
RESTORATION BY A. FORESTIER OF THE RHODESIAN MAN WHOSE
SKULL WAS DISCOVERED IN 1921 176-177
SIDE VIEW OF A PREHISTORIC HUMAN SKULL DISCOVERED IN 1921
IN BROKEN HILL CAVE, NORTHERN RHODESIA 178
Photo: British Museum (Natural History).
A CROMAGNON MAN OR CROMAGNARD, REPRESENTATIVE OF A STRONG
ARTISTIC RACE LIVING IN THE SOUTH OF FRANCE IN THE UPPER
PLEISTOCENE, PERHAPS 25,000 YEARS AGO 178
After the restoration modelled by J. H. McGregor.
PHOTOGRAPH SHOWING A NARROW PASSAGE IN THE CAVERN OF
FONT-DE-GAUME ON THE BEUNE 179
Reproduced by permission from Osborn's
_Men of the Old Stone Age_.
A MAMMOTH DRAWN ON THE WALL OF THE FONT-DE-GAUME CAVERN 179
A GRAZING BISON, DELICATELY AND CAREFULLY DRAWN, ENGRAVED
ON A WALL OF THE ALTAMIRA CAVE, NORTHERN SPAIN 179
PHOTOGRAPH OF A MEDIAN SECTION THROUGH THE SHELL OF THE
PEARLY NAUTILUS 186
PHOTOGRAPH OF THE ENTIRE SHELL OF THE PEARLY NAUTILUS 186
NAUTILUS 186
SHOEBILL 187
Photo: W. S. Berridge.
THE WALKING-FISH OR MUD-SKIPPER (_Periophthalmus_), COMMON
AT THE MOUTHS OF RIVERS IN TROPICAL AFRICA, ASIA, AND
NORTH-WEST AUSTRALIA 190
THE AUSTRALIAN MORE-PORK OR PODARGUS 190
Photo: _The Times_.
PELICAN'S BILL, ADAPTED FOR CATCHING AND STORING FISHES 191
SPOONBILL'S BILL, ADAPTED FOR SIFTING THE MUD AND CATCHING
THE SMALL ANIMALS, E.G. FISHES, CRUSTACEANS, INSECT
LARVÆ, WHICH LIVE THERE 191
AVOCET'S BILL, ADAPTED FOR A CURIOUS SIDEWAYS SCOOPING IN
THE SHORE-POOLS AND CATCHING SMALL ANIMALS 191
HORNBILL'S BILL, ADAPTED FOR EXCAVATING A NEST IN A TREE,
AND ALSO FOR SEIZING AND BREAKING DIVERSE FORMS OF FOOD,
FROM MAMMALS TO TORTOISES, FROM ROOTS TO FRUITS 191
FALCON'S BILL, ADAPTED FOR SEIZING, KILLING, AND TEARING
SMALL MAMMALS AND BIRDS 191
PUFFIN'S BILL, ADAPTED FOR CATCHING SMALL FISHES NEAR THE
SURFACE OF THE SEA, AND FOR HOLDING THEM WHEN CAUGHT AND
CARRYING THEM TO THE NEST 191
LIFE-HISTORY OF A FROG 192
HIND-LEG OF WHIRLIGIG BEETLE WHICH HAS BECOME BEAUTIFULLY
MODIFIED FOR AQUATIC LOCOMOTION 192
Photo: J. J. Ward, F.E.S.
THE BIG ROBBER-CRAB (_Birgus Latro_), THAT CLIMBS THE
COCONUT PALM AND BREAKS OFF THE NUTS 193
EARLY LIFE-HISTORY OF THE SALMON 196
THE SALMON LEAPING AT THE FALL IS A MOST FASCINATING SPECTACLE 197
DIAGRAM OF THE LIFE-HISTORY OF THE COMMON EEL (_Anguilla
Vulgaris_) 200
CASSOWARY 201
Photo: Gambier Bolton.
THE KIWI, ANOTHER FLIGHTLESS BIRD, OF REMARKABLE
APPEARANCE, HABITS, AND STRUCTURE 201
Photo: Gambier Bolton.
THE AUSTRALIAN FRILLED LIZARD, WHICH IS AT PRESENT TRYING
TO BECOME A BIPED 202
A CARPET OF GOSSAMER 202
THE WATER SPIDER 203
JACKDAW BALANCING ON A GATEPOST 208
Photo: O. J. Wilkinson.
TWO OPOSSUMS FEIGNING DEATH 208
From Ingersoll's _The Wit of the Wild_.
MALE OF THREE-SPINED STICKLEBACK, MAKING A NEST OF
WATER-WEED, GLUED TOGETHER BY VISCID THREADS SECRETED
FROM THE KIDNEYS AT THE BREEDING SEASON 209
A FEMALE STICKLEBACK ENTERS THE NEST WHICH THE MALE HAS
MADE, LAYS THE EGGS INSIDE, AND THEN DEPARTS 209
HOMING PIGEON 212
Photo: Imperial War Museum.
CARRIER PIGEON 212
Photo: Imperial War Museum.
YELLOW-CROWNED PENGUIN 213
Photo: James's Press Agency.
PENGUINS ARE "A PECULIAR PEOPLE" 213
Photo: Cagcombe & Co.
HARPY-EAGLE 216
Photo: W. S. Berridge.
THE DINGO OR WILD DOG OF AUSTRALIA, PERHAPS AN INDIGENOUS
WILD SPECIES, PERHAPS A DOMESTICATED DOG THAT HAS GONE
WILD OR FERAL 216
Photo: W. S. Berridge, F.Z.S.
WOODPECKER HAMMERING AT A COTTON-REEL, ATTACHED TO A TREE 217
THE BEAVER 220
THE THRUSH AT ITS ANVIL 221
Photo: F. R. Hinkins & Son.
ALSATIAN WOLF-DOG 226
Photo: Lafayette.
THE POLAR BEAR OF THE FAR NORTH 227
Photo: W. S. Berridge.
AN ALLIGATOR "YAWNING" IN EXPECTATION OF FOOD 227
From the Smithsonian Report, 1914.
BABY ORANG 232
Photo: W. P. Dando.
ORANG-UTAN 232
Photo: Gambier Bolton.
CHIMPANZEE 233
Photo: James's Press Agency.
BABY ORANG-UTAN 233
Photo: James's Press Agency.
ORANG-UTAN 233
Photo: James's Press Agency.
BABY CHIMPANZEES 233
Photo: James's Press Agency.
CHIMPANZEE 238
Photo: W. P. Dando.
YOUNG CHEETAHS, OR HUNTING LEOPARDS 238
Photo: W. S. Berridge.
COMMON OTTER 239
Photo: C. Reid.
SIR ERNEST RUTHERFORD 246
Photo: Elliott & Fry.
J. CLERK-MAXWELL 246
Photo: Rischgitz Collection.
SIR WILLIAM CROOKES 247
Photo: Ernest H. Mills.
PROFESSOR SIR W. H. BRAGG 247
Photo: Photo Press.
COMPARATIVE SIZES OF MOLECULES 250
INCONCEIVABLE NUMBERS AND INCONCEIVABLY SMALL PARTICLES 250
WHAT IS A MILLION? 250
THE BROWNIAN MOVEMENT 251
A SOAP BUBBLE (_Coloured Illustration_) 252
Reproduced from _The Forces of Nature_ (Messrs. Macmillan).
DETECTING A SMALL QUANTITY OF MATTER 254
From _Scientific Ideas of To-day_.
THIS X-RAY PHOTOGRAPH IS THAT OF A HAND OF A SOLDIER
WOUNDED IN THE GREAT WAR 254
Reproduced by permission of X-Rays Ltd.
AN X-RAY PHOTOGRAPH OF A GOLF BALL, REVEALING AN IMPERFECT
CORE 254
Photo: National Physical Laboratory.
A WONDERFUL X-RAY PHOTOGRAPH 255
Reproduced by permission of X-Rays Ltd.
ELECTRIC DISCHARGE IN A VACUUM TUBE 258
THE RELATIVE SIZES OF ATOMS AND ELECTRONS 258
ELECTRONS STREAMING FROM THE SUN TO THE EARTH 259
PROFESSOR SIR J. J. THOMSON 262
ELECTRONS PRODUCED BY PASSAGE OF X-RAYS THROUGH AIR 262
From the Smithsonian Report, 1915.
MAGNETIC DEFLECTION OF RADIUM RAYS 263
PROFESSOR R. A. MILLIKAN'S APPARATUS FOR COUNTING ELECTRONS 263
Reproduced by permission of _Scientific American_.
MAKING THE INVISIBLE VISIBLE 266
THE THEORY OF ELECTRONS 267
ARRANGEMENTS OF ATOMS IN A DIAMOND 267
DISINTEGRATION OF ATOMS 270
SILK TASSEL ELECTRIFIED 270
Reproduced by permission from _The Interpretation of Radium_
(John Murray).
SILK TASSEL DISCHARGED BY THE RAYS FROM RADIUM 270
A HUGE ELECTRIC SPARK 271
ELECTRICAL ATTRACTION BETWEEN COMMON OBJECTS 271
From _Scientific Ideas of To-day_.
AN ELECTRIC SPARK 274
Photo: Leadbeater.
AN ETHER DISTURBANCE AROUND AN ELECTRON CURRENT 275
From _Scientific Ideas of To-day_.
LIGHTNING 278
Photo: H. J. Shepstone.
LIGHT WAVES 279
THE MAGNETIC CIRCUIT OF AN ELECTRIC CURRENT 279
THE MAGNET 279
ROTATING DISC OF SIR ISAAC NEWTON FOR MIXING COLOURS
(_Coloured Illustration_) 280
WAVE SHAPES 282
THE POWER OF A MAGNET 282
THE SPEED OF LIGHT 283
Photo: The Locomotive Publishing Co., Ltd.
ROTATING DISC OF SIR ISAAC NEWTON FOR MIXING COLOURS 283
NIAGARA FALLS 286
TRANSFORMATION OF ENERGY 287
Photo: Stephen Cribb.
"BOILING" A KETTLE ON ICE 287
Photo: Underwood & Underwood.
THE CAUSE OF TIDES 290
THE AEGIR ON THE TRENT 290
Photo: G. Brocklehurst.
A BIG SPRING TIDE, THE AEGIR ON THE TRENT 291
Photo: G. Brocklehurst.
The Outline of Science
INTRODUCTION
There is abundant evidence of a widened and deepened interest in modern
science. How could it be otherwise when we think of the magnitude and
the eventfulness of recent advances?
But the interest of the general public would be even greater than it is
if the makers of new knowledge were more willing to expound their
discoveries in ways that could be "understanded of the people." No one
objects very much to technicalities in a game or on board a yacht, and
they are clearly necessary for terse and precise scientific description.
It is certain, however, that they can be reduced to a minimum without
sacrificing accuracy, when the object in view is to explain "the gist of
the matter." So this OUTLINE OF SCIENCE is meant for the general reader,
who lacks both time and opportunity for special study, and yet would
take an intelligent interest in the progress of science which is making
the world always new.
The story of the triumphs of modern science is one of which Man may well
be proud. Science reads the secret of the distant star and anatomises
the atom; foretells the date of the comet's return and predicts the
kinds of chickens that will hatch from a dozen eggs; discovers the laws
of the wind that bloweth where it listeth and reduces to order the
disorder of disease. Science is always setting forth on Columbus
voyages, discovering new worlds and conquering them by understanding.
For Knowledge means Foresight and Foresight means Power.
The idea of Evolution has influenced all the sciences, forcing us to
think of _everything_ as with a history behind it, for we have travelled
far since Darwin's day. The solar system, the earth, the mountain
ranges, and the great deeps, the rocks and crystals, the plants and
animals, man himself and his social institutions--all must be seen as
the outcome of a long process of Becoming. There are some eighty-odd
chemical elements on the earth to-day, and it is now much more than a
suggestion that these are the outcome of an inorganic evolution, element
giving rise to element, going back and back to some primeval stuff, from
which they were all originally derived, infinitely long ago. No idea has
been so powerful a tool in the fashioning of New Knowledge as this
simple but profound idea of Evolution, that the present is the child of
the past and the parent of the future. And with the picture of a
continuity of evolution from nebula to social systems comes a promise of
an increasing control--a promise that Man will become not only a more
accurate student, but a more complete master of his world.
It is characteristic of modern science that the whole world is seen to
be more vital than before. Everywhere there has been a passage from the
static to the dynamic. Thus the new revelations of the constitution of
matter, which we owe to the discoveries of men like Professor Sir J. J.
Thomson, Professor Sir Ernest Rutherford, and Professor Frederick Soddy,
have shown the very dust to have a complexity and an activity heretofore
unimagined. Such phrases as "dead" matter and "inert" matter have gone
by the board.
The new theory of the atom amounts almost to a new conception of the
universe. It bids fair to reveal to us many of nature's hidden secrets.
The atom is no longer the indivisible particle of matter it was once
understood to be. We know now that there is an atom within the
atom--that what we thought was elementary can be dissociated and broken
up. The present-day theories of the atom and the constitution of matter
are the outcome of the comparatively recent discovery of such things as
radium, the X-rays, and the wonderful revelations of such instruments as
the spectroscope and other highly perfected scientific instruments.
The advent of the electron theory has thrown a flood of light on what
before was hidden or only dimly guessed at. It has given us a new
conception of the framework of the universe. We are beginning to know
and realise of what matter is made and what electric phenomena mean. We
can glimpse the vast stores of energy locked up in matter. The new
knowledge has much to tell us about the origin and phenomena, not only
of our own planet, but other planets, of the stars, and the sun. New
light is thrown on the source of the sun's heat; we can make more than
guesses as to its probable age. The great question to-day is: is there
_one_ primordial substance from which all the varying forms of matter
have been evolved?
But the discovery of electrons is only one of the revolutionary changes
which give modern science an entrancing interest.
As in chemistry and physics, so in the science of living creatures there
have been recent advances that have changed the whole prospect. A good
instance is afforded by the discovery of the "hormones," or chemical
messengers, which are produced by ductless glands, such as the thyroid,
the supra-renal, and the pituitary, and are distributed throughout the
body by the blood. The work of physiologists like Professor Starling and
Professor Bayliss has shown that these chemical messengers regulate what
may be called the "pace" of the body, and bring about that regulated
harmony and smoothness of working which we know as health. It is not too
much to say that the discovery of hormones has changed the whole of
physiology. Our knowledge of the human body far surpasses that of the
past generation.
The persistent patience of microscopists and technical improvements like
the "ultramicroscope" have greatly increased our knowledge of the
invisible world of life. To the bacteria of a past generation have been
added a multitude of microscopic _animal_ microbes, such as that which
causes Sleeping Sickness. The life-histories and the weird ways of many
important parasites have been unravelled; and here again knowledge means
mastery. To a degree which has almost surpassed expectations there has
been a revelation of the intricacy of the stones and mortar of the house
of life, and the microscopic study of germ-cells has wonderfully
supplemented the epoch-making experimental study of heredity which began
with Mendel. It goes without saying that no one can call himself
educated who does not understand the central and simple ideas of
Mendelism and other new departures in biology.
The procession of life through the ages and the factors in the sublime
movement; the peopling of the earth by plants and animals and the
linking of life to life in subtle inter-relations, such as those between
flowers and their insect-visitors; the life-histories of individual
types and the extraordinary results of the new inquiry called
"experimental embryology"--these also are among the subjects with which
this OUTLINE will deal.
The behaviour of animals is another fascinating study, leading to a
provisional picture of the dawn of mind. Indeed, no branch of science
surpasses in interest that which deals with the ways and habits--the
truly wonderful devices, adaptations, and instincts--of insects, birds,
and mammals. We no longer deny a degree of intelligence to some members
of the animal world--even the line between intelligence and reason is
sometimes difficult to find.
Fresh contacts between physiology and the study of man's mental life;
precise studies of the ways of children and wild peoples; and new
methods like those of the psycho-analyst must also receive the attention
they deserve, for they are giving us a "New Psychology" and the claims
of psychical research must also be recognised by the open-minded.
The general aim of the OUTLINE is to give the reader a clear and concise
view of the essentials of present-day science, so that he may follow
with intelligence the modern advance and share appreciatively in man's
continued conquest of his kingdom.
J. ARTHUR THOMSON.
I
THE ROMANCE OF THE HEAVENS
THE SCALE OF THE UNIVERSE--THE SOLAR SYSTEM
§ 1
The story of the triumphs of modern science naturally opens with
Astronomy. The picture of the Universe which the astronomer offers to us
is imperfect; the lines he traces are often faint and uncertain. There
are many problems which have been solved, there are just as many about
which there is doubt, and notwithstanding our great increase in
knowledge, there remain just as many which are entirely unsolved.
The problem of the structure and duration of the universe [said the
great astronomer Simon Newcomb] is the most far-reaching with which
the mind has to deal. Its solution may be regarded as the ultimate
object of stellar astronomy, the possibility of reaching which has
occupied the minds of thinkers since the beginning of civilisation.
Before our time the problem could be considered only from the
imaginative or the speculative point of view. Although we can to-day
attack it to a limited extent by scientific methods, it must be
admitted that we have scarcely taken more than the first step toward
the actual solution.... What is the duration of the universe in
time? Is it fitted to last for ever in its present form, or does it
contain within itself the seeds of dissolution? Must it, in the
course of time, in we know not how many millions of ages, be
transformed into something very different from what it now is? This
question is intimately associated with the question whether the
stars form a system. If they do, we may suppose that system to be
permanent in its general features; if not, we must look further for
our conclusions.
The Heavenly Bodies
The heavenly bodies fall into two very distinct classes so far as their
relation to our Earth is concerned; the one class, a very small one,
comprises a sort of colony of which the Earth is a member. These bodies
are called _planets_, or wanderers. There are eight of them, including
the Earth, and they all circle round the sun. Their names, in the order
of their distance from the sun, are Mercury, Venus, Earth, Mars,
Jupiter, Saturn, Uranus, Neptune, and of these Mercury, the nearest to
the sun, is rarely seen by the naked eye. Uranus is practically
invisible, and Neptune quite so. These eight planets, together with the
sun, constitute, as we have said, a sort of little colony; this colony
is called the Solar System.
The second class of heavenly bodies are those which lie _outside_ the
solar system. Every one of those glittering points we see on a starlit
night is at an immensely greater distance from us than is any member of
the Solar System. Yet the members of this little colony of ours, judged
by terrestrial standards, are at enormous distances from one another. If
a shell were shot in a straight line from one side of Neptune's orbit to
the other it would take five hundred years to complete its journey. Yet
this distance, the greatest in the Solar System as now known (excepting
the far swing of some of the comets), is insignificant compared to the
distances of the stars. One of the nearest stars to the earth that we
know of is Alpha Centauri, estimated to be some twenty-five million
millions of miles away. Sirius, the brightest star in the firmament, is
double this distance from the earth.
We must imagine the colony of planets to which we belong as a compact
little family swimming in an immense void. At distances which would take
our shell, not hundreds, but millions of years to traverse, we reach
the stars--or rather, a star, for the distances between stars are as
great as the distance between the nearest of them and our Sun. The
Earth, the planet on which we live, is a mighty globe bounded by a crust
of rock many miles in thickness; the great volumes of water which we
call our oceans lie in the deeper hollows of the crust. Above the
surface an ocean of invisible gas, the atmosphere, rises to a height of
about three hundred miles, getting thinner and thinner as it ascends.
[Illustration: LAPLACE
One of the greatest mathematical astronomers of all time and the
originator of the nebular theory.]
[Illustration: _Photo: Royal Astronomical Society._
PROFESSOR J. C. ADAMS
who, anticipating the great French mathematician, Le Verrier, discovered
the planet Neptune by calculations based on the irregularities of the
orbit of Uranus. One of the most dramatic discoveries in the history of
Science.]
[Illustration: _Photo: Elliott & Fry, Ltd._
PROFESSOR EDDINGTON
Professor of Astronomy at Cambridge. The most famous of the English
disciples of Einstein.]
[Illustration: FIG. 1.--DIAGRAMS OF THE SOLAR SYSTEM
THE COMPARATIVE DISTANCES OF THE PLANETS
(Drawn approximately to scale)
The isolation of the Solar System is very great. On the above scale the
_nearest_ star (at a distance of 25 trillions of miles) would be over
_one half mile_ away. The hours, days, and years are the measures of
time as we use them; that is: Jupiter's "Day" (one rotation of the
planet) is made in ten of _our hours_; Mercury's "Year" (one revolution
of the planet around the Sun) is eighty-eight of _our days_. Mercury's
"Day" and "Year" are the same. This planet turns always the same side to
the Sun.]
[Illustration: THE COMPARATIVE SIZES OF THE SUN AND THE PLANETS (Drawn
approximately to scale)
On this scale the Sun would be 17-1/2 inches in diameter; it is far
greater than all the planets put together. Jupiter, in turn, is greater
than all the other planets put together.]
Except when the winds rise to a high speed, we seem to live in a very
tranquil world. At night, when the glare of the sun passes out of our
atmosphere, the stars and planets seem to move across the heavens with a
stately and solemn slowness. It was one of the first discoveries of
modern astronomy that this movement is only apparent. The apparent
creeping of the stars across the heavens at night is accounted for by
the fact that the earth turns upon its axis once in every twenty-four
hours. When we remember the size of the earth we see that this implies a
prodigious speed.
In addition to this the earth revolves round the sun at a speed of more
than a thousand miles a minute. Its path round the sun, year in year
out, measures about 580,000,000 miles. The earth is held closely to this
path by the gravitational pull of the sun, which has a mass 333,432
times that of the earth. If at any moment the sun ceased to exert this
pull the earth would instantly fly off into space straight in the
direction in which it was moving at the time, that is to say, at a
tangent. This tendency to fly off at a tangent is continuous. It is the
balance between it and the sun's pull which keeps the earth to her
almost circular orbit. In the same way the seven other planets are held
to their orbits.
Circling round the earth, in the same way as the earth circles round the
sun, is our moon. Sometimes the moon passes directly between us and the
sun, and cuts off the light from us. We then have a total or partial
eclipse of the sun. At other times the earth passes directly between the
sun and the moon, and causes an eclipse of the moon. The great ball of
the earth naturally trails a mighty shadow across space, and the moon is
"eclipsed" when it passes into this.
The other seven planets, five of which have moons of their own, circle
round the sun as the earth does. The sun's mass is immensely larger than
that of all the planets put together, and all of them would be drawn
into it and perish if they did not travel rapidly round it in gigantic
orbits. So the eight planets, spinning round on their axes, follow their
fixed paths round the sun. The planets are secondary bodies, but they
are most important, because they are the only globes in which there can
be life, as we know life.
If we could be transported in some magical way to an immense distance in
space above the sun, we should see our Solar System as it is drawn in
the accompanying diagram (Fig. 1), except that the planets would be mere
specks, faintly visible in the light which they receive from the sun.
(This diagram is drawn approximately to scale.) If we moved still
farther away, trillions of miles away, the planets would fade entirely
out of view, and the sun would shrink into a point of fire, a star. And
here you begin to realize the nature of the universe. _The sun is a
star. The stars are suns._ Our sun looks big simply because of its
comparative nearness to us. The universe is a stupendous collection of
millions of stars or suns, many of which may have planetary families
like ours.
§ 2
The Scale of the Universe
How many stars are there? A glance at a photograph of star-clouds will
tell at once that it is quite impossible to count them. The fine
photograph reproduced in Figure 2 represents a very small patch of that
pale-white belt, the Milky Way, which spans the sky at night. It is true
that this is a particularly rich area of the Milky Way, but the entire
belt of light has been resolved in this way into masses or clouds of
stars. Astronomers have counted the stars in typical districts here and
there, and from these partial counts we get some idea of the total
number of stars. There are estimated to be between two and three
thousand million stars.
Yet these stars are separated by inconceivable distances from each
other, and it is one of the greatest triumphs of modern astronomy to
have mastered, so far, the scale of the universe. For several centuries
astronomers have known the relative distances from each other of the sun
and the planets. If they could discover the actual distance of any one
planet from any other, they could at once tell all the distances within
the Solar System.
The sun is, on the latest measurements, at an average distance of
92,830,000 miles from the earth, for as the orbit of the earth is not a
true circle, this distance varies. This means that in six months from
now the earth will be right at the opposite side of its path round the
sun, or 185,000,000 miles away from where it is now. Viewed or
photographed from two positions so wide apart, the nearest stars show a
tiny "shift" against the background of the most distant stars, and that
is enough for the mathematician. He can calculate the distance of any
star near enough to show this "shift." We have found that the nearest
star to the earth, a recently discovered star, is twenty-five trillion
miles away. Only thirty stars are known to be within a hundred trillion
miles of us.
This way of measuring does not, however, take us very far away in the
heavens. There are only a few hundred stars within five hundred trillion
miles of the earth, and at that distance the "shift" of a star against
the background (parallax, the astronomer calls it) is so minute that
figures are very uncertain. At this point the astronomer takes up a new
method. He learns the different types of stars, and then he is able to
deduce more or less accurately the distance of a star of a known type
from its faintness. He, of course, has instruments for gauging their
light. As a result of twenty years work in this field, it is now known
that the more distant stars of the Milky Way are at least a hundred
thousand trillion (100,000,000,000,000,000) miles away from the sun.
Our sun is in a more or less central region of the universe, or a few
hundred trillion miles from the actual centre. The remainder of the
stars, which are all outside our Solar System, are spread out,
apparently, in an enormous disc-like collection, so vast that even a ray
of light, which travels at the rate of 186,000 miles a second, would
take 50,000 years to travel from one end of it to the other. This, then
is what we call our universe.
Are there other Universes?
Why do we say "our universe"? Why not _the_ universe? It is now believed
by many of our most distinguished astronomers that our colossal family
of stars is only one of many universes. By a universe an astronomer
means any collection of stars which are close enough to control each
other's movements by gravitation; and it is clear that there might be
many universes, in this sense, separated from each other by profound
abysses of space. Probably there are.
For a long time we have been familiar with certain strange objects in
the heavens which are called "spiral nebulæ" (Fig 4). We shall see at a
later stage what a nebula is, and we shall see that some astronomers
regard these spiral nebulæ as worlds "in the making." But some of the
most eminent astronomers believe that they are separate
universes--"island-universes" they call them--or great collections of
millions of stars like our universe. There are certain peculiarities in
the structure of the Milky Way which lead these astronomers to think
that our universe may be a spiral nebula, and that the other spiral
nebulæ are "other universes."
[Illustration: _Photo: Harvard College Observatory._
FIG. 2.--THE MILKY WAY
Note the cloud-like effect.]
[Illustration: FIG. 3--THE MOON ENTERING THE SHADOW CAST BY THE EARTH
The diagram shows the Moon partially eclipsed.]
[Illustration: _From a photograph taken at the Yerkes Observatory_
FIG. 4.--THE GREAT NEBULA IN ANDROMEDA, MESSIER 31]
Vast as is the Solar System, then, it is excessively minute in
comparison with the Stellar System, the universe of the Stars, which is
on a scale far transcending anything the human mind can apprehend.
THE SOLAR SYSTEM
THE SUN
§ 1
But now let us turn to the Solar System, and consider the members of our
own little colony.
Within the Solar System there are a large number of problems that
interest us. What is the size, mass, and distance of each of the
planets? What satellites, like our Moon, do they possess? What are their
temperatures? And those other, sporadic members of our system, comets
and meteors, what are they? What are their movements? How do they
originate? And the Sun itself, what is its composition, what is the
source of its heat, how did it originate? Is it running down?
These last questions introduce us to a branch of astronomy which is
concerned with the physical constitution of the stars, a study which,
not so very many years ago, may well have appeared inconceivable. But
the spectroscope enables us to answer even these questions, and the
answer opens up questions of yet greater interest. We find that the
stars can be arranged in an order of development--that there are stars
at all stages of their life-history. The main lines of the evolution of
the stellar universe can be worked out. In the sun and stars we have
furnaces with temperatures enormously high; it is in such conditions
that substances are resolved into their simplest forms, and it is thus
we are enabled to obtain a knowledge of the most primitive forms of
matter. It is in this direction that the spectroscope (which we shall
refer to immediately) has helped us so much. It is to this wonderful
instrument that we owe our knowledge of the composition of the sun and
stars, as we shall see.
"That the spectroscope will detect the millionth of a milligram of
matter, and on that account has discovered new elements, commands
our admiration; but when we find in addition that it will detect the
nature of forms of matter trillions of miles away, and moreover,
that it will measure the velocities with which these forms of matter
are moving with an absurdly small per cent. of possible error, we
can easily acquiesce in the statement that it is the greatest
instrument ever devised by the brain and hand of man."
Such are some of the questions with which modern astronomy deals. To
answer them requires the employment of instruments of almost incredible
refinement and exactitude and also the full resources of mathematical
genius. Whether astronomy be judged from the point of view of the
phenomena studied, the vast masses, the immense distances, the æons of
time, or whether it be judged as a monument of human ingenuity,
patience, and the rarest type of genius, it is certainly one of the
grandest, as it is also one of the oldest, of the sciences.
The Solar System
In the Solar System we include all those bodies dependent on the sun
which circulate round it at various distances, deriving their light and
heat from the sun--the planets and their moons, certain comets and a
multitude of meteors: in other words, all bodies whose movements in
space are determined by the gravitational pull of the sun.
The Sun
Thanks to our wonderful modern instruments and the ingenious methods
used by astronomers, we have to-day a remarkable knowledge of the sun.
Look at the figure of the sun in the frontispiece. The picture
represents an eclipse of the sun; the dark body of the moon has screened
the sun's shining disc and taken the glare out of our eyes; we see a
silvery halo surrounding the great orb on every side. It is the sun's
atmosphere, or "crown" (corona), stretching for millions of miles into
space in the form of a soft silvery-looking light; probably much of its
light is sunlight reflected from particles of dust, although the
spectroscope shows an element in the corona that has not so far been
detected anywhere else in the universe and which in consequence has been
named Coronium.
We next notice in the illustration that at the base of the halo there
are red flames peeping out from the edges of the hidden disc. When one
remembers that the sun is 866,000 miles in diameter, one hardly needs to
be told that these flames are really gigantic. We shall see what they
are presently.
Regions of the Sun
The astronomer has divided the sun into definite concentric regions or
layers. These layers envelop the nucleus or central body of the sun
somewhat as the atmosphere envelops our earth. It is through these
vapour layers that the bright white body of the sun is seen. Of the
innermost region, the heart or nucleus of the sun, we know almost
nothing. The central body or nucleus is surrounded by a brilliantly
luminous envelope or layer of vaporous matter which is what we see when
we look at the sun and which the astronomer calls the photosphere.
Above--that is, overlying--the photosphere there is a second layer of
glowing gases, which is known as the reversing layer. This layer is
cooler than the underlying photosphere; it forms a veil of smoke-like
haze and is of from 500 to 1,000 miles in thickness.
A third layer or envelope immediately lying over the last one is the
region known as the chromosphere. The chromosphere extends from 5,000
to 10,000 miles in thickness--a "sea" of red tumultuous surging fire.
Chief among the glowing gases is the vapour of hydrogen. The intense
white heat of the photosphere beneath shines through this layer,
overpowering its brilliant redness. From the uppermost portion of the
chromosphere great fiery tongues of glowing hydrogen and calcium vapour
shoot out for many thousands of miles, driven outward by some prodigious
expulsive force. It is these red "prominences" which are such a notable
feature in the picture of the eclipse of the sun already referred to.
During the solar eclipse of 1919 one of these red flames rose in less
than seven hours from a height of 130,000 miles to more than 500,000
miles above the sun's surface. This immense column of red-hot gas, four
or five times the thickness of the earth, was soaring upward at the rate
of 60,000 miles an hour.
These flaming jets or prominences shooting out from the chromosphere are
not to be seen every day by the naked eye; the dazzling light of the sun
obscures them, gigantic as they are. They can be observed, however, by
the spectroscope any day, and they are visible to us for a very short
time during an eclipse of the sun. Some extraordinary outbursts have
been witnessed. Thus the late Professor Young described one on September
7, 1871, when he had been examining a prominence by the spectroscope:
It had remained unchanged since noon of the previous day--a long,
low, quiet-looking cloud, not very dense, or brilliant, or in any
way remarkable except for its size. At 12:30 p.m. the Professor left
the spectroscope for a short time, and on returning half an hour
later to his observations, he was astonished to find the gigantic
Sun flame shattered to pieces. The solar atmosphere was filled with
flying debris, and some of these portions reached a height of
100,000 miles above the solar surface. Moving with a velocity which,
even at the distance of 93,000,000 miles, was almost perceptible to
the eye, these fragments doubled their height in ten minutes. On
January 30, 1885, another distinguished solar observer, the late
Professor Tacchini of Rome, observed one of the greatest prominences
ever seen by man. Its height was no less than 142,000
miles--eighteen times the diameter of the earth. Another mighty
flame was so vast that supposing the eight large planets of the
solar system ranged one on top of the other, the prominence would
still tower above them.[1]
[1] _The Romance of Astronomy_, by H. Macpherson.
[Illustration: FIG. 5.--DIAGRAM SHOWING THE MAIN LAYERS OF THE SUN
Compare with frontispiece.]
[Illustration: _Photo: Royal Observatory, Greenwich._
FIG. 6.--SOLAR PROMINENCES SEEN AT TOTAL SOLAR ECLIPSE, May 29, 1919.
TAKEN AT SOBRAL, BRAZIL.
The small Corona is also visible.]
[Illustration: FIG. 7.--THE VISIBLE SURFACE OF THE SUN
A photograph taken at the Mount Wilson Observatory of the Carnegie
Institution at Washington.]
[Illustration: FIG. 8.--THE SUN
Photographed in the light of glowing hydrogen, at the Mount Wilson
Observatory of the Carnegie Institution of Washington: vortex phenomena
near the spots are especially prominent.]
The fourth and uppermost layer or region is that of the corona, of
immense extent and fading away into the surrounding sky--this we have
already referred to. The diagram (Fig. 5) shows the dispositions of
these various layers of the sun. It is through these several transparent
layers that we see the white light body of the sun.
§ 2
The Surface of the Sun
Here let us return to and see what more we know about the
photosphere--the sun's surface. It is from the photosphere that we have
gained most of our knowledge of the composition of the sun, which is
believed not to be a solid body. Examination of the photosphere shows
that the outer surface is never at rest. Small bright cloudlets come and
go in rapid succession, giving the surface, through contrasts in
luminosity, a granular appearance. Of course, to be visible at all at
92,830,000 miles the cloudlets cannot be small. They imply enormous
activity in the photosphere. If we might speak picturesquely the sun's
surface resembles a boiling ocean of white-hot metal vapours. We have
to-day a wonderful instrument, which will be described later, which
dilutes, as it were, the general glare of the sun, and enables us to
observe these fiery eruptions at any hour. The "oceans" of red-hot gas
and white-hot metal vapour at the sun's surface are constantly driven by
great storms. Some unimaginable energy streams out from the body or
muscles of the sun and blows its outer layers into gigantic shreds, as
it were.
The actual temperature at the sun's surface, or what appears to us to be
the surface--the photosphere--is, of course, unknown, but careful
calculation suggests that it is from 5,000° C. to 7,000° C. The interior
is vastly hotter. We can form no conception of such temperatures as must
exist there. Not even the most obdurate solid could resist such
temperatures, but would be converted almost instantaneously into gas.
But it would not be gas as we know gases on the earth. The enormous
pressures that exist on the sun must convert even gases into thick
treacly fluids. We can only infer this state of matter. It is beyond our
power to reproduce it.
Sun-spots
It is in the brilliant photosphere that the dark areas known as
sun-spots appear. Some of these dark spots--they are dark only by
contrast with the photosphere surrounding them--are of enormous size,
covering many thousands of square miles of surface. What they are we
cannot positively say. They look like great cavities in the sun's
surface. Some think they are giant whirlpools. Certainly they seem to be
great whirling streams of glowing gases with vapours above them and
immense upward and downward currents within them. Round the edges of the
sun-spots rise great tongues of flame.
Perhaps the most popularly known fact about sun-spots is that they are
somehow connected with what we call magnetic storms on earth. These
magnetic storms manifest themselves in interruptions of our telegraphic
and telephonic communications, in violent disturbances of the mariner's
compass, and in exceptional auroral displays. The connection between the
two sets of phenomena cannot be doubted, even although at times there
may be a great spot on the sun without any corresponding "magnetic
storm" effects on the earth.
A surprising fact about sun-spots is that they show definite periodic
variations in number. The best-defined period is one of about eleven
years. During this period the spots increase to a maximum in number and
then diminish to a minimum, the variation being more or less regular.
Now this can only mean one thing. To be periodic the spots must have
some deep-seated connection with the fundamental facts of the sun's
structure and activities. Looked at from this point of view their
importance becomes great.
[Illustration: _Reproduction from "The Forces of Nature"_ (_Messrs.
Macmillan_)
THE AURORA BOREALIS
The aurora borealis is one of the most beautiful spectacles in the sky.
The colours and shape change every instant; sometimes a fan-like cluster
of rays, at other times long golden draperies gliding one over the
other. Blue, green, yellow, red, and white combine to give a glorious
display of colour. The theory of its origin is still, in part, obscure,
but there can be no doubt that the aurora is related to the magnetic
phenomena of the earth and therefore is connected with the electrical
influence of the sun.]
It is from the study of sun-spots that we have learned that the sun's
surface does not appear to rotate all at the same speed. The
"equatorial" regions are rotating quicker than regions farther north or
south. A point forty-five degrees from the equator seems to take about
two and a half days longer to complete one rotation than a point on the
equator. This, of course, confirms our belief that the sun cannot be a
solid body.
What is its composition? We know that there are present, in a gaseous
state, such well-known elements as sodium, iron, copper, zinc, and
magnesium; indeed, we know that there is practically every element in
the sun that we know to be in the earth. How do we know?
It is from the photosphere, as has been said, that we have won most of
our knowledge of the sun. The instrument used for this purpose is the
spectroscope; and before proceeding to deal further with the sun and the
source of its energy it will be better to describe this instrument.
A WONDERFUL INSTRUMENT AND WHAT IT REVEALS
The spectroscope is an instrument for analysing light. So important is
it in the revelations it has given us that it will be best to describe
it fully. Every substance to be examined must first be made to glow,
made luminous; and as nearly everything in the heavens _is_ luminous the
instrument has a great range in Astronomy. And when we speak of
analysing light, we mean that the light may be broken up into waves of
different lengths. What we call light is a series of minute waves in
ether, and these waves are--measuring them from crest to crest, so to
say--of various lengths. Each wave-length corresponds to a colour of the
rainbow. The shortest waves give us a sensation of violet colour, and
the largest waves cause a sensation of red. The rainbow, in fact, is a
sort of natural spectrum. (The meaning of the rainbow is that the
moisture-laden air has sorted out these waves, in the sun's light,
according to their length.) Now the simplest form of spectroscope is a
glass prism--a triangular-shaped piece of glass. If white light
(sunlight, for example) passes through a glass prism, we see a series of
rainbow-tinted colours. Anyone can notice this effect when sunlight is
shining through any kind of cut glass--the stopper of a wine decanter,
for instance. If, instead of catching with the eye the coloured lights
as they emerge from the glass prism, we allow them to fall on a screen,
we shall find that they pass, by continuous gradations, from red at the
one end of the screen, through orange, yellow, green, blue, and indigo,
to violet at the other end. _In other words, what we call white light is
composed of rays of these several colours. They go to make up the effect
which we call white._ And now just as water can be split up into its two
elements, oxygen and hydrogen, so sunlight can be broken up into its
primary colours, which are those we have just mentioned.
This range of colours, produced by the spectroscope, we call the solar
spectrum, and these are, from the spectroscopic point of view, primary
colours. Each shade of colour has its definite position in the spectrum.
That is to say, the light of each shade of colour (corresponding to its
wave-length) is reflected through a certain fixed angle on passing
through the glass prism. Every possible kind of light has its definite
position, and is denoted by a number which gives the wave-length of the
vibrations constituting that particular kind of light.
Now, other kinds of light besides sunlight can be analysed. Light
from any substance which has been made incandescent may be observed with
the spectroscope in the same way, and each element can be thus
separated. It is found that each substance (in the same conditions of
pressure, etc.) gives a constant spectrum of its own. _Each metal
displays its own distinctive colour. It is obvious, therefore, that the
spectrum provides the means for identifying a particular substance._ It
was by this method that we discovered in the sun the presence of such
well-known elements as sodium, iron, copper, zinc, and magnesium.
[Illustration: _Yerkes Observatory._
FIG. 9.--THE GREAT SUN-SPOT OF JULY 17, 1905]
[Illustration: _From photographs taken at the Yerkes Observatory._
FIG. 10.--SOLAR PROMINENCES
These are about 60,000 miles in height. The two photographs show the
vast changes occurring in ten minutes. October 10, 1910.]
[Illustration: _Photo: Mount Wilson Observatory._
FIG. 11.--MARS, October 5, 1909
Showing the dark markings and the Polar Cap.]
[Illustration: FIG. 12.--JUPITER
Showing the belts which are probably cloud formations.]
[Illustration: _Photo: Professor E. E. Barnard, Yerkes Observatory._
FIG. 13.--SATURN, November 19, 1911
Showing the rings, mighty swarms of meteorites.]
Every chemical element known, then, has a distinctive spectrum of its
own when it is raised to incandescence, and this distinctive spectrum is
as reliable a means of identification for the element as a human face is
for its owner. Whether it is a substance glowing in the laboratory or in
a remote star makes no difference to the spectroscope; if the light of
any substance reaches it, that substance will be recognised and
identified by the characteristic set of waves.
The spectrum of a glowing mass of gas will consist in a number of bright
lines of various colours, and at various intervals; corresponding to
each kind of gas, there will be a peculiar and distinctive arrangement
of bright lines. But if the light from such a mass of glowing gas be
made to pass through a cool mass of the _same_ gas it will be found that
dark lines replace the bright lines in the spectrum, the reason for this
being that the cool gas absorbs the rays of light emitted by the hot
gas. Experiments of this kind enable us to reach the important general
statement that every gas, when cold, absorbs the same rays of light
which it emits when hot.
Crossing the solar spectrum are hundreds and hundreds of dark lines.
These could not at first be explained, because this fact of
discriminative absorption was not known. We understand now. The sun's
white light comes from the photosphere, but between us and the
photosphere there is, as we have seen, another solar envelope of
relatively cooler vapours--the reversing layer. Each constituent
element in this outer envelope stops its own kind of light, that is, the
kind of light made by incandescent atoms of the same element in the
photosphere. The "stoppages" register themselves in the solar spectrum
as dark lines placed exactly where the corresponding bright lines would
have been. The explanation once attained, dark lines became as
significant as bright lines. The secret of the sun's composition was
out. We have found practically every element in the sun that we know to
be in the earth. We have identified an element in the sun before we were
able to isolate it on the earth. We have been able even to point to the
coolest places on the sun, the centres of sun-spots, where alone the
temperature seems to have fallen sufficiently low to allow chemical
compounds to form.
It is thus we have been able to determine what the stars, comets, or
nebulæ are made of.
A Unique Discovery
In 1868 Sir Norman Lockyer detected a light coming from the prominences
of the sun which was not given by any substance known on earth, and
attributed this to an unknown gas which he called helium, from the Greek
_helios_, the sun. _In 1895 Sir William Ramsay discovered in certain
minerals the same gas identified by the spectroscope._ We can say,
therefore, that this gas was discovered in the sun nearly thirty years
before it was found on earth; this discovery of the long-lost heir is as
thrilling a chapter in the detective story of science as any in the
sensational stories of the day, and makes us feel quite certain that our
methods really tell us of what elements sun and stars are built up. The
light from the corona of the sun, as we have mentioned indicates a gas
still unknown on earth, which has been christened Coronium.
Measuring the Speed of Light
But this is not all; soon a new use was found for the spectroscope. We
found that we could measure with it the most difficult of all speeds
to measure, speed in the line of sight. Movement at right angles to the
direction in which one is looking is, if there is sufficient of it, easy
to detect, and, if the distance of the moving body is known, easy to
measure. But movement in the line of vision is both difficult to detect
and difficult to measure. Yet, even at the enormous distances with which
astronomers have to deal, the spectroscope can detect such movement and
furnish data for its measurement. If a luminous body containing, say,
sodium is moving rapidly towards the spectroscope, it will be found that
the sodium lines in the spectrum have moved slightly from their usual
definite positions towards the violet end of the spectrum, the amount of
the change of position increasing with the speed of the luminous body.
If the body is moving away from the spectroscope the shifting of the
spectral lines will be in the opposite direction, towards the red end of
the spectrum. In this way we have discovered and measured movements that
otherwise would probably not have revealed themselves unmistakably to us
for thousands of years. In the same way we have watched, and measured
the speed of, tremendous movements on the sun, and so gained proof that
the vast disturbances we should expect there actually do occur.
[Illustration: THE SPECTROSCOPE IS AN INSTRUMENT FOR ANALYSING LIGHT; IT
PROVIDES THE MEANS FOR IDENTIFYING DIFFERENT SUBSTANCES
This pictorial diagram illustrates the principal of Spectrum Analysis,
showing how sunlight is decomposed into its primary colours. What we
call white light is composed of seven different colours. The diagram is
relieved of all detail which would unduly obscure the simple process by
which a ray of light is broken up by a prism into different
wave-lengths. The spectrum rays have been greatly magnified.]
IS THE SUN DYING?
§ 3
Now let us return to our consideration of the sun.
To us on the earth the most patent and most astonishing fact about the
sun is its tremendous energy. Heat and light in amazing quantities pour
from it without ceasing.
Where does this energy come from? Enormous jets of red glowing gases can
be seen shooting outwards from the sun, like flames from a fire, for
thousands of miles. Does this argue fire, as we know fire on the earth?
On this point the scientist is sure. The sun is not burning, and
combustion is not the source of its heat. Combustion is a chemical
reaction between atoms. The conditions that make it possible are known
and the results are predictable and measurable. But no chemical reaction
of the nature of combustion as we know it will explain the sun's energy,
nor indeed will any ordinary chemical reaction of any kind. If the sun
were composed of combustible material throughout and the conditions of
combustion as we understand them were always present, the sun would burn
itself out in some thousands of years, with marked changes in its heat
and light production as the process advanced. There is no evidence of
such changes. There is, instead, strong evidence that the sun has been
emitting light and heat in prodigious quantities, not for thousands, but
for millions of years. Every addition to our knowledge that throws light
on the sun's age seems to make for increase rather than decrease of its
years. This makes the wonder of its energy greater.
And we cannot avoid the issue of the source of the energy by saying
merely that the sun is gradually radiating away an energy that
originated in some unknown manner, away back at the beginning of things.
Reliable calculations show that the years required for the mere cooling
of a globe like the sun could not possibly run to millions. In other
words, the sun's energy must be subject to continuous and more or less
steady renewal. However it may have acquired its enormous energy in the
past, it must have some source of energy in the present.
The best explanation that we have to-day of this continuous accretion of
energy is that it is due to shrinkage of the sun's bulk under the force
of gravity. Gravity is one of the most mysterious forces of nature, but
it is an obvious fact that bodies behave as if they attracted one
another, and Newton worked out the law of this attraction. We may say,
without trying to go too deeply into things, that every particle of
matter attracts every other throughout the universe. If the diameter of
the sun were to shrink by one mile all round, this would mean that all
the millions of tons in the outer one-mile thickness would have a
straight drop of one mile towards the centre. And that is not all,
because obviously the layers below this outer mile would also drop
inwards, each to a less degree than the one above it. What a tremendous
movement of matter, however slowly it might take place! And what a
tremendous energy would be involved! Astronomers calculate that the
above shrinkage of one mile all round would require fifty years for its
completion, assuming, reasonably, that there is close and continuous
relationship between loss of heat by radiation and shrinkage. Even if
this were true we need not feel over-anxious on this theory; before the
sun became too cold to support life many millions of years would be
required.
It was suggested at one time that falls of meteoric matter into the sun
would account for the sun's heat. This position is hardly tenable now.
The mere bulk of the meteoric matter required by the hypothesis, apart
from other reasons, is against it. There is undoubtedly an enormous
amount of meteoric matter moving about within the bounds of the solar
system, but most of it seems to be following definite routes round the
sun like the planets. The stray erratic quantities destined to meet
their doom by collision with the sun can hardly be sufficient to account
for the sun's heat.
Recent study of radio-active bodies has suggested another factor that
may be working powerfully along with the force of gravitation to
maintain the sun's store of heat. In radio-active bodies certain atoms
seem to be undergoing disintegration. These atoms appear to be splitting
up into very minute and primitive constituents. But since matter may be
split up into such constituents, may it not be built up from them?
The question is whether these "radio-active" elements are undergoing
disintegration, or formation, in the sun. If they are undergoing
disintegration--and the sun itself is undoubtedly radio-active--then we
have another source of heat for the sun that will last indefinitely.
THE PLANETS
LIFE IN OTHER WORLDS?
§ 1
It is quite clear that there cannot be life on the stars. Nothing solid
or even liquid can exist in such furnaces as they are. Life exists only
on planets, and even on these its possibilities are limited. Whether all
the stars, or how many of them, have planetary families like our sun, we
cannot positively say. If they have, such planets would be too faint and
small to be visible tens of trillions of miles away. Some astronomers
think that our sun may be exceptional in having planets, but their
reasons are speculative and unconvincing. Probably a large proportion at
least of the stars have planets, and we may therefore survey the globes
of our own solar system and in a general way extend the results to the
rest of the universe.
In considering the possibility of life as we know it we may at once rule
out the most distant planets from the sun, Uranus and Neptune. They are
probably intrinsically too hot. We may also pass over the nearest planet
to the sun, Mercury. We have reason to believe that it turns on its axis
in the same period as it revolves round the sun, and it must therefore
always present the same side to the sun. This means that the heat on the
sunlit side of Mercury is above boiling-point, while the cold on the
other side must be between two and three hundred degrees below
freezing-point.
The Planet Venus
The planet Venus, the bright globe which is known to all as the morning
and evening "star," seems at first sight more promising as regards the
possibility of life. It is of nearly the same size as the earth, and it
has a good atmosphere, but there are many astronomers who believe that,
like Mercury, it always presents the same face to the sun, and it would
therefore have the same disadvantage--a broiling heat on the sunny side
and the cold of space on the opposite side. We are not sure. The
surface of Venus is so bright--the light of the sun is reflected to us
by such dense masses of cloud and dust--that it is difficult to trace
any permanent markings on it, and thus ascertain how long it takes to
rotate on its axis. Many astronomers believe that they have succeeded,
and that the planet always turns the same face to the sun. If it does,
we can hardly conceive of life on its surface, in spite of the
cloud-screen.
[Illustration: FIG. 14.--THE MOON
Showing a great plain and some typical craters. There are thousands of
these craters, and some theories of their origin are explained on page
34.]
[Illustration: FIG. 15.--MARS
1} Drawings by Prof. Lowell to accompany actual photographs of Mars
showing many of the
2} canals. Taken in 1907 by Mr. E. C. Slipher of the Lowell Observatory.
3 Drawing by Prof. Lowell made January 6, 1914.
4 Drawing by Prof. Lowell made January 21, 1914.
Nos. 1 and 2 show the effect of the planet's rotation. Nos. 3 and 4
depict quite different sections. Note the change in the polar snow-caps
in the last two.]
[Illustration: FIG. 16.--THE MOON, AT NINE AND THREE-QUARTER DAYS
Note the mysterious "rays" diverging from the almost perfectly circular
craters indicated by the arrows (Tycho, upper; Copernicus, lower), and
also the mountains to the right with the lunar dawn breaking on them.]
We turn to Mars; and we must first make it clear why there is so much
speculation about life on Mars, and why it is supposed that, if there
_is_ life on Mars, it must be more advanced than life on the earth.
Is there Life on Mars?
The basis of this belief is that if, as we saw, all the globes in our
solar system are masses of metal that are cooling down, the smaller will
have cooled down before the larger, and will be further ahead in their
development. Now Mars is very much smaller than the earth, and must have
cooled at its surface millions of years before the earth did. Hence, if
a story of life began on Mars at all, it began long before the story of
life on the earth. We cannot guess what sort of life-forms would be
evolved in a different world, but we can confidently say that they would
tend toward increasing intelligence; and thus we are disposed to look
for highly intelligent beings on Mars.
But this argument supposes that the conditions of life, namely air and
water, are found on Mars, and it is disputed whether they are found
there in sufficient quantity. The late Professor Percival Lowell, who
made a lifelong study of Mars, maintained that there are hundreds of
straight lines drawn across the surface of the planet, and he claimed
that they are beds of vegetation marking the sites of great channels or
pipes by means of which the "Martians" draw water from their polar
ocean. Professor W. H. Pickering, another high authority, thinks that
the lines are long, narrow marshes fed by moist winds from the poles.
There are certainly white polar caps on Mars. They seem to melt in the
spring, and the dark fringe round them grows broader.
Other astronomers, however, say that they find no trace of water-vapour
in the atmosphere of Mars, and they think that the polar caps may be
simply thin sheets of hoar-frost or frozen gas. They point out that, as
the atmosphere of Mars is certainly scanty, and the distance from the
sun is so great, it may be too cold for the fluid water to exist on the
planet.
If one asks why our wonderful instruments cannot settle these points,
one must be reminded that Mars is never nearer than 34,000,000 miles
from the earth, and only approaches to this distance once in fifteen or
seventeen years. The image of Mars on the photographic negative taken in
a big telescope is very small. Astronomers rely to a great extent on the
eye, which is more sensitive than the photographic plate. But it is easy
to have differences of opinion as to what the eye sees, and so there is
a good deal of controversy.
In August, 1924, the planet will again be well placed for observation,
and we may learn more about it. Already a few of the much-disputed
lines, which people wrongly call "canals," have been traced on
photographs. Astronomers who are sceptical about life on Mars are often
not fully aware of the extraordinary adaptability of life. There was a
time when the climate of the whole earth, from pole to pole, was
semi-tropical for millions of years. No animal could then endure the
least cold, yet now we have plenty of Arctic plants and animals. If the
cold came slowly on Mars, as we have reason to suppose, the population
could be gradually adapted to it. On the whole, it is possible that
there is advanced life on Mars, and it is not impossible, in spite of
the very great difficulties of a code of communication, that our "elder
brothers" may yet flash across space the solution of many of our
problems.
§ 2
Jupiter and Saturn
Next to Mars, going outward from the sun, is Jupiter. Between Mars and
Jupiter, however, there are more than three hundred million miles of
space, and the older astronomers wondered why this was not occupied by a
planet. We now know that it contains about nine hundred "planetoids," or
small globes of from five to five hundred miles in diameter. It was at
one time thought that a planet might have burst into these fragments (a
theory which is not mathematically satisfactory), or it may be that the
material which is scattered in them was prevented by the nearness of the
great bulk of Jupiter from uniting into one globe.
For Jupiter is a giant planet, and its gravitational influence must
extend far over space. It is 1,300 times as large as the earth, and has
nine moons, four of which are large, in attendance on it. It is
interesting to note that the outermost moons of Jupiter and Saturn
revolve round these planets in a direction contrary to the usual
direction taken by moons round planets, and by planets round the sun.
But there is no life on Jupiter.
The surface which we see in photographs (Fig. 12) is a mass of cloud or
steam which always envelops the body of the planet. It is apparently
red-hot. A red tinge is seen sometimes at the edges of its cloud-belts,
and a large red region (the "red spot"), 23,000 miles in length, has
been visible on it for half a century. There may be a liquid or solid
core to the planet, but as a whole it is a mass of seething vapours
whirling round on its axis once in every ten hours. As in the case of
the sun, however, different latitudes appear to rotate at different
rates. The interior of Jupiter is very hot, but the planet is not
self-luminous. The planets Venus and Jupiter shine very brightly, but
they have no light of their own; they reflect the sunlight.
Saturn is in the same interesting condition. The surface in the
photograph (Fig. 13) is steam, and Saturn is so far away from the sun
that the vaporisation of its oceans must necessarily be due to its own
internal heat. It is too hot for water to settle on its surface. Like
Jupiter, the great globe turns on its axis once in ten hours--a
prodigious speed--and must be a swirling, seething mass of metallic
vapours and gases. It is instructive to compare Jupiter and Saturn in
this respect with the sun. They are smaller globes and have cooled down
more than the central fire.
Saturn is a beautiful object in the telescope because it has ten moons
(to include one which is disputed) and a wonderful system of "rings"
round it. The so-called rings are a mighty swarm of meteorites--pieces
of iron and stone of all sorts and sizes, which reflect the light of the
sun to us. This ocean of matter is some miles deep, and stretches from a
few thousand miles from the surface of the planet to 172,000 miles out
in space. Some astronomers think that this is volcanic material which
has been shot out of the planet. Others regard it as stuff which would
have combined to form an eleventh moon but was prevented by the nearness
of Saturn itself. There is no evidence of life on Saturn.
THE MOON
Mars and Venus are therefore the only planets, besides the earth, on
which we may look for life; and in the case of Venus, the possibility is
very faint. But what about the moons which attend the planets? They
range in size from the little ten-miles-wide moons of Mars, to Titan, a
moon of Saturn, and Ganymede, a satellite of Jupiter, which are about
3,000 miles in diameter. May there not be life on some of the larger of
these moons? We will take our own moon as a type of the class.
A Dead World
The moon is so very much nearer to us than any other heavenly body that
we have a remarkable knowledge of it. In Fig. 14 you have a photograph,
taken in one of our largest telescopes, of part of its surface. In a
sense such a telescope brings the moon to within about fifty miles of
us. We should see a city like London as a dark, sprawling blotch on the
globe. We could just detect a Zeppelin or a Diplodocus as a moving speck
against the surface. But we find none of these things. It is true that a
few astronomers believe that they see signs of some sort of feeble life
or movement on the moon. Professor Pickering thinks that he can trace
some volcanic activity. He believes that there are areas of vegetation,
probably of a low order, and that the soil of the moon may retain a
certain amount of water in it. He speaks of a very thin atmosphere, and
of occasional light falls of snow. He has succeeded in persuading some
careful observers that there probably are slight changes of some kind
taking place on the moon.
[Illustration: FIG. 17.--A MAP OF THE CHIEF PLAINS AND CRATERS OF THE
MOON
The plains were originally supposed to be seas: hence the name "Mare."]
[Illustration: FIG. 18.--A DIAGRAM OF A STREAM OF METEORS SHOWING THE
EARTH PASSING THROUGH THEM] [Illustration: _Photo: Royal Observatory,
Greenwich._
FIG. 19.--COMET, September 29, 1908
Notice the tendency to form a number of tails. (See photograph below.)]
[Illustration: _Photo: Royal Observatory, Greenwich._
FIG. 20.--COMET, October 3, 1908
The process has gone further and a number of distinct tails can now be
counted.]
But there are many things that point to absence of air on the moon. Even
the photographs we reproduce tell the same story. The edges of the
shadows are all hard and black. If there had been an appreciable
atmosphere it would have scattered the sun's light on to the edges and
produced a gradual shading off such as we see on the earth. This
relative absence of air must give rise to some surprising effects. There
will be no sounds on the moon, because sounds are merely air waves. Even
a meteor shattering itself to a violent end against the surface of the
moon would make no noise. Nor would it herald its coming by glowing into
a "shooting star," as it would on entering the earth's atmosphere. There
will be no floating dust, no scent, no twilight, no blue sky, no
twinkling of the stars. The sky will be always black and the stars will
be clearly visible by day as by night. The sun's wonderful corona, which
no man on earth, even by seizing every opportunity during eclipses, can
hope to see for more than two hours in all in a long lifetime, will be
visible all day. So will the great red flames of the sun. Of course,
there will be no life, and no landscape effects and scenery effects due
to vegetation.
The moon takes approximately twenty-seven of our days to turn once on
its axis. So for fourteen days there is continuous night, when the
temperature must sink away down towards the absolute cold of space. This
will be followed without an instant of twilight by full daylight. For
another fourteen days the sun's rays will bear straight down, with no
diffusion or absorption of their heat, or light, on the way. It does not
follow, however, that the temperature of the moon's surface must rise
enormously. It may not even rise to the temperature of melting ice.
Seeing there is no air there can be no check on radiation. The heat that
the moon gets will radiate away immediately. We know that amongst the
coldest places on the earth are the tops of very high mountains, the
points that have reared themselves nearest to the sun but farthest out
of the sheltering blanket of the earth's atmosphere. The actual
temperature of the moon's surface by day is a moot point. It may be
below the freezing-point or above the boiling-point of water.
The Mountains of the Moon
The lack of air is considered by many astronomers to furnish the
explanation of the enormous number of "craters" which pit the moon's
surface. There are about a hundred thousand of these strange rings, and
it is now believed by many that they are spots where very large
meteorites, or even planetoids, splashed into the moon when its surface
was still soft. Other astronomers think that they are the remains of
gigantic bubbles which were raised in the moon's "skin," when the globe
was still molten, by volcanic gases from below. A few astronomers think
that they are, as is popularly supposed, the craters of extinct
volcanoes. Our craters, on the earth, are generally deep cups, whereas
these ring-formations on the moon are more like very shallow and broad
saucers. Clavius, the largest of them, is 123 miles across the interior,
yet its encircling rampart is not a mile high.
The mountains on the moon (Fig. 16) rise to a great height, and are
extraordinarily gaunt and rugged. They are like fountains of lava,
rising in places to 26,000 and 27,000 feet. The lunar Apennines have
three thousand steep and weird peaks. Our terrestrial mountains are
continually worn down by frost acting on moisture and by ice and water,
but there are none of these agencies operating on the moon. Its
mountains are comparatively "everlasting hills."
The moon is interesting to us precisely because it is a dead world. It
seems to show how the earth, or any cooling metal globe, will evolve in
the remote future. We do not know if there was ever life on the moon,
but in any case it cannot have proceeded far in development. At the most
we can imagine some strange lowly forms of vegetation lingering here and
there in pools of heavy gas, expanding during the blaze of the sun's
long day, and frozen rigid during the long night.
METEORS AND COMETS
We may conclude our survey of the solar system with a word about
"shooting stars," or meteors, and comets. There are few now who do not
know that the streak of fire which suddenly lights the sky overhead at
night means that a piece of stone or iron has entered our atmosphere
from outer space, and has been burned up by friction. It was travelling
at, perhaps, twenty or thirty miles a second. At seventy or eighty miles
above our heads it began to glow, as at that height the air is thick
enough to offer serious friction and raise it to a white heat. By the
time the meteor reached about twenty miles or so from the earth's
surface it was entirely dissipated, as a rule in fiery vapour.
Millions of Meteorites
It is estimated that between ten and a hundred million meteorites enter
our atmosphere and are cremated, every day. Most of them weigh only an
ounce or two, and are invisible. Some of them weigh a ton or more, but
even against these large masses the air acts as a kind of "torpedo-net."
They generally burst into fragments and fall without doing damage.
It is clear that "empty space" is, at least within the limits of our
solar system, full of these things. They swarm like fishes in the seas.
Like the fishes, moreover, they may be either solitary or gregarious.
The solitary bit of cosmic rubbish is the meteorite, which we have just
examined. A "social" group of meteorites is the essential part of a
comet. The nucleus, or bright central part, of the head of a comet (Fig.
19) consists of a swarm, sometimes thousands of miles wide, of these
pieces of iron or stone. This swarm has come under the sun's
gravitational influence, and is forced to travel round it. From some
dark region of space it has moved slowly into our system. It is not then
a comet, for it has no tail. But as the crowded meteors approach the
sun, the speed increases. They give off fine vapour-like matter and the
fierce flood of light from the sun sweeps this vapour out in an
ever-lengthening tail. Whatever way the comet is travelling, the tail
always points away from the sun.
A Great Comet
The vapoury tail often grows to an enormous length as the comet
approaches the sun. The great comet of 1843 had a tail two hundred
million miles long. It is, however, composed of the thinnest vapours
imaginable. Twice during the nineteenth century the earth passed through
the tail of a comet, and nothing was felt. The vapours of the tail are,
in fact, so attenuated that we can hardly imagine them to be white-hot.
They may be lit by some electrical force. However that may be, the comet
dashes round the sun, often at three or four hundred miles a second,
then may pass gradually out of our system once more. It may be a
thousand years, or it may be fifty years, before the monarch of the
system will summon it again to make its fiery journey round his throne.
[Illustration: _Photo: Harvard College Observatory._
FIG. 21.--TYPICAL SPECTRA
Six main types of stellar spectra. Notice the lines they have in common,
showing what elements are met with in different types of stars. Each of
these spectra corresponds to a different set of physical and chemical
conditions.] [Illustration: _Photo: Mount Wilson Observatory._
FIG. 22.--A NEBULAR REGION SOUTH OF ZETA ORIONIS
Showing a great projection of "dark matter" cutting off the light from
behind.]
[Illustration: _Photo: Astrophysical Observatory, Victoria, British
Columbia._
FIG. 23.--STAR CLUSTER IN HERCULES
A wonderful cluster of stars. It has been estimated that the distance of
this cluster is such that it would take light more than 100,000 years to
reach us.]
THE STELLAR UNIVERSE
§ 1
The immensity of the Stellar Universe, as we have seen, is beyond our
apprehension. The sun is nothing more than a very ordinary star, perhaps
an insignificant one. There are stars enormously greater than the sun.
One such, Betelgeux, has recently been measured, and its diameter is
more than 300 times that of the sun.
The Evolution of Stars
The proof of the similarity between our sun and the stars has come to us
through the spectroscope. The elements that we find by its means in the
sun are also found in the same way in the stars. Matter, says the
spectroscope, is essentially the same everywhere, in the earth and the
sun, in the comet that visits us once in a thousand years, in the star
whose distance is incalculable, and in the great clouds of "fire-mist"
that we call nebulæ.
In considering the evolution of the stars let us keep two points clearly
in mind. The starting-point, the nebula, is no figment of the scientific
imagination. Hundreds of thousands of nebulæ, besides even vaster
irregular stretches of nebulous matter, exist in the heavens. But the
stages of the evolution of this stuff into stars are very largely a
matter of speculation. Possibly there is more than one line of
evolution, and the various theories may be reconciled. And this applies
also to the theories of the various stages through which the stars
themselves pass on their way to extinction.
The light of about a quarter of a million stars has been analysed in the
spectroscope, and it is found that they fall into about a dozen classes
which generally correspond to stages in their evolution (Fig. 21).
The Age of Stars
In its main lines the spectrum of a star corresponds to its colour, and
we may roughly group the stars into red, yellow, and white. This is also
the order of increasing temperature, the red stars being the coolest and
the white stars the hottest. We might therefore imagine that the white
stars are the youngest, and that as they grow older and cooler they
become yellowish, then red, and finally become invisible--just as a
cooling white-hot iron would do. But a very interesting recent research
shows that there are two kinds of red stars; some of them are amongst
the oldest stars and some are amongst the youngest. The facts appear to
be that when a star is first formed it is not very hot. It is an immense
mass of diffuse gas glowing with a dull-red heat. It contracts under the
mutual gravitation of its particles, and as it does so it grows hotter.
It acquires a yellowish tinge. As it continues to contract it grows
hotter and hotter until its temperature reaches a maximum as a white
star. At this point the contraction process does not stop, but the
heating process does. Further contraction is now accompanied by cooling,
and the star goes through its colour changes again, but this time in the
inverse order. It contracts and cools to yellow and finally to red. But
when it again becomes a red star it is enormously denser and smaller
than when it began as a red star. Consequently the red stars are divided
into two classes called, appropriately, Giants and Dwarfs. This theory,
which we owe to an American astronomer, H. N. Russell, has been
successful in explaining a variety of phenomena, and there is
consequently good reason to suppose it to be true. But the question as
to how the red giant stars were formed has received less satisfactory
and precise answers.
The most commonly accepted theory is the nebular theory.
THE NEBULAR THEORY
§ 2
Nebulæ are dim luminous cloud-like patches in the heavens, more like
wisps of smoke in some cases than anything else. Both photography and
the telescope show that they are very numerous, hundreds of thousands
being already known and the number being continually added to. They are
not small. Most of them are immensely large. Actual dimensions cannot be
given, because to estimate these we must first know definitely the
distance of the nebulæ from the earth. The distances of some nebulæ are
known approximately, and we can therefore form some idea of size in
these cases. The results are staggering. The mere visible surface of
some nebulæ is so large that the whole stretch of the solar system would
be too small to form a convenient unit for measuring it. A ray of light
would require to travel for years to cross from side to side of such a
nebula. Its immensity is inconceivable to the human mind.
There appear to be two types of nebulæ, and there is evidence suggesting
that the one type is only an earlier form of the other; but this again
we do not know.
The more primitive nebulæ would seem to be composed of gas in an
extremely rarified form. It is difficult to convey an adequate idea of
the rarity of nebular gases. The residual gases in a vacuum tube are
dense by comparison. A cubic inch of air at ordinary pressure would
contain more matter than is contained in millions of cubic inches of the
gases of nebulæ. The light of even the faintest stars does not seem to
be dimmed by passing through a gaseous nebula, although we cannot be
sure on this point. The most remarkable physical fact about these gases
is that they are luminous. Whence they derive their luminosity we do not
know. It hardly seems possible to believe that extremely thin gases
exposed to the terrific cold of space can be so hot as to be luminous
and can retain their heat and their luminosity indefinitely. A cold
luminosity due to electrification, like that of the aurora borealis,
would seem to fit the case better.
Now the nebular theory is that out of great "fire-mists," such as we
have described, stars are born. We do not know whether gravitation is
the only or even the main force at work in a nebula, but it is supposed
that under the action of gravity the far-flung "fire-mists" would begin
to condense round centres of greatest density, heat being evolved in the
process. Of course the condensation would be enormously slow, although
the sudden irruption of a swarm of meteors or some solid body might
hasten matters greatly by providing large, ready-made centres of
condensation.
Spiral Nebulæ
It is then supposed that the contracting mass of gas would begin to
rotate and to throw off gigantic streamers, which would in their turn
form centres of condensation. The whole structure would thus form a
spiral, having a dense region at its centre and knots or lumps of
condensed matter along its spiral arms. Besides the formless gaseous
nebulæ there are hundreds of thousands of "spiral" nebulæ such as we
have just mentioned in the heavens. They are at all stages of
development, and they are visible to us at all angles--that is to say,
some of them face directly towards us, others are edge on, and some are
in intermediate positions. It appears, therefore, that we have here a
striking confirmation of the nebular hypothesis. But we must not go so
fast. There is much controversy as to the nature of these spiral nebulæ.
Some eminent astronomers think they are other stellar universes,
comparable in size with our own. In any case they are vast structures,
and if they represent stars in process of condensation, they must be
giving birth to huge agglomerations of stars--to star clusters at least.
These vast and enigmatic objects do not throw much light on the origin
of our own solar system. The nebular hypothesis, which was invented
by Laplace to explain the origin of our solar system, has not yet met
with universal acceptance. The explanation offers grave difficulties,
and it is best while the subject is still being closely investigated, to
hold all opinions with reserve. It may be taken as probable, however,
that the universe has developed from masses of incandescent gas.
[Illustration: _Photo: Yerkes Observatory._
FIG. 24.--THE GREAT NEBULA IN ORION
The most impressive nebula in the heavens. It is inconceivably greater
in dimensions than the whole solar system.]
[Illustration: _Photo: Lick Observatory._
FIG. 25--GIANT SPIRAL NEBULA, March 23, 1914
This spiral nebula is seen full on. Notice the central nucleus and the
two spiral arms emerging from its opposite directions. Is matter flowing
out of the nucleus into the arms or along the arms into the nucleus? In
either case we should get two streams in opposite directions within the
nucleus.]
THE BIRTH AND DEATH OF STARS
§ 3
Variable, New, and Dark Stars: Dying Suns
Many astronomers believe that in "variable stars" we have another star,
following that of the dullest red star, in the dying of suns. The light
of these stars varies periodically in so many days, weeks, or years. It
is interesting to speculate that they are slowly dying suns, in which
the molten interior periodically bursts through the shell of thick
vapours that is gathering round them. What we saw about our sun seems to
point to some such stage in the future. That is, however, not the
received opinion about variable stars. It may be that they are stars
which periodically pass through a great swarm of meteors or a region of
space that is rich in cosmic dust of some sort, when, of course, a great
illumination would take place.
One class of these variable stars, which takes its name from the star
Algol, is of special interest. Every third night Algol has its light
reduced for several hours. Modern astronomy has discovered that in this
case there are really two stars, circulating round a common centre, and
that every third night the fainter of the two comes directly between us
and its companion and causes an "eclipse." This was until recently
regarded as a most interesting case in which a dead star revealed itself
to us by passing before the light of another star. But astronomers have
in recent years invented something, the "selenium-cell," which is even
more sensitive than the photographic plate, and on this the supposed
dead star registers itself as very much alive. Algol is, however,
interesting in another way. The pair of stars which we have discovered
in it are hundreds of trillions of miles away from the earth, yet we
know their masses and their distances from each other.
The Death and Birth of Stars
We have no positive knowledge of dead stars; which is not surprising
when we reflect that a dead star means an invisible star! But when we
see so many individual stars tending toward death, when we behold a vast
population of all conceivable ages, we presume that there are many
already dead. On the other hand, there is no reason to suppose that the
universe as a whole is "running down." Some writers have maintained
this, but their argument implies that we know a great deal more about
the universe than we actually do. The scientific man does not know
whether the universe is finite or infinite, temporal or eternal; and he
declines to speculate where there are no facts to guide him. He knows
only that the great gaseous nebulæ promise myriads of worlds in the
future, and he concedes the possibility that new nebulæ may be forming
in the ether of space.
The last, and not the least interesting, subject we have to notice is
the birth of a "new star." This is an event which astronomers now
announce every few years; and it is a far more portentous event than the
reader imagines when it is reported in his daily paper. The story is
much the same in all cases. We say that the star appeared in 1901, but
you begin to realise the magnitude of the event when you learn that the
distant "blaze" had really occurred about the time of the death of
Luther! The light of the conflagration had been speeding toward us
across space at 186,000 miles a second, yet it has taken nearly three
centuries to reach us. To be visible at all to us at that distance the
fiery outbreak must have been stupendous. If a mass of petroleum ten
times the size of the earth were suddenly fired it would not be seen at
such a distance. The new star had increased its light many hundredfold
in a few days.
There is a considerable fascination about the speculation that in such
cases we see the resurrection of a dead world, a means of renewing the
population of the universe. What happens is that in some region of the
sky where no star, or only a very faint star, had been registered on our
charts, we almost suddenly perceive a bright star. In a few days it may
rise to the highest brilliancy. By the spectroscope we learn that this
distant blaze means a prodigious outpour of white-hot hydrogen at
hundreds of miles a second. But the star sinks again after a few months,
and we then find a nebula round it on every side. It is natural to
suppose that a dead or dying sun has somehow been reconverted in whole
or in part into a nebula. A few astronomers think that it may have
partially collided with another star, or approached too closely to
another, with the result we described on an earlier page. The general
opinion now is that a faint or dead star had rushed into one of those
regions of space in which there are immense stretches of nebulous
matter, and been (at least in part) vaporised by the friction.
But the difficulties are considerable, and some astronomers prefer to
think that the blazing star may merely have lit up a dark nebula which
already existed. It is one of those problems on which speculation is
most tempting but positive knowledge is still very incomplete. We may be
content, even proud, that already we can take a conflagration that has
occurred more than a thousand trillion miles away and analyse it
positively into an outflame of glowing hydrogen gas at so many miles a
second.
THE SHAPE OF OUR UNIVERSE
§ 4
Our Universe a Spiral Nebula
What is the shape of our universe, and what are its dimensions? This is
a tremendous question to ask. It is like asking an intelligent insect,
living on a single leaf in the midst of a great Brazilian forest, to say
what is the shape and size of the forest. Yet man's ingenuity has proved
equal to giving an answer even to this question, and by a method exactly
similar to that which would be adopted by the insect. Suppose, for
instance, that the forest was shaped as an elongated oval, and the
insect lived on a tree near the centre of the oval. If the trees were
approximately equally spaced from one another they would appear much
denser along the length of the oval than across its width. This is the
simple consideration that has guided astronomers in determining the
shape of our stellar universe. There is one direction in the heavens
along which the stars appear denser than in the directions at right
angles to it. That direction is the direction in which we look towards
the Milky Way. If we count the number of stars visible all over the
heavens, we find they become more and more numerous as we approach the
Milky Way. As we go farther and farther from the Milky Way the stars
thin out until they reach a maximum sparseness in directions at right
angles to the plane of the Milky Way. We may consider the Milky Way to
form, as it were, the equator of our system, and the line at right
angles to point to the north and south poles.
Our system, in fact, is shaped something like a lens, and our sun is
situated near the centre of this lens. In the remoter part of this lens,
near its edge, or possibly outside it altogether, lies the great series
of star clouds which make up the Milky Way. All the stars are in motion
within this system, but the very remarkable discovery has been made that
these motions are not entirely random. The great majority of the stars
whose motions can be measured fall into two groups drifting past one
another in opposite directions. The velocity of one stream relative to
the other is about twenty-five miles per second. The stars forming these
two groups are thoroughly well mixed; it is not a case of an inner
stream going one way and an outer stream the other. But there are not
quite as many stars going one way as the other. For every two stars in
one stream there are three in the other. Now, as we have said, some
eminent astronomers hold that the spiral nebulæ are universes like our
own, and if we look at the two photographs (Figs. 25 and 26) we see that
these spirals present features which, in the light of what we have just
said about our system, are very remarkable. The nebula in Coma Berenices
is a spiral edge-on to us, and we see that it has precisely the
lens-shaped middle and the general flattened shape that we have found in
our own system. The nebula in Canes Venatici is a spiral facing towards
us, and its shape irresistibly suggests motions along the spiral arms.
This motion, whether it is towards or away from the central, lens-shaped
portion, would cause a double streaming motion in that central portion
of the kind we have found in our own system. Again, and altogether apart
from these considerations, there are good reasons for supposing our
Milky Way to possess a double-armed spiral structure. And the great
patches of dark absorbing matter which are known to exist in the Milky
Way (see Fig. 22) would give very much the mottled appearance we notice
in the arms (which we see edge-on) of the nebula in Coma Berenices. The
hypothesis, therefore, that our universe is a spiral nebula has much to
be said for it. If it be accepted it greatly increases our estimate of
the size of the material universe. For our central, lens-shaped system
is calculated to extend towards the Milky Way for more than twenty
thousand times a million million miles, and about a third of this
distance towards what we have called the poles. If, as we suppose, each
spiral nebula is an independent stellar universe comparable in size with
our own, then, since there are hundreds of thousands of spiral nebulæ,
we see that the size of the whole material universe is indeed beyond our
comprehension.
[Illustration: _Photo: Mount Wilson Observatory._
FIG. 26.--A SPIRAL NEBULA SEEN EDGE-ON
Notice the lens-shaped formation of the nucleus and the arm stretching
as a band across it. See reference in the text to the resemblance
between this and our stellar universe.]
[Illustration: _Photo: H. J. Shepstone._
100-INCH TELESCOPE, MOUNT WILSON
A reflecting telescope: the largest in the world. The mirror is situated
at the base of the telescope.]
[Illustration:
________________________________________________________________
| |
| THE SOLAR SYSTEM |
|________________________________________________________________|
| | | | | |
| | MEAN DISTANCE | PERIOD OF | | |
| NAME | FROM SUN (IN | REVOLUTION | DIAMETER | NUMBER OF |
| | MILLIONS OF | AROUND SUN | (IN MILES) | SATELLITES |
| | MILES) | (IN YEARS) | | |
|_________|_______________|____________|____________|____________|
| | | | | |
| MERCURY | 36.0 | 0.24 | 3030 | 0 |
| VENUS | 67.2 | 0.62 | 7700 | 0 |
| EARTH | 92.9 | 1.00 | 7918 | 1 |
| MARS | 141.5 | 1.88 | 4230 | 2 |
| JUPITER | 483.3 | 11.86 | 86500 | 9 |
| SATURN | 886.0 | 29.46 | 73000 | 10 |
| URANUS | 1781.9 | 84.02 | 31900 | 4 |
| NEPTUNE | 2971.6 | 164.78 | 34800 | 1 |
| SUN | ------ | ------ | 866400 | -- |
| MOON | ------ | ------ | 2163 | -- |
|_________|_______________|____________|____________|____________|
FIG. 27]
[Illustration:
______________________________________
| |
| STAR DISTANCES |
|______________________________________|
| |
| DISTANCE IN |
| STAR LIGHT-YEARS |
| |
| POLARIS 76 |
| CAPELLA 49.4 |
| RIGEL 466 |
| SIRIUS 8.7 |
| PROCYON 10.5 |
| REGULUS 98.8 |
| ARCTURUS 43.4 |
| [ALPHA] CENTAURI 4.29 |
| VEGA 34.7 |
|______________________________________|
| |
| SMALLER MAGELLANIC CLOUD 32,600[A] |
| GREAT CLUSTER IN HERCULES 108,600[A] |
|______________________________________|
[A] ESTIMATED
FIG. 28
The above distances are merely approximate and are subject to further
revision. A "light-year" is the distance that light, travelling at the
rate of 186,000 miles per second, would cover in one year.]
In this simple outline we have not touched on some of the more debatable
questions that engage the attention of modern astronomers. Many of these
questions have not yet passed the controversial stage; out of these will
emerge the astronomy of the future. But we have seen enough to convince
us that, whatever advances the future holds in store, the science of the
heavens constitutes one of the most important stones in the wonderful
fabric of human knowledge.
ASTRONOMICAL INSTRUMENTS
§ 1
The Telescope
The instruments used in modern astronomy are amongst the finest triumphs
of mechanical skill in the world. In a great modern observatory the
different instruments are to be counted by the score, but there are two
which stand out pre-eminent as the fundamental instruments of modern
astronomy. These instruments are the telescope and the spectroscope, and
without them astronomy, as we know it, could not exist.
There is still some dispute as to where and when the first telescope was
constructed; as an astronomical instrument, however, it dates from the
time of the great Italian scientist Galileo, who, with a very small and
imperfect telescope of his own invention, first observed the spots on
the sun, the mountains of the moon, and the chief four satellites of
Jupiter. A good pair of modern binoculars is superior to this early
instrument of Galileo's, and the history of telescope construction, from
that primitive instrument to the modern giant recently erected on Mount
Wilson, California, is an exciting chapter in human progress. But the
early instruments have only an historic interest: the era of modern
telescopes begins in the nineteenth century.
During the last century telescope construction underwent an
unprecedented development. An immense amount of interest was taken in
the construction of large telescopes, and the different countries of the
world entered on an exciting race to produce the most powerful possible
instruments. Besides this rivalry of different countries there was a
rivalry of methods. The telescope developed along two different lines,
and each of these two types has its partisans at the present day. These
types are known as _refractors_ and _reflectors_, and it is necessary to
mention, briefly, the principles employed in each. The _refractor_ is
the ordinary, familiar type of telescope. It consists, essentially, of a
large lens at one end of a tube, and a small lens, called the eye-piece,
at the other. The function of the large lens is to act as a sort of
gigantic eye. It collects a large amount of light, an amount
proportional to its size, and brings this light to a focus within the
tube of the telescope. It thus produces a small but bright image, and
the eye-piece magnifies this image. In the _reflector_, instead of a
large lens at the top of the tube, a large mirror is placed at the
bottom. This mirror is so shaped as to reflect the light that falls on
it to a focus, whence the light is again led to an eye-piece. Thus the
refractor and the reflector differ chiefly in their manner of gathering
light. The powerfulness of the telescope depends on the size of the
light-gatherer. A telescope with a lens four inches in diameter is four
times as powerful as the one with a lens two inches in diameter, for the
amount of light gathered obviously depends on the _area_ of the lens,
and the area varies as the _square_ of the diameter.
The largest telescopes at present in existence are _reflectors_. It is
much easier to construct a very large mirror than to construct a very
large lens; it is also cheaper. A mirror is more likely to get out of
order than is a lens, however, and any irregularity in the shape of a
mirror produces a greater distorting effect than in a lens. A refractor
is also more convenient to handle than is a reflector. For these reasons
great refractors are still made, but the largest of them, the great
Yerkes' refractor, is much smaller than the greatest reflector, the one
on Mount Wilson, California. The lens of the Yerkes' refractor measures
three feet four inches in diameter, whereas the Mount Wilson reflector
has a diameter of no less than eight feet four inches.
[Illustration: THE YERKES 40-INCH REFRACTOR
(The largest _refracting_ telescope in the world. Its big lens weighs
1,000 pounds, and its mammoth tube, which is 62 feet long, weighs about
12,000 pounds. The parts to be moved weigh approximately 22 tons.
The great _100-inch reflector_ of the Mount Wilson reflecting
telescope--the largest _reflecting_ instrument in the world--weighs
nearly 9,000 pounds and the moving parts of the telescope weigh about
100 tons.
The new _72-inch reflector_ at the Dominion Astrophysical Observatory,
near Victoria, B. C., weighs nearly 4,500 pounds, and the moving parts
about 35 tons.)]
[Illustration: _Photo: H. J. Shepstone._
THE DOUBLE-SLIDE PLATE HOLDER ON YERKES 40-INCH REFRACTING TELESCOPE
The smaller telescope at the top of the picture acts as a "finder"; the
field of view of the large telescope is so restricted that it is
difficult to recognise, as it were, the part of the heavens being
surveyed. The smaller telescope takes in a larger area and enables the
precise object to be examined to be easily selected.]
[Illustration: MODERN DIRECT-READING SPECTROSCOPE
(_By A. Hilger, Ltd._)
The light is brought through one telescope, is split up by the prism,
and the resulting spectrum is observed through the other telescope.]
But there is a device whereby the power of these giant instruments,
great as it is, can be still further heightened. That device is the
simple one of allowing the photographic plate to take the place of the
human eye. Nowadays an astronomer seldom spends the night with his eye
glued to the great telescope. He puts a photographic plate there. The
photographic plate has this advantage over the eye, that it builds up
impressions. However long we stare at an object too faint to be seen, we
shall never see it. With the photographic plate, however, faint
impressions go on accumulating. As hour after hour passes, the star
which was too faint to make a perceptible impression on the plate goes
on affecting it until finally it makes an impression which can be made
visible. In this way the photographic plate reveals to us phenomena in
the heavens which cannot be seen even through the most powerful
telescopes.
Telescopes of the kind we have been discussing, telescopes for exploring
the heavens, are mounted _equatorially_; that is to say, they are
mounted on an inclined pillar parallel to the axis of the earth so that,
by rotating round this pillar, the telescope is enabled to follow the
apparent motion of a star due to the rotation of the earth. This motion
is effected by clock-work, so that, once adjusted on a star, and the
clock-work started, the telescope remains adjusted on that star for any
length of time that is desired. But a great official observatory, such
as Greenwich Observatory or the Observatory at Paris, also has _transit_
instruments, or telescopes smaller than the equatorials and without the
same facility of movement, but which, by a number of exquisite
refinements, are more adapted to accurate measurements. It is these
instruments which are chiefly used in the compilation of the _Nautical
Almanac_. They do not follow the apparent motions of the stars. Stars
are allowed to drift across the field of vision, and as each star
crosses a small group of parallel wires in the eye-piece its precise
time of passage is recorded. Owing to their relative fixity of position
these instruments can be constructed to record the _positions_ of stars
with much greater accuracy than is possible to the more general and
flexible mounting of equatorials. The recording of transit is
comparatively dry work; the spectacular element is entirely absent;
stars are treated merely as mathematical points. But these observations
furnish the very basis of modern mathematical astronomy, and without
them such publications as the _Nautical Almanac_ and the _Connaissance
du Temps_ would be robbed of the greater part of their importance.
§ 2
The Spectroscope
We have already learnt something of the principles of the spectroscope,
the instrument which, by making it possible to learn the actual
constitution of the stars, has added a vast new domain to astronomy. In
the simplest form of this instrument the analysing portion consists of a
single prism. Unless the prism is very large, however, only a small
degree of dispersion is obtained. It is obviously desirable, for
accurate analytical work, that the dispersion--that is, the separation
of the different parts of the spectrum--should be as great as possible.
The dispersion can be increased by using a large number of prisms, the
light emerging from the first prism, entering the second, and so on. In
this way each prism produces its own dispersive effect and, when a
number of prisms are employed, the final dispersion is considerable. A
considerable amount of light is absorbed in this way, however, so that
unless our primary source of light is very strong, the final spectrum
will be very feeble and hard to decipher.
Another way of obtaining considerable dispersion is by using a
_diffraction grating_ instead of a prism. This consists essentially of a
piece of glass on which lines are ruled by a diamond point. When the
lines are sufficiently close together they split up light falling on
them into its constituents and produce a spectrum. The modern
diffraction grating is a truly wonderful piece of work. It contains
several thousands of lines to the inch, and these lines have to be
spaced with the greatest accuracy. But in this instrument, again, there
is a considerable loss of light.
We have said that every substance has its own distinctive spectrum, and
it might be thought that, when a list of the spectra of different
substances has been prepared, spectrum analysis would become perfectly
straightforward. In practice, however, things are not quite so simple.
The spectrum emitted by a substance is influenced by a variety of
conditions. The pressure, the temperature, the state of motion of the
object we are observing, all make a difference, and one of the most
laborious tasks of the modern spectroscopist is to disentangle these
effects from one another. Simple as it is in its broad outlines,
spectroscopy is, in reality, one of the most intricate branches of
modern science.
BIBLIOGRAPHY
(The following list of books may be useful to readers wishing to pursue
further the study of Astronomy.)
BALL, _The Story of the Heavens_.
BALL, _The Story of the Sun_.
FORBES, _History of Astronomy_.
HINCKS, _Astronomy_.
KIPPAX, _Call of the Stars_.
LOWELL, _Mars and Its Canals_.
LOWELL, _Evolution of Worlds_.
MCKREADY, _A Beginner's Star-Book_.
NEWCOMB, _Popular Astronomy_.
NEWCOMB, _The Stars: A Study of the Universe_.
OLCOTT, _Field Book of the Stars_.
PRICE, _Essence of Astronomy_.
SERVISS, _Curiosities of the Skies_.
WEBB, _Celestial Objects for Common Telescopes_.
YOUNG, _Text-Book of General Astronomy_.
II
THE STORY OF EVOLUTION
INTRODUCTORY
THE BEGINNING OF THE EARTH--MAKING A HOME FOR LIFE--THE FIRST LIVING
CREATURES
§ 1
The Evolution-idea is a master-key that opens many doors. It is a
luminous interpretation of the world, throwing the light of the past
upon the present. Everything is seen to be an antiquity, with a history
behind it--a _natural history_, which enables us to understand in some
measure how it has come to be as it is. We cannot say more than
"understand in some measure," for while the _fact_ of evolution is
certain, we are only beginning to discern the _factors_ that have been
at work.
The evolution-idea is very old, going back to some of the Greek
philosophers, but it is only in modern times that it has become an
essential part of our mental equipment. It is now an everyday
intellectual tool. It was applied to the origin of the solar system and
to the making of the earth before it was applied to plants and animals;
it was extended from these to man himself; it spread to language, to
folk-ways, to institutions. Within recent years the evolution-idea has
been applied to the chemical elements, for it appears that uranium may
change into radium, that radium may produce helium, and that lead is the
final stable result when the changes of uranium are complete. Perhaps
all the elements may be the outcome of an inorganic evolution. Not less
important is the extension of the evolution-idea to the world within as
well as to the world without. For alongside of the evolution of bodies
and brains is the evolution of feelings and emotions, ideas and
imagination.
Organic evolution means that the present is the child of the past and
the parent of the future. It is not a power or a principle; it is a
process--a process of becoming. It means that the present-day animals
and plants and all the subtle inter-relations between them have arisen
in a natural knowable way from a preceding state of affairs on the whole
somewhat simpler, and that again from forms and inter-relations simpler
still, and so on backwards and backwards for millions of years till we
lose all clues in the thick mist that hangs over life's beginnings.
Our solar system was once represented by a nebula of some sort, and we
may speak of the evolution of the sun and the planets. But since it has
been _the same material throughout_ that has changed in its distribution
and forms, it might be clearer to use some word like genesis. Similarly,
our human institutions were once very different from what they are now,
and we may speak of the evolution of government or of cities. But Man
works with a purpose, with ideas and ideals in some measure controlling
his actions and guiding his achievements, so that it is probably clearer
to keep the good old word history for all processes of social becoming
in which man has been a conscious agent. Now between the genesis of the
solar system and the history of civilisation there comes the vast
process of organic evolution. The word development should be kept for
the becoming of the individual, the chick out of the egg, for instance.
Organic evolution is a continuous natural process of racial change, by
successive steps in a definite direction, whereby distinctively new
individualities arise, take root, and flourish, sometimes alongside of,
and sometimes, sooner or later, in place of, the originative stock. Our
domesticated breeds of pigeons and poultry are the results of
evolutionary change whose origins are still with us in the Rock Dove and
the Jungle Fowl; but in most cases in Wild Nature the ancestral stocks
of present-day forms are long since extinct, and in many cases they are
unknown. Evolution is a long process of coming and going, appearing and
disappearing, a long-drawn-out sublime process like a great piece of
music.
[Illustration: _Photo: Rischgitz Collection._
CHARLES DARWIN
Greatest of naturalists, who made the idea of evolution current
intellectual coin, and in his _Origin of Species_ (1859) made the whole
world new.]
[Illustration: _Photo: Rischgitz Collection._
LORD KELVIN
One of the greatest physicists of the nineteenth century. He estimated
the age of the earth at 20,000,000 years. He had not at his disposal,
however, the knowledge of recent discoveries, which have resulted in
this estimate being very greatly increased.]
[Illustration: _Photo: Lick Observatory._
A GIANT SPIRAL NEBULA
Laplace's famous theory was that the planets and the earth were formed
from great whirling nebulæ.]
[Illustration: _Photo: Natural History Museum._
METEORITE WHICH FELL NEAR SCARBOROUGH, AND IS NOW TO BE SEEN IN THE
NATURAL HISTORY MUSEUM
It weighs about 56 lb., and is a "stony" meteorite, i.e., an aerolite.]
§ 2
The Beginning of the Earth
When we speak the language of science we cannot say "In the beginning,"
for we do not know of and cannot think of any condition of things that
did not arise from something that went before. But we may qualify the
phrase, and legitimately inquire into the beginning of the earth within
the solar system. If the result of this inquiry is to trace the sun and
the planets back to a nebula we reach only a relative beginning. The
nebula has to be accounted for. And even before matter there may have
been a pre-material world. If we say, as was said long ago, "In the
beginning was Mind," we may be expressing or trying to express a great
truth, but we have gone BEYOND SCIENCE.
The Nebular Hypothesis
One of the grandest pictures that the scientific mind has ever thrown
upon the screen is that of the Nebular Hypothesis. According to
Laplace's famous form of this theory (1796), the solar system was once a
gigantic glowing mass, spinning slowly and uniformly around its centre.
As the incandescent world-cloud of gas cooled and its speed of rotation
increased the shrinking mass gave off a separate whirling ring, which
broke up and gathered together again as the first and most distant
planet. The main mass gave off another ring and another till all the
planets, including the earth, were formed. The central mass persisted as
the sun.
Laplace spoke of his theory, which Kant had anticipated forty-one years
before, with scientific caution: "conjectures which I present with all
the distrust which everything not the result of observation or of
calculation ought to inspire." Subsequent research justified his
distrust, for it has been shown that the original nebula need not have
been hot and need not have been gaseous. Moreover, there are great
difficulties in Laplace's theory of the separation of successive rings
from the main mass, and of the condensation of a whirling gaseous ring
into a planet.
So it has come about that the picture of a hot gaseous nebula revolving
as a unit body has given place to other pictures. Thus Sir Norman
Lockyer pointed out (1890) that the earth is gathering to itself
millions of meteorites every day; this has been going on for millions of
years; in distant ages the accretion may have been vastly more rapid and
voluminous; and so the earth has grown! Now the meteoritic contributions
are undoubted, but they require a centre to attract them, and the
difficulty is to account for the beginning of a collecting centre or
planetary nucleus. Moreover, meteorites are sporadic and erratic,
scattered hither and thither rather than collecting into unit-bodies. As
Professor Chamberlin says, "meteorites have rather the characteristics
of the wreckage of some earlier organisation than of the parentage of
our planetary system." Several other theories have been propounded to
account for the origin of the earth, but the one that has found most
favour in the eyes of authorities is that of Chamberlin and Moulton.
According to this theory a great nebular mass condensed to form the sun,
from which under the attraction of passing stars planet after planet,
the earth included, was heaved off in the form of knotted spiral nebulæ,
like many of those now observed in the heavens.
Of great importance were the "knots," for they served as collecting
centres drawing flying matter into their clutches. Whatever part of the
primitive bolt escaped and scattered was drawn out into independent
orbits round the sun, forming the "planetesimals" which behave like
minute planets. These planetesimals formed the food on which the knots
subsequently fed.
The Growth of the Earth
It has been calculated that the newborn earth--the "earth-knot" of
Chamberlin's theory--had a diameter of about 5,500 miles. But it grew
by drawing planetesimals into itself until it had a diameter of over
8,100 miles at the end of its growing period. Since then it has shrunk,
by periodic shrinkages which have meant the buckling up of successive
series of mountains, and it has now a diameter of 7,918 miles. But
during the shrinking the earth became more varied.
A sort of slow boiling of the internally hot earth often forced molten
matter through the cold outer crust, and there came about a gradual
assortment of lighter materials nearer the surface and heavier materials
deeper down. The continents are built of the lighter materials, such as
granites, while the beds of the great oceans are made of the heavier
materials such as basalts. In limited areas land has often become sea,
and sea has often given place to land, but the probability is that the
distinction of the areas corresponding to the great continents and
oceans goes back to a very early stage.
The lithosphere is the more or less stable crust of the earth, which may
have been, to begin with, about fifty miles in thickness. It seems that
the young earth had no atmosphere, and that ages passed before water
began to accumulate on its surface--before, in other words, there was
any hydrosphere. The water came from the earth itself, to begin with,
and it was long before there was any rain dissolving out saline matter
from the exposed rocks and making the sea salt. The weathering of the
high grounds of the ancient crust by air and water furnished the
material which formed the sandstones and mudstones and other sedimentary
rocks, which are said to amount to a thickness of over fifty miles in
all.
§ 3
Making a Home for Life
It is interesting to inquire how the callous, rough-and-tumble
conditions of the outer world in early days were replaced by others that
allowed of the germination and growth of that tender plant we call
LIFE. There are very tough living creatures, but the average organism is
ill suited for violence. Most living creatures are adapted to mild
temperatures and gentle reactions. Hence the fundamental importance of
the early atmosphere, heavy with planetesimal dust, in blanketing the
earth against intensities of radiance from without, as Chamberlin says,
and inequalities of radiance from within. This was the first preparation
for life, but it was an atmosphere without free oxygen. Not less
important was the appearance of pools and lakelets, of lakes and seas.
Perhaps the early waters covered the earth. And water was the second
preparation for life--water, that can dissolve a larger variety of
substances in greater concentration than any other liquid; water, that
in summer does not readily evaporate altogether from a pond, nor in
winter freeze throughout its whole extent; water, that is such a mobile
vehicle and such a subtle cleaver of substances; water, that forms over
80 per cent. of living matter itself.
Of great significance was the abundance of carbon, hydrogen, and oxygen
(in the form of carbonic acid and water) in the atmosphere of the
cooling earth, for these three wonderful elements have a unique
_ensemble_ of properties--ready to enter into reactions and relations,
making great diversity and complexity possible, favouring the formation
of the plastic and permeable materials that build up living creatures.
We must not pursue the idea, but it is clear that the stones and mortar
of the inanimate world are such that they built a friendly home for
life.
Origin of Living Creatures upon the Earth
During the early chapters of the earth's history, no living creature
that we can imagine could possibly have lived there. The temperature was
too high; there was neither atmosphere nor surface water. Therefore it
follows that at some uncertain, but inconceivably distant date, living
creatures appeared upon the earth. No one knows how, but it is
interesting to consider possibilities.
[Illustration: _Reproduced from the Smithsonian Report, 1915._
A LIMESTONE CANYON
Many fossils of extinct animals have been found in such rock
formations.]
[Illustration: GENEALOGICAL TREE OF ANIMALS
Showing in order of evolution the general relations of the chief classes
into which the world of living things is divided. This scheme represents
the present stage of our knowledge, but is admittedly provisional.]
[Illustration: DIAGRAM OF AMOEBA
(Greatly magnified.)
The amoeba is one of the simplest of all animals, and gives us a hint
of the original ancestors. It looks like a tiny irregular speck of
greyish jelly, about 1/100th of an inch in diameter. It is commonly
found gliding on the mud or weeds in ponds, where it engulfs its
microscopic food by means of out-flowing lobes (PS). The food vacuole
(FV) contains ingested food. From the contractile vacuole (CV) the waste
matter is discharged. N is the nucleus, GR, granules.]
From ancient times it has been a favourite answer that the dust of the
earth may have become living in a way which is outside scientific
description. This answer forecloses the question, and it is far too soon
to do that. Science must often say "Ignoramus": Science should be slow
to say "Ignorabimus."
A second position held by Helmholtz, Lord Kelvin, and others, suggests
that minute living creatures may have come to the earth from elsewhere,
in the cracks of a meteorite or among cosmic dust. It must be remembered
that seeds can survive prolonged exposure to very low temperatures; that
spores of bacteria can survive high temperature; that seeds of plants
and germs of animals in a state of "latent life" can survive prolonged
drought and absence of oxygen. It is possible, according to Berthelot,
that as long as there is not molecular disintegration vital activities
may be suspended for a time, and may afterwards recommence when
appropriate conditions are restored. Therefore, one should be slow to
say that a long journey through space is impossible. The obvious
limitation of Lord Kelvin's theory is that it only shifts the problem of
the origin of organisms (i.e. living creatures) from the earth to
elsewhere.
The third answer is that living creatures of a very simple sort may have
emerged on the earth's surface from not-living material, e.g. from some
semi-fluid carbon compounds activated by ferments. The tenability of
this view is suggested by the achievements of the synthetic chemists,
who are able artificially to build up substances such as oxalic acid,
indigo, salicylic acid, caffeine, and grape-sugar. We do not know,
indeed, what in Nature's laboratory would take the place of the clever
synthetic chemist, but there seems to be a tendency to complexity.
Corpuscles form atoms, atoms form molecules, small molecules large
ones.
Various concrete suggestions have been made in regard to the possible
origin of living matter, which will be dealt with in a later chapter. So
far as we know of what goes on to-day, there is no evidence of
spontaneous generation; organisms seem always to arise from pre-existing
organisms of the same kind; where any suggestion of the contrary has
been fancied, there have been flaws in the experimenting. But it is one
thing to accept the verdict "omne vivum e vivo" as a fact to which
experiment has not yet discovered an exception and another thing to
maintain that this must always have been true or must always remain
true.
If the synthetic chemists should go on surpassing themselves, if
substances like white of egg should be made artificially, and if we
should get more light on possible steps by which simple living creatures
may have arisen from not-living materials, this would not greatly affect
our general outlook on life, though it would increase our appreciation
of what is often libelled as "inert" matter. If the dust of the earth
did naturally give rise very long ago to living creatures, if they are
in a real sense born of her and of the sunshine, then the whole world
becomes more continuous and more vital, and all the inorganic groaning
and travailing becomes more intelligible.
§ 4
The First Organisms upon the Earth
We cannot have more than a speculative picture of the first living
creatures upon the earth or, rather, in the waters that covered the
earth. A basis for speculation is to be found, however, in the simplest
creatures living to-day, such as some of the bacteria and one-celled
animalcules, especially those called Protists, which have not taken any
very definite step towards becoming either plants or animals. No one can
be sure, but there is much to be said for the theory that the first
creatures were microscopic globules of living matter, not unlike the
simplest bacteria of to-day, but able to live on air, water, and
dissolved salts. From such a source may have originated a race of
one-celled marine organisms which were able to manufacture chlorophyll,
or something like chlorophyll, that is to say, the green pigment which
makes it possible for plants to utilise the energy of the sunlight in
breaking up carbon dioxide and in building up (photosynthesis) carbon
compounds like sugars and starch. These little units were probably
encased in a cell-wall of cellulose, but their boxed-in energy expressed
itself in the undulatory movement of a lash or flagellum, by means of
which they propelled themselves energetically through the water. There
are many similar organisms to-day, mostly in water, but some of
them--simple one-celled plants--paint the tree-stems and even the
paving-stones green in wet weather. According to Prof. A. H. Church
there was a long chapter in the history of the earth when the sea that
covered everything teemed with these green flagellates--the originators
of the Vegetable Kingdom.
On another tack, however, there probably evolved a series of simple
predatory creatures, not able to build up organic matter from air,
water, and salts, but devouring their neighbours. These units were not
closed in with cellulose, but remained naked, with their living matter
or protoplasm flowing out in changeful processes, such as we see in the
Amoebæ in the ditch or in our own white blood corpuscles and other
amoeboid cells. These were the originators of the animal kingdom. Thus
from very simple Protists the first animals and the first plants may
have arisen. All were still very minute, and it is worth remembering
that had there been any scientific spectator after our kind upon the
earth during these long ages, he would have lamented the entire absence
of life, although the seas were teeming. The simplest forms of life and
the protoplasm which Huxley called the physical basis of life will be
dealt with in the chapter on Biology in a later section of this work.
FIRST GREAT STEPS IN EVOLUTION
THE FIRST PLANTS--THE FIRST ANIMALS--BEGINNINGS OF BODIES--EVOLUTION OF
SEX--BEGINNING OF NATURAL DEATH
§ 1
The Contrast between Plants and Animals
However it may have come about, there is no doubt at all that one of the
first great steps in Organic Evolution was the forking of the
genealogical tree into Plants and Animals--the most important parting of
the ways in the whole history of Nature.
Typical plants have chlorophyll; they are able to feed at a low chemical
level on air, water, and salts, using the energy of the sunlight in
their photosynthesis. They have their cells boxed in by cellulose walls,
so that their opportunities for motility are greatly restricted. They
manufacture much more nutritive material than they need, and live far
below their income. They have no ready way of getting rid of any
nitrogenous waste matter that they may form, and this probably helps to
keep them sluggish.
Animals, on the other hand, feed at a high chemical level, on the
carbohydrates (e.g. starch and sugar), fats, and proteins (e.g. gluten,
albumin, casein) which are manufactured by other animals, or to begin
with, by plants. Their cells have not cellulose walls, nor in most cases
much wall of any kind, and motility in the majority is unrestricted.
Animals live much more nearly up to their income. If we could make for
an animal and a plant of equal weight two fractions showing the ratio of
the upbuilding, constructive, chemical processes to the down-breaking,
disruptive, chemical processes that go on in their respective bodies,
the ratio for the plant would be much greater than the corresponding
ratio for the animal. In other words, animals take the munitions which
plants laboriously manufacture and explode them in locomotion and
work; and the entire system of animate nature depends upon the
photosynthesis that goes on in green plants.
[Illustration: _From the Smithsonian Report, 1917_
A PIECE OF A REEF-BUILDING CORAL, BUILT UP BY A LARGE COLONY OF SMALL
SEA-ANEMONE-LIKE POLYPS, EACH OF WHICH FORMS FROM THE SALTS OF THE SEA A
SKELETON OR SHELL OF LIME
The wonderful mass of corals, which are very beautiful, are the skeleton
remains of hundreds of these little creatures.]
[Illustration: _Photo: J. J. Ward, F.E.S._
THE INSET CIRCLE SHOWS A GROUP OF CHALK-FORMING ANIMALS, OR
FORAMINIFERA, EACH ABOUT THE SIZE OF A VERY SMALL PIN'S HEAD
They form a great part of the chalk cliffs of Dover and similar deposits
which have been raised from the floor of an ancient sea.
THE ENORMOUSLY ENLARGED ILLUSTRATION IS THAT OF A COMMON FORAMINIFER
(POLYSTOMELLA) SHOWING THE SHELL IN THE CENTRE AND THE OUTFLOWING
NETWORK OF LIVING MATTER, ALONG WHICH GRANULES ARE CONTINUALLY
TRAVELLING, AND BY WHICH FOOD PARTICLES ARE ENTANGLED AND DRAWN IN
_Reproduced by permission of the Natural History Museum_ (_after Max
Schultze_).]
As the result of much more explosive life, animals have to deal with
much in the way of nitrogenous waste products, the ashes of the living
fire, but these are usually got rid of very effectively, e.g. in the
kidney filters, and do not clog the system by being deposited as
crystals and the like, as happens in plants. Sluggish animals like
sea-squirts which have no kidneys are exceptions that prove the rule,
and it need hardly be said that the statements that have been made in
regard to the contrasts between plants and animals are general
statements. There is often a good deal of the plant about the animal, as
in sedentary sponges, zoophytes, corals, and sea-squirts, and there is
often a little of the animal about the plant, as we see in the movements
of all shoots and roots and leaves, and occasionally in the parts of the
flower. But the important fact is that on the early forking of the
genealogical tree, i.e. the divergence of plants and animals, there
depended and depends all the higher life of the animal kingdom, not to
speak of mankind. The continuance of civilisation, the upkeep of the
human and animal population of the globe, and even the supply of oxygen
to the air we breathe, depend on the silent laboratories of the green
leaves, which are able with the help of the sunlight to use carbonic
acid, water, and salts to build up the bread of life.
§ 2
The Beginnings of Land Plants
It is highly probable that for long ages the waters covered the earth,
and that all the primeval vegetation consisted of simple Flagellates in
the universal Open Sea. But contraction of the earth's crust brought
about elevations and depressions of the sea-floor, and in places the
solid substratum was brought near enough the surface to allow the
floating plants to begin to settle down without getting out of the
light. This is how Professor Church pictures the beginning of a fixed
vegetation--a very momentous step in evolution. It was perhaps among
this early vegetation that animals had their first successes. As the
floor of the sea in these shallow areas was raised higher and higher
there was a beginning of dry land. The sedentary plants already spoken
of were the ancestors of the shore seaweeds, and there is no doubt that
when we go down at the lowest tide and wade cautiously out among the
jungle of vegetation only exposed on such occasions we are getting a
glimpse of very ancient days. _This_ is the forest primeval.
The Protozoa
Animals below the level of zoophytes and sponges are called Protozoa.
The word obviously means "First Animals," but all that we can say is
that the very simplest of them may give us some hint of the simplicity
of the original first animals. For it is quite certain that the vast
majority of the Protozoa to-day are far too complicated to be thought of
as primitive. Though most of them are microscopic, each is an animal
complete in itself, with the same fundamental bodily attributes as are
manifested in ourselves. They differ from animals of higher degree in
not being built up of the unit areas or corpuscles called cells. They
have no cells, no tissues, no organs, in the ordinary acceptation of
these words, but many of them show a great complexity of internal
structure, far exceeding that of the ordinary cells that build up the
tissues of higher animals. They are complete living creatures which have
not gone in for body-making.
In the dim and distant past there was a time when the only animals were
of the nature of Protozoa, and it is safe to say that one of the great
steps in evolution was the establishment of three great types of
Protozoa: (_a_) Some were very active, the Infusorians, like the slipper
animalcule, the night-light (Noctiluca), which makes the seas
phosphorescent at night, and the deadly Trypanosome, which causes
Sleeping Sickness. (_b_) Others were very sluggish, the parasitic
Sporozoa, like the malaria organism which the mosquito introduces into
man's body. (_c_) Others were neither very active nor very passive, the
Rhizopods, with out-flowing processes of living matter. This amoeboid
line of evolution has been very successful; it is represented by the
Rhizopods, such as Amoebæ and the chalk-forming Foraminifera and the
exquisitely beautiful flint-shelled Radiolarians of the open sea. They
have their counterparts in the amoeboid cells of most multicellular
animals, such as the phagocytes which migrate about in the body,
engulfing and digesting intruding bacteria, serving as sappers and
miners when something has to be broken down and built up again, and
performing other useful offices.
§ 3
The Making of a Body
The great naturalist Louis Agassiz once said that the biggest gulf in
Organic Nature was that between the unicellular and the multicellular
animals (Protozoa and Metazoa). But the gulf was bridged very long ago
when sponges, stinging animals, and simple worms were evolved, and
showed, for the first time, a "body." What would one not give to be able
to account for the making of a body, one of the great steps in
evolution! No one knows, but the problem is not altogether obscure.
When an ordinary Protozoon or one-celled animal divides into two or
more, which is its way of multiplying, the daughter-units thus formed
float apart and live independent lives. But there are a few Protozoa in
which the daughter-units are not quite separated off from one another,
but remain coherent. Thus Volvox, a beautiful green ball, found in some
canals and the like, is a colony of a thousand or even ten thousand
cells. It has almost formed a body! But in this "colony-making"
Protozoon, and in others like it, the component cells are all of one
kind, whereas in true multicellular animals there are different kinds
of cells, showing division of labour. There are some other Protozoa in
which the nucleus or kernel divides into many nuclei within the cell.
This is seen in the Giant Amoeba (Pelomyxa), sometimes found in
duck-ponds, or the beautiful Opalina, which always lives in the hind
part of the frog's food-canal. If a portion of the living matter of
these Protozoa should gather round each of the nuclei, then _that would
be the beginning of a body_. It would be still nearer the beginning of a
body if division of labour set in, and if there was a setting apart of
egg-cells and sperm-cells distinct from body-cells.
It was possibly in some such way that animals and plants with a body
were first evolved. Two points should be noticed, that body-making is
not essentially a matter of size, though it made large size possible.
For the body of a many-celled Wheel Animalcule or Rotifer is no bigger
than many a Protozoon. Yet the Rotifer--we are thinking of Hydatina--has
nine hundred odd cells, whereas the Protozoon has only one, except in
forms like Volvox. Secondly, it is a luminous fact that _every
many-celled animal from sponge to man that multiplies in the ordinary
way begins at the beginning again as a "single cell,"_ the fertilised
egg-cell. It is, of course, not an ordinary single cell that develops
into an earthworm or a butterfly, an eagle, or a man; it is a cell in
which a rich inheritance, the fruition of ages, is somehow condensed;
but it is interesting to bear in mind the elementary fact that every
many-celled creature, reproduced in the ordinary way and not by budding
or the like, starts as a fertilised egg-cell. The coherence of the
daughter-cells into which the fertilised egg-cell divides is a
reminiscence, as it were, of the primeval coherence of daughter-units
that made the first body possible.
The Beginning of Sexual Reproduction
A freshwater Hydra, growing on the duckweed usually multiplies by
budding. It forms daughter-buds, living images of itself; a check comes
to nutrition and these daughter-buds go free. A big sea-anemone may
divide in two or more parts, which become separate animals. This is
asexual reproduction, which means that the multiplication takes place by
dividing into two or many portions, and not by liberating egg-cells and
sperm-cells. Among animals as among plants, asexual reproduction is very
common. But it has great disadvantages, for it is apt to be
physiologically expensive, and it is beset with difficulties when the
body shows great division of labour, and is very intimately bound into
unity. Thus, no one can think of a bee or a bird multiplying by division
or by budding. Moreover, if the body of the parent has suffered from
injury or deterioration, the result of this is bound to be handed on to
the next generation if asexual reproduction is the only method.
[Illustration: _Photos: J. J. Ward, F.E.S._
A PLANT-LIKE ANIMAL, OR ZOOPHYTE, CALLED OBELIA
Consisting of a colony of small polyps, whose stinging tentacles are
well shown greatly enlarged in the lower photograph.]
[Illustration: _Reproduced by permission of "The Quart. Journ. Mic.
Sci."_
TRYPANOSOMA GAMBIENSE
(Very highly magnified.)
The microscopic animal Trypanosome, which causes Sleeping Sickness. The
study of these organisms has of late years acquired an immense
importance on account of the widespread and dangerous maladies to which
some of them give rise. It lives in the blood of man, who is infected by
the bite of a Tse-tse fly which carries the parasite from some other
host.]
[Illustration: VOLVOX
The Volvox is found in some canals and the like. It is one of the first
animals to suggest the beginning of a body. It is a colony of a thousand
or even ten thousand cells, but they are all cells of one kind. In
_multicellular_ animals the cells are of _different_ kinds with
different functions. Each of the ordinary cells (marked 5) has two
lashes or flagella. Daughter colonies inside the Parent colony are being
formed at 3, 4, and 2. The development of germ-cells is shown at 1.]
[Illustration: PROTEROSPONGIA
One of the simplest multicellular animals, illustrating the beginning of
a body. There is a setting apart of egg-cells and sperm-cells, distinct
from body-cells; the collared lashed cells on the margin are different
in kind from those farther in. Thus, as in indubitable multicellular
animals, division of labour has begun.]
Splitting into two or many parts was the old-fashioned way of
multiplying, but one of the great steps in evolution was the discovery
of a better method, namely, sexual reproduction. The gist of this is
simply that during the process of body-building (by the development of
the fertilised egg-cell) certain units, _the germ-cells_, do not share
in forming ordinary tissues or organs, but remain apart, continuing the
full inheritance which was condensed in the fertilised egg-cell. _These
cells kept by themselves are the originators of the future reproductive
cells of the mature animal_; they give rise to the egg-cells and the
sperm-cells.
The advantages of this method are great. (1) The new generation is
started less expensively, for it is easier to shed germ-cells into the
cradle of the water than to separate off half of the body. (2) It is
possible to start a great many new lives at once, and this may be of
vital importance when the struggle for existence is very keen, and when
parental care is impossible. (3) The germ-cells are little likely to be
prejudicially affected by disadvantageous dints impressed on the body of
the parent--little likely unless the dints have peculiarly penetrating
consequences, as in the case of poisons. (4) A further advantage is
implied in the formation of two kinds of germ-cells--the ovum or
egg-cell, with a considerable amount of building material and often with
a legacy of nutritive yolk; the spermatozoon or sperm-cell, adapted to
move in fluids and to find the ovum from a distance, thus securing
change-provoking cross-fertilisation.
§ 4
The Evolution of Sex
Another of the great steps in organic evolution was the differentiation
of two different physiological types, the male or sperm-producer and the
female or egg-producer. It seems to be a deep-seated difference in
constitution, which leads one egg to develop into a male, and another,
lying beside it in the nest, into a female. In the case of pigeons it
seems almost certain, from the work of Professor Oscar Riddle, that
there are two kinds of egg, a male-producing egg and a female-producing
egg, which differ in their yolk-forming and other physiological
characters.
In sea-urchins we often find two creatures superficially
indistinguishable, but the one is a female with large ovaries and the
other is a male with equally large testes. Here the physiological
difference does not affect the body as a whole, but the reproductive
organs or gonads only, though more intimate physiology would doubtless
discover differences in the blood or in the chemical routine
(metabolism). In a large number of cases, however, there are marked
superficial differences between the sexes, and everyone is familiar with
such contrasts as peacock and peahen, stag and hind. In such cases the
physiological difference between the sperm-producer and the
ovum-producer, for this is the essential difference, saturates through
the body and expresses itself in masculine and feminine structures and
modes of behaviour. The expression of the masculine and feminine
characters is in some cases under the control of hormones or chemical
messengers which are carried by the blood from the reproductive organs
throughout the body, and pull the trigger which brings about the
development of an antler or a wattle or a decorative plume or a capacity
for vocal and saltatory display. In some cases it is certain that the
female carries in a latent state the masculine features, but these are
kept from expressing themselves by other chemical messengers from the
ovary. Of these chemical messengers more must be said later on.
Recent research has shown that while the difference between male and
female is very deep-rooted, corresponding to a difference in gearing, it
is not always clear-cut. Thus a hen-pigeon may be very masculine, and a
cock-pigeon very feminine. The difference is in degree, not in kind.
§ 5
What is the meaning of the universal or almost universal inevitableness
of death? A Sequoia or "Big Tree" of California has been known to live
for over two thousand years, but eventually it died. A centenarian
tortoise has been known, and a sea-anemone sixty years of age; but
eventually they die. What is the meaning of this apparently inevitable
stoppage of bodily life?
The Beginning of Natural Death
There are three chief kinds of death, (_a_) The great majority of
animals come to a violent end, being devoured by others or killed by
sudden and extreme changes in their surroundings. (_b_) When an animal
enters a new habitat, or comes into new associations with other
organisms, it may be invaded by a microbe or by some larger parasite to
which it is unaccustomed and to which it can offer no resistance. With
many parasites a "live-and-let-live" compromise is arrived at, but new
parasites are apt to be fatal, as man knows to his cost when he is
bitten by a tse-tse fly which infects him with the microscopic animal (a
Trypanosome) that causes Sleeping Sickness. In many animals the
parasites are not troublesome as long as the host is vigorous, but if
the host is out of condition the parasites may get the upper hand, as in
the so-called "grouse disease," and become fatal. (_c_) But besides
violent death and microbic (or parasitic) death, there is natural death.
This is in great part to be regarded as the price paid for a body. A
body worth having implies complexity or division of labour, and this
implies certain internal furnishings of a more or less stable kind in
which the effects of wear and tear are apt to accumulate. It is not the
living matter itself that grows old so much as the framework in which it
works--the furnishings of the vital laboratory. There are various
processes of rejuvenescence, e.g. rest, repair, change, reorganisation,
which work against the inevitable processes of senescence, but sooner or
later the victory is with ageing. Another deep reason for natural death
is to be found in the physiological expensiveness of reproduction, for
many animals, from worms to eels, illustrate natural death as the
nemesis of starting new lives. Now it is a very striking fact that to a
large degree the simplest animals or Protozoa are exempt from natural
death. They are so relatively simple that they can continually
recuperate by rest and repair; they do not accumulate any bad debts.
Moreover, their modes of multiplying, by dividing into two or many
units, are very inexpensive physiologically. It seems that in some
measure this bodily immortality of the Protozoa is shared by some simple
many-celled animals like the freshwater Hydra and Planarian worms. Here
is an interesting chapter in evolution, the evolution of means of
evading or staving off natural death. Thus there is the well-known case
of the Paloloworm of the coral-reefs where the body breaks up in
liberating the germ-cells, but the head-end remains fixed in a crevice
of the coral, and buds out a new body at leisure.
Along with the evolution of the ways of avoiding death should be
considered also the gradual establishment of the length of life best
suited to the welfare of the species, and the punctuation of the
life-history to suit various conditions.
[Illustration: _Photo: J. J. Ward, F.E.S._
GREEN HYDRA
A little freshwater polyp, about half an inch long, with a crown of
tentacles round the mouth. It is seen giving off a bud, a clear
illustration of asexual reproduction. When a tentacle touches some small
organism the latter is paralysed and drawn into the mouth.]
[Illustration: _Photo: J. J. Ward, F.E.S._
EARTHWORM
Earthworms began the profitable habit of moving with one end of the body
always in front, and from worms to man the great majority of animals
have bilateral symmetry.]
[Illustration: DIAGRAM ILLUSTRATING THE BEGINNING OF INDIVIDUAL LIFE
1. An immature _sperm_-cell, with 4 chromosomes (nuclear bodies)
represented as rods.
2. A mature sperm-cell, with 2 chromosomes.
3. An immature _egg_-cell, with 4 chromosomes represented as curved
bodies.
4. A mature egg-cell, with 2 chromosomes.
5. The spermatozoon fertilises the ovum, introducing 2 chromosomes.
6. The fertilised ovum, with 4 chromosomes, 2 of paternal origin and 2
of maternal origin.
7. The chromosomes lie at the equator, and each is split longitudinally.
The centrosome introduced by the spermatozoon has divided into two
centrosomes, one at each pole of the nucleus. These play an important
part in the division or segmentation of the egg.
8. The fertilised egg has divided into two cells. Each cell has 2
paternal and 2 maternal chromosomes.]
[Illustration: _Reproduced from the Smithsonian Report, 1917._
GLASS MODEL OF A SEA-ANEMONE
A long tubular sea-anemone, with a fine crown of tentacles around the
mouth. The suggestion of a flower is very obvious. By means of stinging
lassoes on the tentacles minute animals on which it feeds are paralysed
and captured for food.]
[Illustration: THIS DRAWING SHOWS THE EVOLUTION OF THE BRAIN FROM FISH
TO MAN
The Cerebrum, the seat of intelligence, increases in proportion to the
other parts. In mammals it becomes more and more convoluted. The brain,
which lies in one plane in fishes, becomes gradually curved on itself.
In birds it is more curved than the drawing shows.]
§ 6
Great Acquisitions
In animals like sea-anemones and jellyfishes the general symmetry of the
body is radial; that is to say, there is no right or left, and the body
might be halved along many planes. It is a kind of symmetry well suited
for sedentary or for drifting life. But worms began the profitable habit
of moving with one end of the body always in front, and from worms to
man the great majority of animals have bilateral symmetry. They have a
right and a left side, and there is only one cut that halves the body.
This kind of symmetry is suited for a more strenuous life than radial
animals show; it is suited for pursuing food, for avoiding enemies, for
chasing mates. And _with the establishment of bilateral symmetry must be
associated the establishment of head-brains_, the beginning of which is
to be found in some simple worm-types.
Among the other great acquisitions gradually evolved we may notice: a
well-developed head with sense-organs, the establishment of large
internal surfaces such as the digestive and absorptive wall of the
food-canal, the origin of quickly contracting striped muscle and of
muscular appendages, the formation of blood as a distributing medium
throughout the body, from which all the parts take what they need and to
which they also contribute.
Another very important acquisition, almost confined (so far as is known)
to backboned animals, was the evolution of what are called glands of
internal secretion, such as the thyroid and the supra-renal. These
manufacture subtle chemical substances which are distributed by the
blood throughout the body, and have a manifold influence in regulating
and harmonising the vital processes. Some of these chemical messengers
are called hormones, which stimulate organs and tissues to greater
activity; others are called chalones, which put on a brake. Some
regulate growth and others rapidly alter the pressure and composition
of the blood. Some of them call into active development certain parts of
the body which have been, as it were, waiting for an appropriate
trigger-pulling. Thus, at the proper time, the milk-glands of a
mammalian mother are awakened from their dormancy. This very interesting
outcome of evolution will be dealt with in another portion of this work.
THE INCLINED PLANE OF ANIMAL BEHAVIOUR
§ 1
Before passing to a connected story of the gradual emergence of higher
and higher forms of life in the course of the successive ages--the
procession of life, as it may be called--it will be useful to consider
the evolution of animal behaviour.
Evolution of Mind
A human being begins as a microscopic fertilised egg-cell, within which
there is condensed the long result of time--Man's inheritance. The long
period of nine months before birth, with its intimate partnership
between mother and offspring, is passed as it were in sleep, and no one
can make any statement in regard to the mind of the unborn child. Even
after birth the dawn of mind is as slow as it is wonderful. To begin
with, there is in the ovum and early embryo no nervous system at all,
and it develops very gradually from simple beginnings. Yet as mentality
cannot come in from outside, we seem bound to conclude that the
potentiality of it--whatever that means--resides in the individual from
the very first. The particular kind of activity known to us as thinking,
feeling, and willing is the most intimate part of our experience, known
to us directly apart from our senses, and the possibility of that must
be implicit in the germ-cell just as the genius of Newton was implicit
in a very miserable specimen of an infant. Now what is true of the
individual is true also of the race--there is a gradual evolution of
that aspect of the living creature's activity which we call mind. We
cannot put our finger on any point and say: Before this stage there was
no mind. Indeed, many facts suggest the conclusion that wherever there
is life there is some degree of mind--even in the plants. Or it might be
more accurate to put the conclusion in another way, that the activity we
call life has always in some degree an inner or mental aspect.
[Illustration: OKAPI AND GIRAFFE
The Okapi is one of the great zoölogical discoveries. It gives a good
idea of what the Giraffe's ancestors were like. The Okapi was unknown
until discovered in 1900 by Sir Harry Johnston in Central Africa, where
these strange animals have probably lived in dense forests from time
immemorial.]
In another part of this book there is an account of the dawn of mind in
backboned animals; what we aim at here is an outline of what may be
called the inclined plane of animal behaviour.
A very simple animal accumulates a little store of potential energy, and
it proceeds to expend this, like an explosive, by acting on its
environment. It does so in a very characteristic self-preservative
fashion, so that it burns without being consumed and explodes without
being blown to bits. It is characteristic of the organism that it
remains a going concern for a longer or shorter period--its length of
life. Living creatures that expended their energy ineffectively or
self-destructively would be eliminated in the struggle for existence.
When a simple one-celled organism explores a corner of the field seen
under a microscope, behaving to all appearance very like a dog scouring
a field seen through a telescope, it seems permissible to think of
something corresponding to mental endeavour associated with its
activity. This impression is strengthened when an amoeba pursues
another amoeba, overtakes it, engulfs it, loses it, pursues it again,
recaptures it, and so on. What is quite certain is that the behaviour of
the animalcule is not like that of a potassium pill fizzing about in a
basin of water, nor like the lurching movements of a gun that has got
loose and "taken charge" on board ship. Another feature is that the
locomotor activity of an animalcule often shows a distinct
individuality: it may swim, for instance, in a loose spiral.
But there is another side to vital activity besides acting upon the
surrounding world; the living creature is acted on by influences from
without. The organism acts on its environment; that is the one side of
the shield: the environment acts upon the organism; that is the other
side. If we are to see life whole we must recognise these two sides of
what we call living, and it is missing an important part of the history
of animal life if we fail to see that evolution implies becoming more
advantageously sensitive to the environment, making more of its
influences, shutting out profitless stimuli, and opening more gateways
to knowledge. The bird's world is a larger and finer world than an
earthworm's; the world means more to the bird than to the worm.
The Trial and Error Method
Simple creatures act with a certain degree of spontaneity on their
environment, and they likewise react effectively to surrounding stimuli.
Animals come to have definite "answers back," sometimes several,
sometimes only one, as in the case of the Slipper Animalcule, which
reverses its cilia when it comes within the sphere of some disturbing
influence, retreats, and, turning upon itself tentatively, sets off
again in the same general direction as before, but at an angle to the
previous line. If it misses the disturbing influence, well and good; if
it strikes it again, the tactics are repeated until a satisfactory way
out is discovered or the stimulation proves fatal.
It may be said that the Slipper Animalcule has but one answer to every
question, but there are many Protozoa which have several enregistered
reactions. When there are alternative reactions which are tried one
after another, the animal is pursuing what is called the trial-and-error
method, and a higher note is struck.
There is an endeavour after satisfaction, and a trial of answers. When
the creature profits by experience to the extent of giving the right
answer first, there is the beginning of learning.
[Illustration: DIAGRAM OF A SIMPLE REFLEX ARC IN A BACKBONELESS ANIMAL
LIKE AN EARTHWORM
1. A sensory nerve-cell (S.C.) on the surface receives a stimulus.
2. The stimulus travels along the sensatory nerve-fibre (S.F.)
3. The sensory nerve-fibre branches in the nerve-cord.
4. Its branches come into close contact (SY^{1}) with those of an
associative or communicating nerve-cell (A.C.).
5. Other branches of the associative cell come into close contact
(SY^{2}) with the branches or dendrites of a motor nerve-cell (M.C.).
6. An impulse or command travels along the motor nerve-fibre or axis
cylinder of the motor nerve-cell.
7. The motor nerve-fibre ends on a muscle-fibre (M.F.) near the surface.
This moves and the reflex action is complete.]
[Illustration: _Photo: British Museum_ (_Natural History_).
THE YUCCA MOTH
The Yucca Moth, emerging from her cocoon, flies at night to a Yucca
flower and collects pollen from the stamens, holding a little ball of it
in her mouth-parts. She then visits another flower and lays an egg in
the seed-box. After this she applies the pollen to the tip of the
pistil, thus securing the fertilisation of the flower and the growth of
the ovules in the pod. Yucca flowers in Britain do not produce seeds
because there are no Yucca Moths.]
[Illustration: INCLINED PLANE OF ANIMAL BEHAVIOUR
Diagram illustrating animal behaviour. The main line represents the
general life of the creature. On the upper side are activities implying
initiative; on the lower side actions which are almost automatic.
_Upper Side._--I. Energetic actions. II. Simple tentatives. III.
Trial-and-error methods. IV. Non-intelligent experiments. V.
Experiential "learning." VI. Associative "learning." VII. Intelligent
behaviour. VIII. Rational conduct (man).
_Lower Side._--1. Reactions to environment. 2. Enregistered reactions.
3. Simple reflex actions. 4. Compound reflex actions. 5. Tropisms. 6.
Enregistered rhythms. 7. Simple instincts. 8. Chain instincts. 9.
Instinctive activities influenced by intelligence. 10. Subconscious
cerebration at a high level (man).]
[Illustration: _Photo: J. J. Ward, F.E.S._
VENUS' FLY-TRAP
One of the most remarkable plants in the world, which captures its prey
by means of a trap formed from part of its leaf. It has been induced to
snap at and hold a bristle. If an insect lighting on the leaf touches
one of six very sensitive hairs, which pull the trigger of the movement,
the two halves of the leaf close rapidly and the fringing teeth on the
margin interlock, preventing the insect's escape. Then follows an
exudation of digestive juice.]
[Illustration: _Reproduced by permission from "The Wonders of Instinct"
by J. H. Fabre._
A SPIDER SUNNING HER EGGS
A kind of spider, called Lycosa, lying head downwards at the edge of her
nest, and holding her silken cocoon--the bag containing the eggs--up
towards the sun in her hindmost pair of legs. This extraordinary
proceeding is believed to assist in the hatching.]
Reflex Actions
Among simple multicellular animals, such as sea-anemones, we find the
beginnings of reflex actions, and a considerable part of the behaviour
of the lower animals is reflex. That is to say, there are laid down in
the animal in the course of its development certain pre-arrangements of
nerve-cells and muscle-cells which secure that a fit and proper answer
is given to a frequently recurrent stimulus. An earthworm half out of
its burrow becomes aware of the light tread of a thrush's foot, and
jerks itself back into its hole before anyone can say "reflex action."
What is it that happens?
Certain sensory nerve-cells in the earthworm's skin are stimulated by
vibrations in the earth; the message travels down a sensory nerve-fibre
from each of the stimulated cells and enters the nerve-cord. The sensory
fibres come into vital connection with branches of intermediary,
associative, or communicating cells, which are likewise connected with
motor nerve-cells. To these the message is thus shunted. From the motor
nerve-cells an impulse or command travels by motor nerve-fibres, one
from each cell, to the muscles, which contract. If this took as long to
happen as it takes to describe, even in outline, it would not be of much
use to the earthworm. But the motor answer follows the sensory stimulus
almost instantaneously. The great advantage of establishing or
enregistering these reflex chains is that the answers are practically
ready-made or inborn, not requiring to be learned. It is not necessary
that the brain should be stimulated if there is a brain; nor does the
animal will to act, though in certain cases it may by means of higher
controlling nerve-centres keep the natural reflex response from being
given, as happens, for instance, when we control a cough or a sneeze on
some solemn occasion. The evolutionary method, if we may use the
expression, has been to enregister ready-made responses; and as we
ascend the animal kingdom, we find reflex actions becoming complicated
and often linked together, so that the occurrence of one pulls the
trigger of another, and so on in a chain. The behaviour of the
insectivorous plant called Venus's fly-trap when it shuts on an insect
is like a reflex action in an animal, but plants have no definite
nervous system.
What are Called Tropisms
A somewhat higher level on the inclined plane is illustrated by what are
called "tropisms," obligatory movements which the animal makes,
adjusting its whole body so that physiological equilibrium results in
relation to gravity, pressure, currents, moisture, heat, light,
electricity, and surfaces of contact. A moth is flying past a candle;
the eye next the light is more illumined than the other; a physiological
inequilibrium results, affecting nerve-cells and muscle-cells; the
outcome is that the moth automatically adjusts its flight so that both
eyes become equally illumined; in doing this it often flies into the
candle.
It may seem bad business that the moth should fly into the candle, but
the flame is an utterly artificial item in its environment to which no
one can expect it to be adapted. These tropisms play an important rôle
in animal behaviour.
§ 2
Instinctive Behaviour
On a higher level is instinctive behaviour, which reaches such
remarkable perfection in ants, bees, and wasps. In its typical
expression instinctive behaviour depends on inborn capacities; it does
not require to be learned; it is independent of practice or experience,
though it may be improved by both; it is shared equally by all members
of the species of the same sex (for the female's instincts are often
different from the male's); it refers to particular conditions of life
that are of vital importance, though they may occur only once in a
lifetime. The female Yucca Moth emerges from the cocoon when the Yucca
flower puts forth its bell-like blossoms. She flies to a flower,
collects some pollen from the stamens, kneads it into a pill-like ball,
and stows this away under her chin. She flies to an older Yucca flower
and lays her eggs in some of the ovules within the seed-box, but before
she does so she has to deposit on the stigma the ball of pollen. From
this the pollen-tubes grow down and the pollen-nucleus of a tube
fertilises the egg-cell in an ovule, so that the possible seeds become
real seeds, for it is only a fraction of them that the Yucca Moth has
destroyed by using them as cradles for her eggs. Now it is plain that
the Yucca Moth has no individual experience of Yucca flowers, yet she
secures the continuance of her race by a concatenation of actions which
form part of her instinctive repertory.
From a physiological point of view instinctive behaviour is like a chain
of compound reflex actions, but in some cases, at least, there is reason
to believe that the behaviour is suffused with awareness and backed by
endeavour. This is suggested in exceptional cases where the stereotyped
routine is departed from to meet exceptional conditions. It should also
be noted that just as ants, hive bees, and wasps exhibit in most cases
purely instinctive behaviour, but move on occasion on the main line of
trial and error or of experimental initiative, so among birds and
mammals the intelligent behaviour is sometimes replaced by instinctive
routine. Perhaps there is no instinctive behaviour without a spice of
intelligence, and no intelligent behaviour without an instinctive
element. The old view that instinctive behaviour was originally
intelligent, and that instinct is "lapsed intelligence," is a tempting
one, and is suggested by the way in which habitual intelligent actions
cease in the individual to require intelligent control, but it rests on
the unproved hypothesis that the acquisitions of the individual can be
entailed on the race. It is almost certain that instinct is on a line of
evolution quite different from intelligence, and that it is nearer to
the inborn inspirations of the calculating boy or the musical genius
than to the plodding methods of intelligent learning.
Animal Intelligence
The higher reaches of the inclined plane of behaviour show intelligence
in the strict sense. They include those kinds of behaviour which cannot
be described without the suggestion that the animal makes some sort of
perceptual inference, not only profiting by experience but learning by
ideas. Such intelligent actions show great individual variability; they
are plastic and adjustable in a manner rarely hinted at in connection
with instincts where routine cannot be departed from without the
creature being nonplussed; they are not bound up with particular
circumstances as instinctive actions are, but imply an appreciative
awareness of relations.
When there is an experimenting with general ideas, when there is
_conceptual_ as contrasted with _perceptual_ inference, we speak of
Reason, but there is no evidence of this below the level of man. It is
not, indeed, always that we can credit man with rational conduct, but he
has the possibility of it ever within his reach.
Animal instinct and intelligence will be illustrated in another part of
this work. We are here concerned simply with the general question of the
evolution of behaviour. There is a main line of tentative experimental
behaviour both below and above the level of intelligence, and it has
been part of the tactics of evolution to bring about the hereditary
enregistration of capacities of effective response, the advantages being
that the answers come more rapidly and that the creature is left free,
if it chooses, for higher adventures.
There is no doubt as to the big fact that in the course of evolution
animals have shown an increasing complexity and masterfulness of
behaviour, that they have become at once more controlled and more
definitely free agents, and that the inner aspect of the
behaviour--experimenting, learning, thinking, feeling, and willing--has
come to count for more and more.
§ 3
Evolution of Parental Care
Mammals furnish a crowning instance of a trend of evolution which
expresses itself at many levels--the tendency to bring forth the young
at a well-advanced stage and to an increase of parental care associated
with a decrease in the number of offspring. There is a British starfish
called _Luidia_ which has two hundred millions of eggs in a year, and
there are said to be several millions of eggs in conger-eels and some
other fishes. These illustrate the spawning method of solving the
problem of survival. Some animals are naturally prolific, and the number
of eggs which they sow broadcast in the waters allows for enormous
infantile mortality and obviates any necessity for parental care.
But some other creatures, by nature less prolific, have found an
entirely different solution of the problem. They practise parental care
and they secure survival with greatly economised reproduction. This is a
trend of evolution particularly characteristic of the higher animals. So
much so that Herbert Spencer formulated the generalisation that the size
and frequency of the animal family is inverse ratio to the degree of
evolution to which the animal has attained.
Now there are many different methods of parental care which secure the
safety of the young, and one of these is called viviparity. The young
ones are not liberated from the parent until they are relatively well
advanced and more or less able to look after themselves. This gives the
young a good send-off in life, and their chances of death are greatly
reduced. In other words, the animals that have varied in the direction
of economised reproduction may keep their foothold in the struggle for
existence if they have varied at the same time in the direction of
parental care. In other cases it may have worked the other way round.
In the interesting archaic animal called _Peripatus_, which has to face
a modern world too severe for it, one of the methods of meeting the
environing difficulties is the retention of the offspring for many
months within the mother, so that it is born a fully-formed creature.
There are only a few offspring at a time, and, although there are
exceptional cases like the summer green-flies, which are very prolific
though viviparous, the general rule is that viviparity is associated
with a very small family. The case of flowering plants stands by itself,
for although they illustrate a kind of viviparity, the seed being
embryos, an individual plant may have a large number of flowers and
therefore a huge family.
Viviparity naturally finds its best illustrations among terrestrial
animals, where the risks to the young life are many, and it finds its
climax among mammals.
Now it is an interesting fact that the three lowest mammals, the
Duckmole and two Spiny Ant-eaters, lay eggs, i.e. are oviparous; that
the Marsupials, on the next grade, bring forth their young, as it were,
prematurely, and in most cases stow them away in an external pouch;
while all the others--the Placentals--show a more prolonged ante-natal
life and an intimate partnership between the mother and the unborn
young.
§ 4
There is another way of looking at the sublime process of evolution. It
has implied a mastery of all the possible haunts of life; it has been a
progressive conquest of the environment.
1. It is highly probable that living organisms found their foothold in
the stimulating conditions of the shore of the sea--the shallow water,
brightly illumined, seaweed-growing shelf fringing the Continents. This
littoral zone was a propitious environment where sea and fresh water,
earth and air all meet, where there is stimulating change, abundant
oxygenation and a copious supply of nutritive material in what the
streams bring down and in the rich seaweed vegetation.
[Illustration: THE HOATZIN INHABITS BRITISH GUIANA
The newly hatched bird has claws on its thumb and first finger and so is
enabled to climb on the branches of trees with great dexterity until
such time as the wings are strong enough to sustain it in flight.]
[Illustration: _Photograph, from the British Museum (Natural History),
of a drawing by Mr. E. Wilson._
PERIPATUS
A widely distributed old-fashioned type of animal, somewhat like a
permanent caterpillar. It has affinities both with worms and with
insects. It has a velvety skin, minute diamond-like eyes, and short
stump-like legs. A defenceless, weaponless animal, it comes out at
night, and is said to capture small insects by squirting jets of slime
from its mouth.]
[Illustration: _Photo: W. S. Berridge, F.Z.S._
ROCK KANGAROO CARRYING ITS YOUNG IN A POUCH
The young are born so helpless that they cannot even suck. The mother
places them in the external pouch, and fitting their mouths on the teats
injects the milk. After a time the young ones go out and in as they
please.]
It is not an easy haunt of life, but none the worse for that, and it is
tenanted to-day by representatives of practically every class of animals
from infusorians to seashore birds and mammals.
The Cradle of the Open Sea
2. The open-sea or pelagic haunt includes all the brightly illumined
surface waters beyond the shallow water of the shore area.
It is perhaps the easiest of all the haunts of life, for there is no
crowding, there is considerable uniformity, and an abundance of food for
animals is afforded by the inexhaustible floating "sea-meadows" of
microscopic Algæ. These are reincarnated in minute animals like the
open-sea crustaceans, which again are utilised by fishes, these in turn
making life possible for higher forms like carnivorous turtles and
toothed whales. It is quite possible that the open sea was the original
cradle of life and perhaps Professor Church is right in picturing a long
period of pelagic life before there was any sufficiently shallow water
to allow the floating plants to anchor. It is rather in favour of this
view that many shore animals such as crabs and starfishes, spend their
youthful stages in the relatively safe cradle of the open sea, and only
return to the more strenuous conditions of their birthplace after they
have gained considerable strength of body. It is probably safe to say
that the honour of being the original cradle of life lies between the
shore of the sea and the open sea.
The Great Deeps
3. A third haunt of life is the floor of the Deep Sea, the abyssal area,
which occupies more than a half of the surface of the globe. It is a
region of extreme cold--an eternal winter; of utter darkness--an eternal
night--relieved only by the fitful gleams of "phosphorescent" animals;
of enormous pressure--2-1/2 tons on the square inch at a depth of 2,500
fathoms; of profound calm, unbroken silence, immense monotony. And as
there are no plants in the great abysses, the animals must live on one
another, and, in the long run, on the rain of moribund animalcules which
sink from the surface through the miles of water. It seems a very
unpromising haunt of life, but it is abundantly tenanted, and it gives
us a glimpse of the insurgent nature of the living creature that the
difficulties of the Deep Sea should have been so effectively conquered.
It is probable that the colonising of the great abysses took place in
relatively recent times, for the fauna does not include many very
antique types. It is practically certain that the colonisation was due
to littoral animals which followed the food-débris, millennium after
millennium, further and further down the long slope from the shore.
The Freshwaters
4. A fourth haunt of life is that of the freshwaters, including river
and lake, pond and pool, swamp and marsh. It may have been colonised by
gradual migration up estuaries and rivers, or by more direct passage
from the seashore into the brackish swamp. Or it may have been in some
cases that partially landlocked corners of ancient seas became gradually
turned into freshwater basins. The animal population of the freshwaters
is very representative, and is diversely adapted to meet the
characteristic contingencies--the risk of being dried up, the risk of
being frozen hard in winter, and the risk of being left high and dry
after floods or of being swept down to the sea.
Conquest of the Dry Land
5. The terrestrial haunt has been invaded age after age by contingents
from the sea or from the freshwaters. We must recognise the worm
invasion, which led eventually to the making of the fertile soil, the
invasion due to air-breathing Arthropods, which led eventually to the
important linkage between flowers and their insect visitors, and the
invasion due to air-breathing Amphibians, which led eventually to the
higher terrestrial animals and to the development of intelligence and
family affection. Besides these three great invasions, there were minor
ones such as that leading to land-snails, for there has been a
widespread and persistent tendency among aquatic animals to try to
possess the dry land.
Getting on to dry land had a manifold significance.
It implied getting into a medium with a much larger supply of oxygen
than there is dissolved in the water. But the oxygen of the air is more
difficult to capture, especially when the skin becomes hard or well
protected, as it is almost bound to become in animals living on dry
ground. Thus this leads to the development of _internal surfaces_, such
as those of lungs, where the oxygen taken into the body may be absorbed
by the blood. In most animals the blood goes to the surface of
oxygen-capture; but in insects and their relatives there is a different
idea--of taking the air to the blood or in greater part to the area of
oxygen-combustion, the living tissues. A system of branching air-tubes
takes air into every hole and corner of the insect's body, and this
thorough aeration is doubtless in part the secret of the insect's
intense activity. The blood never becomes impure.
The conquest of the dry land also implied a predominance of that kind of
locomotion which may be compared to punting, when the body is pushed
along by pressing a lever against a hard substratum. And it also
followed that with few exceptions the body of the terrestrial animal
tended to be compact, readily lifted off the ground by the limbs or
adjusted in some other way so that there may not be too large a surface
trailing on the ground. An animal like a jellyfish, easily supported in
the water, would be impossible on land. Such apparent exceptions as
earthworms, centipedes, and snakes are not difficult to explain, for the
earthworm is a burrower which eats its way through the soil, the
centipede's long body is supported by numerous hard legs, and the snake
pushes itself along by means of the large ventral scales to which the
lower ends of very numerous ribs are attached.
Methods of Mastering the Difficulties of Terrestrial Life
A great restriction attendant on the invasion of the dry land is that
locomotion becomes limited to one plane, namely, the surface of the
earth. This is in great contrast to what is true in the water, where the
animal can move up or down, to right or to left, at any angle and in
three dimensions. It surely follows from this that the movements of land
animals must be rapid and precise, unless, indeed, safety is secured in
some other way. Hence it is easy to understand why most land animals
have very finely developed striped muscles, and why a beetle running on
the ground has far more numerous muscles than a lobster swimming in the
sea.
Land animals were also handicapped by the risks of drought and of frost,
but these were met by defences of the most diverse description, from the
hairs of woolly caterpillars to the fur of mammals, from the carapace of
tortoises to the armour of armadillos. In other cases, it is hardly
necessary to say, the difficulties may be met in other ways, as frogs
meet the winter by falling into a lethargic state in some secluded
retreat.
Another consequence of getting on to dry land is that the eggs or young
can no longer be set free anyhow, as is possible when the animal is
surrounded by water, which is in itself more or less of a cradle. If the
eggs were laid or the young liberated on dry ground, the chances are
many that they would be dried up or devoured. So there are numerous ways
in which land animals secure the safety of their young, e.g. by burying
them in the ground, or by hiding them in nests, or by carrying them
about for a prolonged period either before or after birth. This may mean
great safety for the young, this may make it possible to have only a
small family, and this may tend to the evolution of parental care and
the kindly emotions. Thus it may be understood that from the conquest of
the land many far-reaching consequences have followed.
[Illustration: _Photo: Rischgitz._
PROFESSOR THOMAS HENRY HUXLEY (1825-95)
One of the most distinguished of zoologists, with unsurpassed gifts as a
teacher and expositor. He did great service in gaining a place for
science in ordinary education and in popular estimation. No one
championed Evolutionism with more courage and skill.]
[Illustration: BARON CUVIER, 1769-1832
One of the founders of modern Comparative Anatomy. A man of gigantic
intellect, who came to Paris as a youth from the provinces, and became
the director of the higher education of France and a peer of the Empire.
He was opposed to Evolutionist ideas, but he had anatomical genius.]
[Illustration: AN ILLUSTRATION SHOWING VARIOUS METHODS OF FLYING AND
SWOOPING
Gull, with a feather-wing, a true flier. Fox-bat, with a skin-wing, a
true flier. Flying Squirrel, with a parachute of skin, able to swoop
from tree to tree, but not to fly. Flying Fish, with pectoral fins used
as volplanes in a great leap due to the tail. To some extent able to
sail in albatros fashion.]
Finally, it is worth dwelling on the risks of terrestrial life, because
they enable us better to understand why so many land animals have become
burrowers and others climbers of trees, why some have returned to the
water and others have taken to the air. It may be asked, perhaps, why
the land should have been colonised at all when the risks and
difficulties are so great. The answer must be that necessity and
curiosity are the mother and father of invention. Animals left the water
because the pools dried up, or because they were overcrowded, or because
of inveterate enemies, but also because of that curiosity and spirit of
adventure which, from first to last, has been one of the spurs of
progress.
Conquering the Air
6. The last great haunt of life is the air, a mastery of which must be
placed to the credit of insects, Pterodactyls, birds, and bats. These
have been the successes, but it should be noted that there have been
many brilliant failures, which have not attained to much more than
parachuting. These include the Flying Fishes, which take leaps from the
water and are carried for many yards and to considerable heights,
holding their enlarged pectoral fins taut or with little more than a
slight fluttering. There is a so-called Flying Frog (_Rhacophorus_) that
skims from branch to branch, and the much more effective Flying Dragon
(_Draco volans_) of the Far East, which has been mentioned already.
Among mammals there are Flying Phalangers, Flying Lemurs, and more
besides, all attaining to great skill as parachutists, and illustrating
the endeavour to master the air which man has realised in a way of his
own.
The power of flight brings obvious advantages. A bird feeding on the
ground is able to evade the stalking carnivore by suddenly rising into
the air; food and water can be followed rapidly and to great distances;
the eggs or the young can be placed in safe situations; and birds in
their migrations have made a brilliant conquest both of time and space.
Many of them know no winter in their year, and the migratory flight of
the Pacific Golden Plover from Hawaii to Alaska and back again does not
stand alone.
THE PROCESSION OF LIFE THROUGH THE AGES
§ 1
The Rock Record
How do we know when the various classes of animals and plants were
established on the earth? How do we know the order of their appearance
and the succession of their advances? The answer is: by reading the Rock
Record. In the course of time the crust of the earth has been elevated
into continents and depressed into ocean-troughs, and the surface of the
land has been buckled up into mountain ranges and folded in gentler
hills and valleys. The high places of the land have been weathered by
air and water in many forms, and the results of the weathering have been
borne away by rivers and seas, to be laid down again elsewhere as
deposits which eventually formed sandstones, mudstones, and similar
sedimentary rocks. Much of the material of the original crust has thus
been broken down and worked up again many times over, and if the total
thickness of the sedimentary rocks is added up it amounts, according to
some geologists, to a total of 67 miles. In most cases, however, only a
small part of this thickness is to be seen in one place, for the
deposits were usually formed in limited areas at any one time.
The Use of Fossils
When the sediments were accumulating age after age, it naturally came
about that remains of the plants and animals living at the time were
buried, and these formed the fossils by the aid of which it is possible
to read the story of the past. By careful piecing together of evidence
the geologist is able to determine the order in which the different
sedimentary rocks were laid down, and thus to say, for instance, that
the Devonian period was the time of the origin of Amphibians. In other
cases the geologist utilises the fossils in his attempt to work out the
order of the strata when these have been much disarranged. For the
simpler fossil forms of any type must be older than those that are more
complex. There is no vicious circle here, for the general succession of
strata is clear, and it is quite certain that there were fishes before
there were amphibians, and amphibians before there were reptiles, and
reptiles before there were birds and mammals. In certain cases, e.g. of
fossil horses and elephants, the actual historical succession has been
clearly worked out.
If the successive strata contained good samples of all the plants and
animals living at the time when the beds were formed, then it would be
easy to read the record of the rocks, but many animals were too soft to
become satisfactory fossils, many were eaten or dissolved away, many
were destroyed by heat and pressure, so that the rock record is like a
library very much damaged by fire and looting and decay.
§ 2
The Geological Time-table
The long history of the earth and its inhabitants is conveniently
divided into eras. Thus, just as we speak of the ancient, mediæval, and
modern history of mankind, so we may speak of Palæozoic, Mesozoic and
Cenozoic eras in the history of the earth as a whole.
Geologists cannot tell us except in an approximate way how long the
process of evolution has taken. One of the methods is to estimate how
long has been required for the accumulation of the salts of the sea,
for all these have been dissolved out of the rocks since rain began to
fall on the earth. Dividing the total amount of saline matter by what is
contributed every year in modern times, we get about a hundred million
years as the age of the sea. But as the present rate of
salt-accumulation is probably much greater than it was during many of
the geological periods, the prodigious age just mentioned is in all
likelihood far below the mark. Another method is to calculate how long
it would take to form the sedimentary rocks, like sandstones and
mudstones, which have a _total_ thickness of over fifty miles, though
the _local_ thickness is rarely over a mile. As most of the materials
have come from the weathering of the earth's crust, and as the annual
amount of weathering now going on can be estimated, the time required
for the formation of the sedimentary rocks of the world can be
approximately calculated. There are some other ways of trying to tell
the earth's age and the length of the successive periods, but no
certainty has been reached.
The eras marked on the table (page 92) as _before the Cambrian_
correspond to about thirty-two miles of thickness of strata; and all the
subsequent eras with fossil-bearing rocks to a thickness of about
twenty-one miles--in itself an astounding fact. Perhaps thirty million
years must be allotted to the Pre-Cambrian eras, eighteen to the
Palæozoic, nine to the Mesozoic, three to the Cenozoic, making a grand
total of sixty millions.
The Establishment of Invertebrate Stocks
It is an astounding fact that at least half of geological time (the
Archæozoic and Proterozoic eras) passed before there were living
creatures with parts sufficiently hard to form fossils. In the latter
part of the Proterozoic era there are traces of one-celled marine
animals (Radiolarians) with shells of flint, and of worms that wallowed
in the primal mud. It is plain that as regards the most primitive
creatures the rock record tells us little.
[Illustration: _From Knipe's "Nebula to Man."_
ANIMALS OF THE CAMBRIAN PERIOD e.g. Sponges, Jellyfish, Starfish,
Sea-lilies, Water-fleas, and Trilobites]
[Illustration: _Photo: J. J. Ward, F.E.S._
A TRILOBITE
Trilobites were ancient seashore animals, abundant from the Upper
Cambrian to the Carboniferous eras. They have no direct descendants
to-day. They were jointed-footed animals, allied to Crustaceans and
perhaps also to King-crabs. They were able to roll themselves up in
their ring-armour.]
[Illustration: _Photo: British Museum (Natural History)._
THE GAMBIAN MUD-FISH, PROTOPTERUS
It can breathe oxygen dissolved in water by its gills; it can also
breathe dry air by means of its swim-bladder, which has become a lung.
It is a _double-breather_, showing evolution in process. For seven
months of the year, the dry season, it can remain inert in the mud,
getting air through an open pipe to the surface. When water fills the
pools it can use its gills again. Mud-nests or mud encasements with the
lung-fish inside have often been brought to Britain and the fish when
liberated were quite lively.]
[Illustration: THE ARCHÆOPTERYX
(_After William Leche of Stockholm._)
A good restoration of the oldest known bird, Archæopteryx (Jurassic
Era). It was about the size of a crow; it had teeth on both jaws; it had
claws on the thumb and two fingers; and it had a long lizard-like tail.
But it had feathers, proving itself a true bird.]
[Illustration: WING OF A BIRD, SHOWING THE ARRANGEMENT OF THE FEATHERS
The longest feathers or primaries (PR) are borne by the two fingers (2
and 3), and their palm-bones (CMC); the second longest or secondaries
are borne by the ulna bone (U) of the fore-arm; there is a separate tuft
(AS) on the thumb (TH).]
The rarity of direct traces of life in the oldest rocks is partly due to
the fact that the primitive animals would be of delicate build, but it
must also be remembered that the ancient rocks have been profoundly and
repeatedly changed by pressure and heat, so that the traces which did
exist would be very liable to obliteration. And if it be asked what
right we have to suppose the presence of living creatures in the absence
or extreme rarity of fossils, we must point to great accumulations of
limestone which indicate the existence of calcareous algæ, and to
deposits of iron which probably indicate the activity of iron-forming
Bacteria. Ancient beds of graphite similarly suggest that green plants
flourished in these ancient days.
§ 3
The Era of Ancient Life (Palæozoic)
The _Cambrian_ period was the time of the establishment of the chief
stocks of backboneless animals such as sponges, jellyfishes, worms,
sea-cucumbers, lamp-shells, trilobites, crustaceans, and molluscs. There
is something very eloquent in the broad fact that the peopling of the
seas had definitely begun some thirty million years ago, for Professor
H. F. Osborn points out that in the Cambrian period there was already a
colonisation of the shore of the sea, the open sea, and the deep waters.
The _Ordovician_ period was marked by abundant representation of the
once very successful class of Trilobites--jointed-footed,
antenna-bearing, segmented marine animals, with numerous appendages and
a covering of chitin. They died away entirely with the end of the
Palæozoic era. Also very notable was the abundance of predatory
cuttlefishes, the bullies of the ancient seas. But it was in this period
that the first backboned animals made their appearance--an epoch-making
step in evolution. In other words, true fishes were evolved--destined in
the course of ages to replace the cuttlefishes (which are mere molluscs)
in dominating the seas.
_______________________________________________________________________
_RECENT TIMES_ Human civilisation.
_______________________________________________________________________
{PLEISTOCENE OR GLACIAL TIME Last great Ice Age.
_CENOZOIC ERA_ {MIOCENE AND PLIOCENE TIMES Emergence of Man.
{EOCENE AND OLIGOCENE TIMES Rise of higher mammals.
_______________________________________________________________________
{CRETACEOUS PERIOD Rise of primitive mammals,
{ flowering plants,
{ and higher insects.
_MESOZOIC ERA_ {JURASSIC PERIOD Rise of birds and flying
{ reptiles.
{TRIASSIC PERIOD Rise of dinosaur reptiles.
_______________________________________________________________________
{PERMIAN PERIOD Rise of reptiles.
{CARBONIFEROUS PERIOD Rise of insects.
{DEVONIAN PERIOD First amphibians.
_PALÆOZOIC ERA_ {SILURIAN PERIOD Land animals began.
{ORDOVICIAN PERIOD First fishes.
{CAMBRIAN PERIOD Peopling of the sea.
_______________________________________________________________________
_PROTEROZOIC AGES_ Many of the Backboneless stocks began.
_ARCHÆOZOIC AGES_ Living creatures began to be upon the earth.
_______________________________________________________________________
{Making of continents and ocean-basins.
{Beginnings of atmosphere and hydrosphere.
_FORMATIVE TIMES_ {Cooling of the earth.
{Establishment of the solar system.
_______________________________________________________________________
In the _Silurian_ period in which the peopling of the seas went on
apace, there was the first known attempt at colonising the dry land. For
in Silurian rocks there are fossil scorpions, and that implies ability
to breathe dry air--by means of internal surfaces, in this case known as
lungbooks. It was also towards the end of the Silurian, when a period of
great aridity set in, that fishes appeared related to our mud-fishes or
double-breathers (Dipnoi), which have lungs as well as gills. This,
again, meant utilising dry air, just as the present-day mud-fishes do
when the water disappears from the pools in hot weather. The lung-fishes
or mud-fishes of to-day are but three in number, one in Queensland, one
in South America, and one in Africa, but they are extremely
interesting "living fossils," binding the class of fishes to that of
amphibians. It is highly probable that the first invasion of the dry
land should be put to the credit of some adventurous worms, but the
second great invasion was certainly due to air-breathing Arthropods,
like the pioneer scorpion we mentioned.
[Illustration: PICTORIAL REPRESENTATION OF THE SUCCESSIVE STRATA OF THE
EARTH'S CRUST, WITH SUGGESTIONS OF CHARACTERISTIC FOSSILS
E.g. Fish and Trilobite in the Devonian (red), a large Amphibian in the
Carboniferous (blue), Reptiles in Permian (light red), the first Mammal
in the Triassic (blue), the first Bird in the Jurassic (yellow), Giant
Reptiles in the Cretaceous (white), then follow the Tertiary strata with
progressive mammals, and Quaternary at the top with man and mammoth.]
The _Devonian_ period, including that of the Old Red Sandstone, was one
of the most significant periods in the earth's history. For it was the
time of the establishment of flowering plants upon the earth and of
terrestrial backboned animals. One would like to have been the
discoverer of the Devonian foot-print of _Thinopus_, the first known
Amphibian foot-print--an eloquent vestige of the third great invasion of
the dry land. It was probably from a stock of Devonian lung-fishes that
the first Amphibians sprang, but it was not till the next period that
they came to their own. While they were still feeling their way, there
was a remarkable exuberance of shark-like and heavily armoured fishes in
the Devonian seas.
EVOLUTION OF LAND ANIMALS
§ 1
Giant Amphibians and Coal-measures
The _Carboniferous_ period was marked by a mild moist climate and a
luxuriant vegetation in the swampy low grounds. It was a much less
strenuous time than the Devonian period; it was like a very long summer.
There were no trees of the type we see now, but there were forests of
club-mosses and horsetails which grew to a gigantic size compared with
their pigmy representatives of to-day. In these forests the
jointed-footed invaders of the dry land ran riot in the form of
centipedes, spiders, scorpions, and insects, and on these the primeval
Amphibians fed. The appearance of insects made possible a new linkage of
far-reaching importance, namely, the cross-fertilisation of flowering
plants by their insect visitors, and from this time onwards it may be
said that flowers and their visitors have evolved hand in hand.
Cross-fertilisation is much surer by insects than by the wind, and
cross-fertilisation is more advantageous than self-fertilisation because
it promotes both fertility and plasticity. It was probably in this
period that _coloured_ flowers--attractive to insect-visitors--began to
justify themselves as beauty became useful, and began to relieve the
monotonous green of the horsetail and club-moss forests, which covered
great tracts of the earth for millions of years. In the Carboniferous
forests there were also land-snails, representing one of the minor
invasions of the dry land, tending on the whole to check vegetation.
They, too, were probably preyed upon by the Amphibians, some of which
attained a large size. Each age has had its giants, and those of the
Carboniferous were Amphibians called Labyrinthodonts, some of which were
almost as big as donkeys. It need hardly be said that it was in this
period that most of the Coal-measures were laid down by the immense
accumulation of the spores and debris of the club-moss forests. Ages
afterwards, it was given to man to tap this great source of
energy--traceable back to the sunshine of millions of years ago. Even
then it was true that no plant or animal lives or dies to itself!
The Acquisitions of Amphibians.
As Amphibians had their Golden Age in the Carboniferous period we may
fitly use this opportunity of indicating the advances in evolution which
the emergence of Amphibians implied. (1) In the first place the passage
from water to dry land was the beginning of a higher and more promiseful
life, taxed no doubt by increased difficulties. The natural question
rises why animals should have migrated from water to dry land at all
when great difficulties were involved in the transition. The answers
must be: (_a_) that local drying up of water-basins or elevations of the
land surface often made the old haunts untenable; (_b_) that there may
have been great congestion and competition in the old quarters; and
(_c_) that there has been an undeniable endeavour after well-being
throughout the history of animal life. In the same way with mankind,
migrations were prompted by the setting in of prolonged drought, by
over-population, and by the spirit of adventure. (2) In Amphibians for
the first time the non-digitate paired fins of fishes were replaced by
limbs with fingers and toes. This implied an advantageous power of
grasping, of holding firm, of putting food into the mouth, of feeling
things in three dimensions. (3) We cannot be positive in regard to the
soft parts of the ancient Amphibians known only as fossils, but if they
were in a general way like the frogs and toads, newts and salamanders of
the present day, we may say that they made among other acquisitions the
following: true ventral lungs, a three-chambered heart, a movable
tongue, a drum to the ear, and lids to the eyes. It is very interesting
to find that though the tongue of the tadpole has some muscle-fibres in
it, they are not strong enough to effect movement, recalling the tongue
of fishes, which has not any muscles at all. Gradually, as the tadpole
becomes a frog, the muscle-fibres grow in strength, and make it possible
for the full-grown creature to shoot out its tongue upon insects. This
is probably a recapitulation of what was accomplished in the course of
millennia in the history of the Amphibian race. (4) Another acquisition
made by Amphibians was a voice, due, as in ourselves, to the rapid
passage of air over taut membranes (vocal cords) stretched in the
larynx. It is an interesting fact that for millions of years there was
upon the earth no sound of life at all, only the noise of wind and wave,
thunder and avalanche. Apart from the instrumental music of some
insects, perhaps beginning in the Carboniferous, the first vital sounds
were due to Amphibians, and theirs certainly was the first voice--surely
one of the great steps in organic evolution.
[Illustration: _Photo: British Museum (Natural History)._
FOSSIL OF A PTERODACTYL OR EXTINCT FLYING DRAGON
The wing is made of a web of skin extended on the enormously elongated
outermost finger. The long tail served for balancing and steering. The
Pterodactyls varied from the size of sparrows to a wing-span of fifteen
feet--the largest flying creatures.]
[Illustration: _From Knipe's "Nebula to Man."_
PARIASAURUS: AN EXTINCT VEGETARIAN TRIASSIC REPTILE
Total length about 9 feet. (Remains found in Cape Colony, South
Africa.)]
[Illustration: _From Knipe's "Nebula to Man."_
TRICERATOPS: A HUGE EXTINCT REPTILE
(From remains found in Cretaceous strata of Wyoming, U.S.A.)
This Dinosaur, about the size of a large rhinoceros, had a huge
three-horned skull with a remarkable bony collar over the neck. But, as
in many other cases, its brain was so small that it could have passed
down the spinal canal in which the spinal cord lies. Perhaps this partly
accounts for the extinction of giant reptiles.]
[Illustration: _Photo: "Daily Mail."_
THE DUCKMOLE OR DUCK-BILLED PLATYPUS OF AUSTRALIA
The Duckmole or Duck-billed Platypus of Australia is a survivor of the
most primitive mammals. It harks back to reptiles, e.g. in being an
egg-layer, in having comparatively large eggs, and in being imperfectly
warm-blooded. It swims well and feeds on small water-animals. It can
also burrow.]
Evolution of the Voice
The first use of the voice was probably that indicated by our frogs and
toads--it serves as a sex-call. That is the meaning of the trumpeting
with which frogs herald the spring, and it is often only in the males
that the voice is well developed. But if we look forward, past
Amphibians altogether, we find the voice becoming a maternal call
helping to secure the safety of the young--a use very obvious when young
birds squat motionless at the sound of the parent's danger-note. Later
on, probably, the voice became an infantile call, as when the unhatched
crocodile pipes from within the deeply buried egg, signalling to the
mother that it is time to be unearthed. Higher still the voice expresses
emotion, as in the song of birds, often outside the limits of the
breeding time. Later still, particular sounds become words, signifying
particular things or feelings, such as "food," "danger," "home,"
"anger," and "joy." Finally words become a medium of social intercourse
and as symbols help to make it possible for man to reason.
§ 2
The Early Reptiles
In the _Permian_ period reptiles appeared, or perhaps one should say,
began to assert themselves. That is to say, there was an emergence of
backboned animals which were free from water and relinquished the method
of breathing by gills, which Amphibians retained in their young stages
at least. The unhatched or unborn reptile breathes by means of a
vascular hood spread underneath the egg-shell and absorbing dry air from
without. It is an interesting point that this vascular hood, called the
allantois, is represented in the Amphibians by an unimportant bladder
growing out from the hind end of the food-canal. A great step in
evolution was implied in the origin of this ante-natal hood or foetal
membrane and another one--of protective significance--called the amnion,
which forms a water-bag over the delicate embryo. The step meant total
emancipation from the water and from gill-breathing, and the two
foetal membranes, the amnion and the allantois, persist not only in
all reptiles but in birds and mammals as well. These higher Vertebrates
are therefore called Amniota in contrast to the Lower Vertebrates or
Anamnia (the Amphibians, Fishes, and primitive types).
It is a suggestive fact that the embryos of all reptiles, birds, and
mammals show gill-clefts--_a tell-tale evidence of their distant aquatic
ancestry_. But these embryonic gill-clefts are not used for respiration
and show no trace of gills except in a few embryonic reptiles and birds
where their dwindled vestiges have been recently discovered. As to the
gill-clefts, they are of no use in higher Vertebrates except that the
first becomes the Eustachian tube leading from the ear-passage to the
back of the mouth. The reason why they persist when only one is of any
use, and that in a transformed guise, would be difficult to interpret
except in terms of the Evolution theory. They illustrate the lingering
influence of a long pedigree, the living hand of the past, the tendency
that individual development has to recapitulate racial evolution. In a
condensed and telescoped manner, of course, for what took the race a
million years may be recapitulated by the individual in a week!
In the Permian period the warm moist climate of most of the
Carboniferous period was replaced by severe conditions, culminating in
an Ice Age which spread from the Southern Hemisphere throughout the
world. With this was associated a waning of the Carboniferous flora, and
the appearance of a new one, consisting of ferns, conifers, ginkgos, and
cycads, which persisted until near the end of the Mesozoic era. The
Permian Ice Age lasted for millions of years, and was most severe in the
Far South. Of course, it was a very different world then, for North
Europe was joined to North America, Africa to South America, and
Australia to Asia. It was probably during the Permian Ice Age that many
of the insects divided their life-history into two main chapters--the
feeding, growing, moulting, immature, larval stages, e.g. caterpillars,
and the more ascetic, non-growing, non-moulting, winged phase, adapted
for reproduction. Between these there intervened the quiescent,
well-protected pupa stage or chrysalis, probably adapted to begin with
as a means of surviving the severe winter. For it is easier for an
animal to survive when the vital processes are more or less in abeyance.
Disappearance of many Ancient Types
We cannot leave the last period of the Palæozoic era and its prolonged
ice age without noticing that it meant the entire cessation of a large
number of ancient types, especially among plants and backboneless
animals, which now disappear for ever. It is necessary to understand
that the animals of ancient days stand in three different relations to
those of to-day. (_a_) There are ancient types that have living
representatives, sometimes few and sometimes many, sometimes much
changed and sometimes but slightly changed. The lamp-shell,
_Lingulella_, of the Cambrian and Ordovician period has a very near
relative in the _Lingula_ of to-day. There are a few extremely
conservative animals. (_b_) There are ancient types which have no living
representatives, except in the guise of transformed descendants, as the
King-crab (_Limulus_) may be said to be a transformed descendant of the
otherwise quite extinct race to which Eurypterids or Sea-scorpions
belonged. (_c_) There are altogether extinct types--_lost races_--which
have left not a wrack behind. For there is not any representation to-day
of such races as Graptolites and Trilobites.
Looking backwards over the many millions of years comprised in the
Palæozoic era, what may we emphasise as the most salient features? There
was in the _Cambrian_ the establishment of the chief classes of
backboneless animals; in the _Ordovician_ the first fishes and perhaps
the first terrestrial plants; in the _Silurian_ the emergence of
air-breathing Invertebrates and mud-fishes; in the _Devonian_ the
appearance of the first Amphibians, from which all higher land animals
are descended, and the establishment of a land flora; in the
_Carboniferous_ the great Club-moss forests and an exuberance of
air-breathing insects and their allies; in the _Permian_ the first
reptiles and a new flora.
THE GEOLOGICAL MIDDLE AGES
§ 1
The Mesozoic Era
In a broad way the Mesozoic era corresponds with the Golden Age of
reptiles, and with the climax of the Conifer and Cycad flora, which was
established in the Permian. But among the Conifers and Cycads our modern
flowering plants were beginning to show face tentatively, just like
birds and mammals among the great reptiles.
In the _Triassic_ period the exuberance of reptilian life which marked
the Permian was continued. Besides Turtles which still persist, there
were Ichthyosaurs, Plesiosaurs, Dinosaurs, and Pterosaurs, none of which
lasted beyond the Mesozoic era. Of great importance was the rise of the
Dinosaurs in the Triassic, for it is highly probable that within the
limits of this vigorous and plastic stock--some of them bipeds--we must
look for the ancestors of both birds and mammals. Both land and water
were dominated by reptiles, some of which attained to gigantic size. Had
there been any zoologist in those days, he would have been very
sagacious indeed if he had suspected that reptiles did not represent the
climax of creation.
The Flying Dragons
The _Jurassic_ period showed a continuance of the reptilian splendour.
They radiated in many directions, becoming adapted to many haunts. Thus
there were many Fish Lizards paddling in the seas, many types of
terrestrial dragons stalking about on land, many swiftly gliding
alligator-like forms, and the Flying Dragons which began in the Triassic
attained to remarkable success and variety. Their wing was formed by the
extension of a great fold of skin on the enormously elongated outermost
finger, and they varied from the size of a sparrow to a spread of over
five feet. A soldering of the dorsal vertebræ as in our Flying Birds was
an adaptation to striking the air with some force, but as there is not
more than a slight keel, if any, on the breast-bone, it is unlikely that
they could fly far. For we know from our modern birds that the power of
flight may be to some extent gauged from the degree of development of
the keel, which is simply a great ridge for the better insertion of the
muscles of flight. It is absent, of course, in the Running Birds, like
the ostrich, and it has degenerated in an interesting way in the
burrowing parrot (_Stringops_) and a few other birds that have "gone
back."
The First Known Bird
But the Jurassic is particularly memorable because its strata have
yielded two fine specimens of the first known bird, _Archæopteryx_.
These were entombed in the deposits which formed the fine-grained
lithographic stones of Bavaria, and practically every bone in the body
is preserved except the breast-bone. Even the feathers have left their
marks with distinctness. This oldest known bird--too far advanced to be
the first bird--was about the size of a crow and was probably of
arboreal habits. Of great interest are its reptilian features, so
pronounced that one cannot evade the evolutionist suggestion. It had
teeth in both jaws, which no modern bird has; it had a long lizard-like
tail, which no modern bird has; it had claws on three fingers, and a
sort of half-made wing. That is to say, it does not show, what all
modern birds show, a fusion of half the wrist-bones with the whole of
the palm-bones, the well-known carpo-metacarpus bone which forms a basis
for the longest pinions. In many reptiles, such as Crocodiles, there are
peculiar bones running across the abdomen beneath the skin, the
so-called "abdominal ribs," and it seems an eloquent detail to find
these represented in _Archæopteryx_, the earliest known bird. No modern
bird shows any trace of them. [Illustration: SKELETON OF AN EXTINCT
FLIGHTLESS TOOTHED BIRD, HESPERORNIS
(_After Marsh._)
The bird was five or six feet high, something like a swimming ostrich,
with a very powerful leg but only a vestige of a wing. There were sharp
teeth in a groove. The modern divers come nearest to this ancient
type.]
[Illustration: SIX STAGES IN THE EVOLUTION OF THE HORSE, SHOWING GRADUAL
INCREASE IN SIZE
(_After Lull and Matthew._)
1. Four-toed horse, Eohippus, about one foot high. Lower Eocene, N.
America.
2. Another four-toed horse, Orohippus, a little over a foot high. Middle
Eocene, N. America.
3. Three-toed horse, Mesohippus, about the size of a sheep. Middle
Oligocene, N. America.
4. Three-toed horse, Merychippus, Miocene, N. America. Only one toe
reaches the ground on each foot, but the remains of two others are
prominent.
5. The first one-toed horse, Pliohippus, about forty inches high at the
shoulder. Pliocene, N. America.
6. The modern horse, running on the third digit of each foot.]
There is no warrant for supposing that the flying reptiles or
Pterodactyls gave rise to birds, for the two groups are on different
lines, and the structure of the wings is entirely different. Thus the
long-fingered Pterodactyl wing was a parachute wing, while the secret of
the bird's wing has its centre in the feathers. It is highly probable
that birds evolved from certain Dinosaurs which had become bipeds, and
it is possible that they were for a time swift runners that took "flying
jumps" along the ground. Thereafter, perhaps, came a period of arboreal
apprenticeship during which there was much gliding from tree to tree
before true flight was achieved. It is an interesting fact that the
problem of flight has been solved four times among animals--by insects,
by Pterodactyls, by birds, and by bats; and that the four solutions are
on entirely different lines.
In the _Cretaceous_ period the outstanding events included the waning of
giant reptiles, the modernising of the flowering plants, and the
multiplication of small mammals. Some of the Permian reptiles, such as
the dog-toothed Cynodonts, were extraordinarily mammal-like, and it was
probably from among them that definite mammals emerged in the Triassic.
Comparatively little is known of the early Triassic mammals save that
their back-teeth were marked by numerous tubercles on the crown, but
they were gaining strength in the late Triassic when small arboreal
insectivores, not very distant from the modern tree-shrews (_Tupaia_),
began to branch out in many directions indicative of the great divisions
of modern mammals, such as the clawed mammals, hoofed mammals, and the
race of monkeys or Primates. In the Upper Cretaceous there was an
exuberant "radiation" of mammals, adaptive to the conquest of all sorts
of haunts, and this was vigorously continued in Tertiary times.
There is no difficulty in the fact that the earliest remains of definite
mammals in the Triassic precede the first-known bird in the Jurassic.
For although we usually rank mammals as higher than birds (being mammals
ourselves, how could we do otherwise?), there are many ways in which
birds are pre-eminent, e.g. in skeleton, musculature, integumentary
structures, and respiratory system. The fact is that birds and mammals
are on two quite different tacks of evolution, not related to one
another, save in having a common ancestry in extinct reptiles. Moreover,
there is no reason to believe that the Jurassic _Archæopteryx_ was the
first bird in any sense except that it is the first of which we have any
record. In any case it is safe to say that birds came to their own
before mammals did.
Looking backwards, we may perhaps sum up what is most essential in the
Mesozoic era in Professor Schuchert's sentence: "The Mesozoic is the Age
of Reptiles, and yet the little mammals and the toothed birds are
storing up intelligence and strength to replace the reptiles when the
cycads and conifers shall give way to the higher flowering plants."
§ 2
The Cenozoic or Tertiary Era
In the _Eocene_ period there was a replacement of the small-brained
archaic mammals by big-brained modernised types, and with this must be
associated the covering of the earth with a garment of grass and dry
pasture. Marshes were replaced by meadows and browsing by grazing
mammals. In the spreading meadows an opportunity was also offered for a
richer evolution of insects and birds.
During the _Oligocene_ the elevation of the land continued, the climate
became much less moist, and the grazing herds extended their range.
The _Miocene_ was the mammalian Golden Age and there were crowning
examples of what Osborn calls "adaptive radiation." That is to say,
mammals, like the reptiles before them, conquer every haunt of life.
There are flying bats, volplaning parachutists, climbers in trees like
sloths and squirrels, quickly moving hoofed mammals, burrowers like the
moles, freshwater mammals, like duckmole and beaver, shore-frequenting
seals and manatees, and open-sea cetaceans, some of which dive far more
than full fathoms five. It is important to realise the perennial
tendency of animals to conquer every corner and to fill every niche of
opportunity, and to notice that this has been done by successive sets of
animals in succeeding ages. _Most notably the mammals repeat all the
experiments of reptiles on a higher turn of the spiral._ Thus arises
what is called convergence, the superficial resemblance of unrelated
types, like whales and fishes, the resemblance being due to the fact
that the different types are similarly adapted to similar conditions of
life. Professor H. F. Osborn points out that mammals may seek any one of
the twelve different habitat-zones, and that in each of these there may
be six quite different kinds of food. Living creatures penetrate
everywhere like the overflowing waters of a great river in flood.
§ 3
The _Pliocene_ period was a more strenuous time, with less genial
climatic conditions, and with more intense competition. Old land bridges
were broken and new ones made, and the geographical distribution
underwent great changes. Professor R. S. Lull describes the _Pliocene_
as "a period of great unrest." "Many migrations occurred the world over,
new competitions arose, and the weaker stocks began to show the effects
of the strenuous life. One momentous event seems to have occurred in the
Pliocene, and that was the transformation of the precursor of humanity
into man--the culmination of the highest line of evolution."
The _Pleistocene_ period was a time of sifting. There was a continued
elevation of the continental masses, and Ice Ages set in, relieved by
less severe interglacial times when the ice-sheets retreated northwards
for a time. Many types, like the mammoth, the woolly rhinoceros, the
sabre-toothed tiger, the cave-lion, and the cave-bear, became extinct.
Others which formerly had a wide range became restricted to the Far
North or were left isolated here and there on the high mountains, like
the Snow Mouse, which now occurs on isolated Alpine heights above the
snow-line. Perhaps it was during this period that many birds of the
Northern Hemisphere learned to evade the winter by the sublime device of
migration.
Looking backwards we may quote Professor Schuchert again:
"The lands in the Cenozoic began to bloom with more and more
flowering plants and grand hardwood forests, the atmosphere is
scented with sweet odours, a vast crowd of new kinds of insects
appear, and the places of the once dominant reptiles of the lands
and seas are taken by the mammals. Out of these struggles there
rises a greater intelligence, seen in nearly all of the mammal
stocks, but particularly in one, the monkey-ape-man. Brute man
appears on the scene with the introduction of the last glacial
climate, a most trying time for all things endowed with life, and
finally there results the dominance of reasoning man over all his
brute associates."
In man and human society the story of evolution has its climax.
The Ascent of Man
Man stands apart from animals in his power of building up general ideas
and of using these in the guidance of his behaviour and the control of
his conduct. This is essentially wrapped up with his development of
language as an instrument of thought. Some animals have words, but man
has language (Logos). Some animals show evidence of _perceptual_
inference, but man often gets beyond this to _conceptual_ inference
(Reason). Many animals are affectionate and brave, self-forgetful and
industrious, but man "thinks the ought," definitely guiding his conduct
in the light of ideals, which in turn are wrapped up with the fact that
he is "a social person."
Besides his big brain, which may be three times as heavy as that of a
gorilla, man has various physical peculiarities. He walks erect, he
plants the sole of his foot flat on the ground, he has a chin and a good
heel, a big forehead and a non-protrusive face, a relatively uniform set
of teeth without conspicuous canines, and a relatively naked body.
[Illustration: DIAGRAM SHOWING SEVEN STAGES IN THE EVOLUTION OF THE
FORE-LIMBS AND HIND-LIMBS OF THE ANCESTORS OF THE MODERN HORSE,
BEGINNING WITH THE EARLIEST KNOWN PREDECESSORS OF THE HORSE AND
CULMINATING WITH THE HORSE OF TO-DAY
(_After Marsh and Lull._)
1 and 1A, fore-limb and hind-limb of Eohippus; 2 and 2A, Orohippus; 3
and 3A, Mesohippus; 4 and 4A, Hypohippus; 5 and 5A, Merychippus; 6 and
6A, Hipparion; 7 and 7A, the modern horse. Note how the toes shorten and
disappear.]
[Illustration: A. Fore-limb of Monkey B. Fore-limb of Whale
WHAT IS MEANT BY HOMOLOGY? ESSENTIAL SIMILARITY OF ARCHITECTURE, THOUGH
THE APPEARANCES MAY BE VERY DIFFERENT
This is seen in comparing these two fore-limbs, A, of Monkey, B, of
Whale. They are as different as possible, yet they show the same bones,
e.g. SC, the scapula or shoulder-blade; H, the humerus or upper arm; R
and U, the radius and ulna of the fore-arm; CA, the wrist; MC, the palm;
and then the fingers.]
But in spite of man's undeniable apartness, there is no doubt as to his
solidarity with the rest of creation. There is an "all-pervading
similitude of structure," between man and the Anthropoid Apes, though it
is certain that it is not from any living form that he took his origin.
None of the anatomical distinctions, except the heavy brain, could be
called momentous. Man's body is a veritable museum of relics (vestigial
structures) inherited from pre-human ancestors. In his everyday bodily
life and in some of its disturbances, man's pedigree is often revealed.
Even his facial expression, as Darwin showed, is not always human. Some
fossil remains bring modern man nearer the anthropoid type.
It is difficult not to admit the ring of truth in the closing words of
Darwin's _Descent of Man_:
"We must, however, acknowledge, as it seems to me, that man, with
all his noble qualities, with sympathy which feels for the most
debased, with benevolence which extends not only to other men but to
the humblest living creature, with his God-like intellect which has
penetrated into the movements and constitution of the solar
system--with all these exalted powers--man still bears in his bodily
frame the indelible stamp of his lowly origin."
THE EVOLVING SYSTEM OF NATURE
There is another side of evolution so obvious that it is often
overlooked, the tendency to link lives together in vital
inter-relations. Thus flowers and their insect visitors are often
vitally interlinked in mutual dependence. Many birds feed on berries and
distribute the seeds. The tiny freshwater snail is the host of the
juvenile stages of the liver-fluke of the sheep. The mosquito is the
vehicle of malaria from man to man, and the tse-tse fly spreads sleeping
sickness. The freshwater mussel cannot continue its race without the
unconscious co-operation of the minnow, and the freshwater fish called
the bitterling cannot continue its race without the unconscious
co-operation of the mussel. There are numerous mutually beneficial
partnerships between different kinds of creatures, and other
inter-relations where the benefit is one-sided, as in the case of
insects that make galls on plants. There are also among kindred animals
many forms of colonies, communities, and societies. Nutritive chains
bind long series of animals together, the cod feeding on the whelk, the
whelk on the worm, the worm on the organic dust of the sea. There is a
system of successive incarnations and matter is continually passing from
one embodiment to another. These instances must suffice to illustrate
the central biological idea of the web of life, the interlinked System
of Animate Nature. Linnæus spoke of the Systema Naturæ, meaning the
orderly hierarchy of classes, orders, families, genera, and species; but
we owe to Darwin in particular some knowledge of a more dynamic Systema
Naturæ, the network of vital inter-relations. This has become more and
more complex as evolution has continued, and man's web is most complex
of all. It means making Animate Nature more of a unity; it means an
external method of registering steps of progress; it means an evolving
set of sieves by which new variations are sifted, and living creatures
are kept from slipping down the steep ladder of evolution.
Parasitism
It sometimes happens that the inter-relation established between one
living creature and another works in a retrograde direction. This is the
case with many thoroughgoing internal parasites which have sunk into an
easygoing kind of life, utterly dependent on their host for food,
requiring no exertions, running no risks, and receiving no spur to
effort. Thus we see that evolution is not necessarily progressive;
everything depends on the conditions in reference to which the living
creatures have been evolved. When the conditions are too easygoing, the
animal may be thoroughly well adapted to them--as a tapeworm certainly
is--but it slips down the rungs of the ladder of evolution.
This is an interesting minor chapter in the story of evolution--the
establishment of different kinds of parasites, casual and constant,
temporary and lifelong, external hangers-on and internal unpaying
boarders, those that live in the food-canal and depend on the host's
food and those that inhabit the blood or the tissues and find their food
there. It seems clear that ichneumon grubs and the like which hatch
inside a caterpillar and eat it alive are not so much parasites as
"beasts of prey" working from within.
But there are two sides to this minor chapter: there is the evolution of
the parasite, and there is also the evolution of counteractive measures
on the part of the host. Thus there is the maintenance of a bodyguard of
wandering amoeboid cells, which tackle the microbes invading the body
and often succeed in overpowering and digesting them. Thus, again, there
is the protective capacity the blood has of making antagonistic
substances or "anti-bodies" which counteract poisons, including the
poisons which the intruding parasites often make.
THE EVIDENCES OF EVOLUTION--HOW IT CAME ABOUT
§ 1
Progress in Evolution
There has often been slipping back and degeneracy in the course of
evolution, but the big fact is that there has been progress. For
millions of years Life has been slowly creeping upwards, and if we
compare the highest animals--Birds and Mammals--with their predecessors,
we must admit that they are more controlled, more masters of their
fate, with more mentality. Evolution is on the whole _integrative_; that
is to say, it makes against instability and disorder, and towards
harmony and progress. Even in the rise of Birds and Mammals we can
discern that the evolutionary process was making towards a fuller
embodiment or expression of what Man values most--control, freedom,
understanding, and love. The advance of animal life through the ages has
been chequered, but on the whole it has been an advance towards
increasing fullness, freedom, and fitness of life. In the study of this
advance--the central fact of Organic Evolution--there is assuredly much
for Man's instruction and much for his encouragement.
Evidences of Evolution
In all this, it may be said, the fact of evolution has been taken for
granted, but what are the evidences? Perhaps it should be frankly
answered that the idea of evolution, that the present is the child of
the past and the parent of the future, cannot be _proved_ as one may
prove the Law of Gravitation. All that can be done is to show that it is
a key--a way of looking at things--that fits the facts. There is no lock
that it does not open.
But if the facts that the evolution theory vividly interprets be called
the evidences of its validity, there is no lack of them. There is
_historical_ evidence; and what is more eloquent than the general fact
that fishes emerge before amphibians, and these before reptiles, and
these before birds, and so on? There are wonderfully complete fossil
series, e.g. among cuttlefishes, in which we can almost see evolution in
process. The pedigree of horse and elephant and crocodile is in general
very convincing, though it is to be confessed that there are other cases
in regard to which we have no light. Who can tell, for instance, how
Vertebrates arose or from what origin?
There is _embryological_ evidence, for the individual development often
reads like an abbreviated recapitulation of the presumed evolution of
the race. The mammal's visceral clefts are tell-tale evidence of remote
aquatic ancestors, breathing by gills. Something is known in regard to
the historical evolution of antlers in bygone ages; the Red Deer of
to-day recapitulates at least the general outlines of the history. The
individual development of an asymmetrical flat-fish, like a plaice or
sole, which rests and swims on one side, tells us plainly that its
ancestors were symmetrical fishes.
There is what might be called _physiological_ evidence, for many plants
and animals are variable before our eyes, and evolution is going on
around us to-day. This is familiarly seen among domesticated animals and
cultivated plants, but there is abundant flux in Wild Nature. It need
hardly be said that some organisms are very conservative, and that
change need not be expected when a position of stable equilibrium has
been secured.
There is also _anatomical_ evidence of a most convincing quality. In the
fore-limbs of backboned animals, say, the paddle of a turtle, the wing
of a bird, the flipper of a whale, the fore-leg of a horse, and the arm
of a man; the same essential bones and muscles are used to such diverse
results! What could it mean save blood relationship? And as to the two
sets of teeth in whalebone whales, which never even cut the gum, is
there any alternative but to regard them as relics of useful teeth which
ancestral forms possessed? In short, the evolution theory is justified
by the way in which it works.
§ 2
Factors in Evolution
If it be said "So much for the _fact_ of evolution, but what of the
_factors_?" the answer is not easy. For not only is the problem the
greatest of all scientific problems, but the inquiry is still very
young. The scientific study of evolution practically dates from the
publication of _The Origin of Species_ in 1859.
Heritable novelties or variations often crop up in living creatures, and
these form the raw material of evolution. These variations are the
outcome of expression of changes in the germ-cells that develop into
organisms. But why should there be changes in the constitution of the
germ-cells? Perhaps because the living material is very complex and
inherently liable to change; perhaps because it is the vehicle of a
multitude of hereditary items among which there are very likely to be
reshufflings or rearrangements; perhaps because the germ-cells have very
changeful surroundings (the blood, the body-cavity fluid, the
sea-water); perhaps because deeply saturating outside influences, such
as change of climate and habitat, penetrate through the body to its
germ-cells and provoke them to vary. But we must be patient with the
wearisome reiteration of "perhaps." Moreover, every many-celled organism
reproduced in the usual way, arises from an egg-cell fertilised by a
sperm-cell, and the changes involved in and preparatory to this
fertilisation may make new permutations and combinations of the living
items and hereditary qualities not only possible but necessary. It is
something like shuffling a pack of cards, but the cards are living. As
to the changes wrought on the body during its lifetime by peculiarities
in nurture, habits, and surroundings, these dents or modifications are
often very important for the individual, but it does not follow that
they are directly important for the race, since it is not certain that
they are transmissible.
Given a crop of variations or new departures or mutations, whatever the
inborn novelties may be called, we have then to inquire how these are
sifted. The sifting, which means the elimination of the relatively less
fit variations and the selection of the relatively more fit, effected in
many different ways in the course of the struggle for existence. The
organism plays its new card in the game of life, and the consequences
may determine survival. The relatively less fit to given conditions
will tend to be eliminated, while the relatively more fit will tend to
survive. If the variations are hereditary and reappear, perhaps
increased in amount, generation after generation, and if the process of
sifting continue consistently, the result will be the evolution of the
species. The sifting process may be helped by various forms of
"isolation" which lessen the range of free intercrossing between members
of a species, e.g. by geographical barriers. Interbreeding of similar
forms tends to make a stable stock; out-breeding among dissimilars tends
to promote variability. But for an outline like this it is enough to
suggest the general method of organic evolution: Throughout the ages
organisms have been making tentatives--new departures of varying
magnitude--and these tentatives have been tested. The method is that of
testing all things and holding fast that which is good.
BIBLIOGRAPHY
(The following short list may be useful to readers who desire to have
further books recommended to them.)
CLODD, _Story of Creation: A Plain Account of Evolution._
DARWIN, _Origin of Species, Descent of Man._
DEPERET, _Transformation of the Animal World_ (Internat. Sci. Series).
GEDDES AND THOMSON, _Evolution_ (Home University Library).
GOODRICH, _Evolution_ (The People's Books).
HEADLEY, _Life and Evolution._
HUTCHINSON, H. NEVILLE, _Extinct Monsters_ (1892).
LULL, _Organic Evolution._
MCCABE, _A B C of Evolution._
METCALF, _Outline of the Theory of Organic Evolution._
OSBORN, H. F., _The Evolution of Life_ (1921).
THOMSON, _Darwinism and Human Life._
WALLACE, _Darwinism._
III
ADAPTATIONS TO ENVIRONMENT
ADAPTATIONS TO ENVIRONMENT
We saw in a previous chapter how the process of evolution led to a
mastery of all the haunts of life. But it is necessary to return to
these haunts or homes of animals in some detail, so as to understand the
peculiar circumstances of each, and to see how in the course of ages of
struggle all sorts of self-preserving and race-continuing adaptations or
fitnesses have been wrought out and firmly established. Living creatures
have spread over all the earth and in the waters under the earth; some
of them have conquered the underground world and others the air. It is
possible, however, as has been indicated, to distinguish six great
haunts of life, each tenanted by a distinctive fauna, namely, the shore
of the sea, the open sea, the depths of the sea, the freshwaters, the
dry land, and the air. In the deep sea there are no plants at all; in
the air the only plants are floating bacteria, though there is a sense
in which a tree is very aerial, and the orchid perched on its branches
still more so; in the other four haunts there is a flora as well as a
fauna--the two working into one another's hands in interesting and often
subtle inter-relations--the subject of a separate study.
I. THE SHORE OF THE SEA
The Seaweed Area
By the shore of the sea the zoologist means much more than the narrow
zone between tide-marks; he means the whole of the relatively shallow,
well-illumined, seaweed-growing shelf around the continents and
continental islands. Technically, this is called the littoral area, and
it is divisible into zones, each with its characteristic population. It
may be noted that the green seaweeds are highest up on the shore; the
brown ones come next; the beautiful red ones are lowest. All of them
have got green chlorophyll, which enables them to utilise the sun's rays
in photosynthesis (i.e. building up carbon compounds from air, water,
and salts), but in the brown and red seaweeds the green pigment is
masked by others. It is maintained by some botanists that these other
pigments enable their possessors to make more of the scantier light in
the deeper waters. However this may be, we must always think of the
shore-haunt as the seaweed-growing area. Directly and indirectly the
life of the shore animals is closely wrapped up with the seaweeds, which
afford food and foothold, and temper the force of the waves. The minute
fragments broken off from seaweeds and from the sea-grass (a flowering
plant called Zostera) form a sort of nutritive sea-dust which is swept
slowly down the slope from the shore, to form a very useful deposit in
the quietness of deepish water. It is often found in the stomachs of
marine animals living a long way offshore.
Conditions of Shore Life
The littoral area as defined is not a large haunt of life; it occupies
only about 9 million square miles, a small fraction of the 197,000,000
of the whole earth's surface. But it is a very long haunt, some 150,000
miles, winding in and out by bay and fiord, estuary and creek. Where
deep water comes close to cliffs there may be no shore at all; in other
places the relatively shallow water, with seaweeds growing over the
bottom, may extend outwards for miles. The nature of the shore varies
greatly according to the nature of the rocks, according to what the
streams bring down from inland, and according to the jetsam that is
brought in by the tides. The shore is a changeful place; there is, in
the upper reaches, a striking difference between "tide in" and "tide
out"; there are vicissitudes due to storms, to freshwater floods, to
wind-blown sand, and to slow changes of level, up and down. The shore is
a very crowded haunt, for it is comparatively narrow, and every niche
among the rocks may be precious.
[Illustration: AN EIGHT-ARMED CUTTLEFISH OR OCTOPUS ATTACKING A SMALL
CRAB
These molluscs are particularly fond of crustaceans, which they crunch
with their parrot's beak-like jaws. Their salivary juice has a
paralysing effect on their prey. To one side, below the eye, may be seen
the funnel through which water is very forcibly ejected in the process
of locomotion.]
[Illustration: A COMMON STARFISH, WHICH HAS LOST THREE ARMS AND IS
REGROWING THEM
The lowest arm is being regrown double.
(_After Professor W. C. McIntosh._)]
[Illustration: A PHOTOGRAPH SHOWING A STARFISH (_Asterias Forreri_)
WHICH HAS CAPTURED A LARGE FISH
The suctorial tube-feet are seen gripping the fish firmly. (After an
observation on the Californian coast.)]
[Illustration: _Photo: J. J. Ward, F.E.S._
THE PAPER NAUTILUS (ARGONAUTA), AN ANIMAL OF THE OPEN SEA
The delicate shell is made by the female only, and is used as a shelter
for the eggs and young ones. It is secreted by two of the arms, not by
the mantle as other mollusc shells are. It is a single-chambered shell,
very different from that of the Pearly Nautilus.]
Keen Struggle for Existence
It follows that the shore must be the scene of a keen struggle for
existence--which includes all the answers-back that living creatures
make to environing difficulties and limitations. There is struggle for
food, accentuated by the fact that small items tend to be swept away by
the outgoing tide or to sink down the slope to deep water. Apart from
direct competition, e.g. between hungry hermit-crabs, it often involves
hard work to get a meal. This is true even of apparently sluggish
creatures. Thus the Crumb-of-Bread Sponge, or any other seashore sponge,
has to lash large quantities of water through the intricate canal system
of its body before it can get a sufficient supply of the microscopic
organisms and organic particles on which it feeds. An index of the
intensity of the struggle for food is afforded by the nutritive chains
which bind animals together. The shore is almost noisy with the
conjugation of the verb to eat in its many tenses. One pound of rock-cod
requires for its formation ten pounds of whelk; one pound of whelk
requires ten pounds of sea-worms; and one pound of worms requires ten
pounds of sea-dust. Such is the circulation of matter, ever passing from
one embodiment or incarnation to another.
Besides struggle for food there is struggle for foothold and for fresh
air, struggle against the scouring tide and against the pounding
breakers. The risk of dislodgment is often great and the fracture of
limbs is a common accident. Of kinds of armour--the sea-urchin's
hedgehog-like test, the crab's shard, the limpet's shell--there is great
variety, surpassed only by that of weapons--the sea-anemone's
stinging-cells, the sea-urchin's snapping-blades, the hermit-crab's
forceps, the grappling tentacles and parrot's-beak jaws of the octopus.
Shifts for a Living
We get another glimpse of the intensity of the seashore struggle for
existence in the frequency of "shifts for a living," adaptations of
structure or of behaviour which meet frequently recurrent vicissitudes.
The starfish is often in the dilemma of losing a limb or its life; by a
reflex action it jettisons the captured arm and escapes. And what is
lost is gradually regrown. The crab gets its leg broken past all
mending; it casts off the leg across a weak breakage plane near the
base, and within a preformed bandage which prevents bleeding a new leg
is formed in miniature. Such is the adaptive device--more reflex than
reflective--which is called self-mutilation or autotomy.
In another part of this book there is a discussion of camouflaging and
protective resemblance; how abundantly these are illustrated on the
shore! But there are other "shifts for a living." Some of the
sand-hoppers and their relatives illustrate the puzzling phenomenon of
"feigning death," becoming suddenly so motionless that they escape the
eyes of their enemies. Cuttlefishes, by discharging sepia from their
ink-bags, are able to throw dust in the eyes of their enemies. Some
undisguised shore-animals, e.g. crabs, are adepts in a hide-and-seek
game; some fishes, like the butterfish or gunnel, escape between stones
where there seemed no opening and are almost uncatchable in their
slipperiness. Subtlest of all, perhaps, is the habit some hermit-crabs
have of entering into mutually beneficial partnership (commensalism)
with sea-anemones, which mask their bearers and also serve as mounted
batteries, getting transport as their reward and likewise crumbs from
the frequently spread table. But enough has been said to show that the
shore-haunt exhibits an extraordinary variety of shifts for a living.
Parental Care on the Shore
According to Darwin, the struggle for existence, as a big fact in the
economy of Animate Nature, includes not only competition but all the
endeavours which secure the welfare of the offspring, and give them a
good send-off in life. So it is without a jolt that we pass from
struggle for food and foothold to parental care. The marine leech called
Pontobdella, an interesting greenish warty creature fond of fixing
itself to skate, places its egg-cocoons in the empty shell of a bivalve
mollusc, and guards them for weeks, removing any mud that might injure
their development. We have seen a British starfish with its fully-formed
young ones creeping about on its body, though the usual mode of
development for shore starfishes is that the young ones pass through a
free-swimming larval period in the open water. The father sea-spider
carries about the eggs attached to two of his limbs; the father
sea-horse puts his mate's eggs into his breast pocket and carries them
there in safety until they are hatched; the father stickleback of the
shore-pools makes a seaweed nest and guards the eggs which his wives are
induced to lay there; the father lumpsucker mounts guard over the bunch
of pinkish eggs which his mate has laid in a nook of a rocky shore-pool,
and drives off intruders with zest. He also aerates the developing eggs
by frequent paddling with his pectoral fins and tail, as the Scots name
Cock-paidle probably suggests. It is interesting that the salient
examples of parental care in the shore-haunt are mostly on the male
parent's side. But there is maternal virtue as well.
[Illustration: TEN-ARMED CUTTLEFISH OR SQUID IN THE ACT OF CAPTURING A
FISH
The arms bear numerous prehensile suckers, which grip the prey. In the
mouth there are strong jaws shaped like a parrot's beak. The
cuttlefishes are molluscs and may be regarded as the highest of the
backboneless or Invertebrate animals. Many occur near shore, others in
the open sea, and others in the great depths.]
[Illustration: GREENLAND WHALE
Showing the double blowhole or nostrils on the top of the head and the
whalebone plates hanging down from the roof of the mouth.]
[Illustration: MINUTE TRANSPARENT EARLY STAGE OF A SEA-CUCUMBER
It swims in the open sea by means of girdles of microscopic cilia shown
in the figure. After a period of free swimming and a remarkable
metamorphosis, the animal settles down on the floor of the sea in
relatively shallow water.]
[Illustration: _Photo: British Museum (Natural History)_
AN INTRICATE COLONY OF OPEN-SEA ANIMALS (_Physophora Hydrostatica_)
RELATED TO THE PORTUGUESE MAN-OF-WAR
There is great division of labor in the colony. At the top are floating
and swimming "persons"; the long ones below are offensive "persons"
bearing batteries of stinging cells; in the middle zone there are
nutritive, reproductive, and other "persons." The color of the colony is
a fine translucent blue. Swimmers and bathers are often badly stung by
this strange animal and its relatives.]
[Illustration: A SCENE IN THE GREAT DEPTHS
Showing a deep-sea fish of large gape, two feather-stars on the end of
long stalks, a "sea-spider" (or Pycnogon) walking on lanky legs on the
treacherous ooze, likewise a brittle-star, and some deep-sea corals.]
The fauna of the shore is remarkably _representative_--from unicellular
Protozoa to birds like the oyster-catcher and mammals like the seals.
Almost all the great groups of animals have apparently served an
apprenticeship in the shore-haunt, and since lessons learned for
millions of years sink in and become organically enregistered, it is
justifiable to look to the shore as a great school in which were gained
racial qualities of endurance, patience, and alertness.
II. THE OPEN SEA
In great contrast to the narrow, crowded, difficult conditions of the
shore-haunt (littoral area) are the spacious, bountiful, and relatively
easygoing conditions of the open sea (pelagic area), which means the
well-lighted surface waters quite away from land. Many small organisms
have their maximum abundance at about fifty fathoms, so that the word
"surface" is to be taken generously. The light becomes very dim at 250
fathoms, and the open sea, as a zoological haunt, stops with the light.
It is hardly necessary to say that the pelagic plants are more abundant
near the surface, and that below a certain depth the population consists
almost exclusively of animals. Not a few of the animals sink and rise in
the water periodically; there are some that come near the surface by
day, and others that come near the surface by night. Of great interest
is the habit of the extremely delicate Ctenophores or
"sea-gooseberries," which the splash of a wave would tear into shreds.
Whenever there is any hint of a storm they sink beyond its reach, and
the ocean's surface must have remained flat as a mirror for many hours
before they can be lured upwards from the calm of their deep retreat.
The Floating Sea-meadows
To understand the vital economy of the open sea, we must recognise the
incalculable abundance of minute unicellular plants, for they form the
fundamental food-supply. Along with these must also be included numerous
microscopic animals which have got possession of chlorophyll, or have
entered into internal partnership with unicellular Algæ (symbiosis).
These green or greenish plants and animals are the _producers_, using
the energy of the sunlight to help them in building up carbon compounds
out of air, water, and salts. The animals which feed on the producers,
or on other animals, are the _consumers_. Between the two come those
open-sea bacteria that convert nitrogenous material, e.g. from dead
plants or animals that other bacteria have rotted, into forms, e.g.
nitrates, which plants can re-utilise. The importance of these
_middlemen_ is great in keeping "the circulation of matter" agoing.
[Illustration: 1. SEA-HORSE IN SARGASSO WEED. In its frond-like tags of
skin and in its colouring this kind of sea-horse is well concealed among
the floating seaweed of the so-called Sargasso Sea.
2. THE LARGE MARINE LAMPREYS (_PETROMYZON MARINUS_), WHICH MAY BE AS
LONG AS ONE'S ARM, SPAWN IN FRESH WATER. Stones and pebbles, gripped in
the suctorial mouth, are removed from a selected spot and piled around
the circumference, so that the eggs, which are laid within the circle,
are not easily washed away.
3. THE DEEP-SEA FISH _CHIASMODON NIGER_ IS FAMOUS FOR ITS VORACITY. It
sometimes manages to swallow a fish larger than itself, which causes an
extraordinary protrusion of the stomach.
4. DEEP-SEA FISHES. Two of them--_Melanocetus murrayi_ and _Melanocetus
indicus_--are related to the Angler of British coasts, but adapted to
life in the great abysses. They are very dark in colour, and delicately
built; they possess well-developed luminous organs. The third form is
called Chauliodus, a predatory animal with large gape and formidable
teeth.]
[Illustration: FLINTY SKELETON OF VENUS FLOWER BASKET (EUPLECTELLA), A
JAPANESE DEEP-SEA SPONGE]
[Illustration: EGG DEPOSITORY OF _Semotilus Atromaculatus_
In the building of this egg depository, the male fish takes stones from
the bottom of the stream, gripping them in his mouth, and heaps them up
into the dam. In the egg depository he arranges the stones so that when
the eggs are deposited in the interstices they are thoroughly protected,
and cannot be washed down-stream.
1, dam of stones; 2, egg depository; 3, hillock of sand. The arrow shows
the direction of the stream. Upper fish, male; lower, female.]
The "floating sea-meadows," as Sir John Murray called them, are always
receiving contributions from inshore waters, where the conditions are
favourable for the prolific multiplication of unicellular Algæ, and
there is also a certain amount of non-living sea-dust always being swept
out from the seaweed and sea-grass area.
Swimmers and Drifters
The animals of the open sea are conveniently divided into the active
swimmers (Nekton) and the more passive drifters (Plankton). The swimmers
include whales great and small, such birds as the storm petrel, the
fish-eating turtles and sea-snakes, such fishes as mackerel and herring,
the winged snails or sea-butterflies on which whalebone whales largely
feed, some of the active cuttles or squids, various open-sea prawns and
their relatives, some worms like the transparent arrow-worm, and such
active Protozoa as Noctiluca, whose luminescence makes the waves sparkle
in the short summer darkness. Very striking as an instance of the
insurgence of life are the sea-skimmers (Halobatidæ), wingless insects
related to the water-measurers in the ditch. They are found hundreds of
miles from land, skimming on the surface of the open sea, and diving in
stormy weather. They feed on floating dead animals.
The drifters or easygoing swimmers--for there is no hard and fast
line--are represented, for instance, by the flinty-shelled Radiolarians
and certain of the chalk-forming animals (Globigerinid Foraminifera); by
jellyfishes, swimming-bells, and Portuguese men-of-war; by the
comb-bearers or Ctenophores; by legions of minute Crustaceans; by
strange animals called Salps, related to the sedentary sea-squirts; and
by some sluggish fishes like globe-fishes, which often float idly on the
surface.
Open-sea animals tend to be delicately built, with a specific gravity
near that of the sea-water, with adaptations, such as projecting
filaments, which help flotation, and with capacities of rising and
sinking according to the surrounding conditions. Many of them are
luminescent, and many of them are very inconspicuous in the water owing
to their transparency or their bluish colour. In both cases the
significance is obscure.
Hunger and Love
Hunger is often very much in evidence in the open sea, especially in
areas where the Plankton is poor. For there is great diversity in this
respect, most of the Mediterranean, for instance, having a scanty
Plankton as compared with the North Sea. In the South Pacific, west of
Patagonia, there is said to be an immense "sea desert" where there is
little Plankton, and therefore little in the way of fishes. The success
of fisheries in the North, e.g. on the Atlantic cod-banks, is due to the
richness of the floating sea-meadows and the abundance of the smaller
constituents of the animal Plankton.
Hunger is plain enough when the Baleen Whale rushes through the water
with open jaws, engulfing in the huge cavern of its mouth, where the
pendent whalebone plates form a huge sieve, incalculable millions of
small fry.
But there is love as well as hunger in the open sea. The maternal care
exhibited by the whale reaches a very high level, and the delicate shell
of the female Paper Nautilus or Argonaut, in which the eggs and the
young ones are sheltered, may well be described as "the most beautiful
cradle in the world."
Besides the permanent inhabitants of the open sea, there are the larval
stages of many shore-animals which are there only for a short time. For
there is an interesting give and take between the shore-haunt and the
open sea. From the shore come nutritive contributions and minute
organisms which multiply quickly in the open waters. But not less
important is the fact that the open waters afford a safe cradle or
nursery for many a delicate larva, e.g. of crab and starfish,
acorn-shell and sea-urchin, which could not survive for a day in the
rough-and-tumble conditions of the shore and the shallow water. After
undergoing radical changes and gaining strength, the young creatures
return to the shore in various ways.
III. THE DEEP SEA
Very different from all the other haunts are the depths of the sea,
including the floor of the abysses and the zones of water near the
bottom. This haunt, forever unseen, occupies more than a third of the
earth's surface, and it is thickly peopled. It came into emphatic notice
in connection with the mending of telegraph cables, but the results of
the _Challenger_ expedition (1873-6) gave the first impressive picture
of what was practically a new world.
Physical Conditions
The average depth of the ocean is about two and a half miles; therefore,
since many parts are relatively shallow, there must be enormous depths.
A few of these, technically called "deeps," are about six miles deep, in
which Mount Everest would be engulfed. There is enormous pressure in
such depths; even at 2,500 fathoms it is two and a half tons on the
square inch. The temperature is on and off the freezing-point of fresh
water (28°-34° Fahr.), due to the continual sinking down of cold water
from the Poles, especially from the South. Apart from the fitful gleams
of luminescent animals, there is utter darkness in the deep waters. The
rays of sunlight are practically extinguished at 250 fathoms, though
very sensitive bromogelatine plates exposed at 500 fathoms have shown
faint indications even at that depth. It is a world of absolute calm and
silence, and there is no scenery on the floor. A deep, cold, dark,
silent, monotonous world!
Biological Conditions
While some parts of the floor of the abysses are more thickly peopled
than others, there is no depth limit to the distribution of life.
Wherever the long arm of the dredge has reached, animals have been
found, e.g. Protozoa, sponges, corals, worms, starfishes, sea-urchins,
sea-lilies, crustaceans, lamp-shells, molluscs, ascidians, and fishes--a
very representative fauna. In the absence of light there can be no
chlorophyll-possessing plants, and as the animals cannot all be eating
one another there must be an extraneous source of food-supply. This is
found in the sinking down of minute organisms which are killed on the
surface by changes of temperature and other causes. What is left of
them, before or after being swallowed, and of sea-dust and mineral
particles of various kinds forms the diversified "ooze" of the
sea-floor, a soft muddy precipitate, which is said to have in places the
consistence of butter in summer weather.
There seems to be no bacteria in the abysses, so there can be no
rotting. Everything that sinks down, even the huge carcase of a whale,
must be nibbled away by hungry animals and digested, or else, in the
case of most bones, slowly dissolved away. Of the whale there are left
only the ear-bones, of the shark his teeth.
Adaptations to Deep-sea Life
In adaptation to the great pressure the bodies of deep-sea animals are
usually very permeable, so that the water gets through and through them,
as in the case of Venus' Flower Basket, a flinty sponge which a child's
finger would shiver. But when the pressure inside is the same as that
outside nothing happens. In adaptation to the treacherous ooze, so apt
to smother, many of the active deep-sea animals have very long,
stilt-like legs, and many of the sedentary types are lifted into safety
on the end of long stalks which have their bases embedded in the mud. In
adaptation to the darkness, in which there is only luminescence that
eyes could use, there is a great development of tactility. The
interesting problem of luminescence will be discussed elsewhere.
As to the origin of the deep-sea fauna, there seems no doubt that it
has arisen by many contributions from the various shore-haunts.
Following the down-drifting food, many shore-animals have in the course
of many generations reached the world of eternal night and winter, and
become adapted to its strange conditions. For the animals of the
deep-sea are as fit, beautiful, and vigorous as those elsewhere. There
are no slums in Nature.
[Illustration: THE BITTERLING (_Rhodeus Amarus_)
A Continental fish which lays its eggs by means of a long ovipositor
inside the freshwater mussel. The eggs develop inside the mollusc's
gill-plates.]
[Illustration: _Photo: W. S. Berridge._
WOOLLY OPOSSUM CARRYING HER FAMILY
One of the young ones is clinging to its mother and has its long
prehensile tail coiled round hers.]
[Illustration: SURINAM TOAD (_Pipa Americana_) WITH YOUNG ONES HATCHING
OUT OF LITTLE POCKETS ON HER BACK]
[Illustration: STORM PETREL OR MOTHER CAREY'S CHICKEN
(_Procellaria Pelagica_)
This characteristic bird of the open sea does not come to land at all
except to nest. It is the smallest web-footed bird, about four inches
long. The legs are long and often touch the water as the bird flies. The
storm petrel is at home in the Atlantic, and often nests on islands off
the west coast of Britain.]
IV. THE FRESH WATERS
Of the whole earth's surface the freshwaters form a very small fraction,
about a hundredth, but they make up for their smallness by their
variety. We think of deep lake and shallow pond, of the great river and
the purling brook, of lagoon and swamp, and more besides. There is a
striking resemblance in the animal population of widely separated
freshwater basins: and this is partly because birds carry many small
creatures on their muddy feet from one water-shed to another; partly
because some of the freshwater animals are descended from types which
make their way from the sea and the seashore through estuaries and
marshes, and only certain kinds of constitution could survive the
migration; and partly because some lakes are landlocked dwindling relics
of ancient seas, and similar forms again would survive the change.
A typical assemblage of freshwater animals would include many Protozoa,
like Amoebæ and the Bell-Animalcules, a representative of one family
of sponges (Spongillidæ), the common Hydra, many unsegmented worms
(notably Planarians and Nematodes), many Annelids related to the
earthworms, many crustaceans, insects, and mites, many bivalves and
snails, various fishes, a newt or two, perhaps a little mud-turtle or in
warm countries a huge Crocodilian, various interesting birds like the
water-ouzel or dipper, and mammals like the water-vole and the
water-shrew.
Freshwater animals have to face certain difficulties, the greatest of
which are drought, frost, and being washed away in times of flood.
There is no more interesting study in the world than an inquiry into the
adaptations by which freshwater animals overcome the difficulties of the
situation. We cannot give more than a few illustrations.
(1) Drought is circumvented by the capacity that many freshwater animals
have of lying low and saying nothing. Thus the African mudfish may spend
half the year encased in the mud, and many minute crustaceans can
survive being dried up for years. (2) Escape from the danger of being
frozen hard in the pool is largely due to the almost unique property of
water that it expands as it approaches the freezing-point. Thus the
colder water rises to the surface and forms or adds to the protecting
blanket of ice. The warmer water remains unfrozen at the bottom, and the
animals live on. (3) The risk of being washed away, e.g. to the sea, is
lessened by all sorts of gripping, grappling, and anchoring structures,
and by shortening the juvenile stages when the risks are greatest.
V. THE DRY LAND
Over and over again in the history of animal life there have been
attempts to get out of the water on to terra firma, and many of these
have been successful, notably those made (1) by worms, (2) by
air-breathing Arthropods, and (3) by amphibians.
In thinking of the conquest of the dry land by animals, we must
recognise the indispensable rôle of plants in preparing the way. The dry
ground would have proved too inhospitable had not terrestrial plants
begun to establish themselves, affording food, shelter, and humidity.
There had to be plants before there could be earthworms, which feed on
decaying leaves and the like, but how soon was the debt repaid when the
earthworms began their worldwide task of forming vegetable mould,
opening up the earth with their burrows, circulating the soil by means
of their castings, and bruising the particles in their
gizzard--certainly the most important mill in the world.
Another important idea is that littoral haunts, both on the seashore and
in the freshwaters, afforded the necessary apprenticeship and
transitional experience for the more strenuous life on dry land. Much
that was perfected on land had its beginnings on the shore. Let us
inquire, however, what the passage from water to dry land actually
implied. This has been briefly discussed in a previous article (on
Evolution), but the subject is one of great interest and importance.
Difficulties and Results of the Transition from Water to Land
Leaving the water for dry land implied a loss in freedom of movement,
for the terrestrial animal is primarily restricted to the surface of the
earth. Thus it became essential that movements should be very rapid and
very precise, needs with which we may associate the acquisition of fine
cross-striped, quickly contracting muscles, and also, in time, their
multiplication into very numerous separate engines. We exercise
fifty-four muscles in the half-second that elapses between raising the
heel of our foot in walking and planting it firmly on the ground again.
Moreover, the need for rapid precisely controlled movements implied an
improved nervous system, for the brain was a movement-controlling organ
for ages before it did much in the way of thinking. The transition to
terra firma also involved a greater compactness of body, so that there
should not be too great friction on the surface. An animal like the
jellyfish is unthinkable on land, and the elongated bodies of some land
animals like centipedes and snakes are specially adapted so that they do
not "sprawl." They are exceptions that prove the rule.
Getting on to dry land meant entering a kingdom where the differences
between day and night, between summer and winter are more felt than in
the sea. This made it advantageous to have protections against
evaporation and loss of heat and other such dangers. Hence a variety of
ways in which the surface of the body acquired a thickened skin, or a
dead cuticle, or a shell, or a growth of hair, and so forth. In many
cases there is an increase of the protection before the winter sets in,
e.g. by growing thicker fur or by accumulating a layer of fat below the
skin.
But the thickening or protection of the skin involved a partial or total
loss of the skin as a respiratory surface. There is more oxygen
available on dry land than in the water, but it is not so readily
captured. Thus we see the importance of moist internal surfaces for
capturing the oxygen which has been drawn into the interior of the body
into some sort of lung. A unique solution was offered by Tracheate
Arthropods, such as Peripatus, Centipedes, Millipedes, and Insects,
where the air is carried to every hole and corner of the body by a
ramifying system of air-tubes or tracheæ. In most animals the blood goes
to the air, in insects the air goes to the blood. In the Robber-Crab,
which has migrated from the shore inland, the dry air is absorbed by
vascular tufts growing under the shelter of the gill-cover.
The problem of disposing of eggs or young ones is obviously much more
difficult on land than in the water. For the water offers an immediate
cradle, whereas on the dry land there were many dangers, e.g. of
drought, extremes of temperature, and hungry sharp-eyed enemies, which
had to be circumvented. So we find all manner of ways in which land
animals hide their eggs or their young ones in holes and nests, on herbs
and on trees. Some carry their young ones about after they are born,
like the Surinam toad and the kangaroo, while others have prolonged the
period of ante-natal life during which the young ones develop in safety
within their mother, and in very intimate partnership with her in the
case of the placental mammals. It is very interesting to find that the
pioneer animal called Peripatus, which bridges the gap between worms and
insects, carries its young for almost a year before birth.
Enough has been said to show that the successive conquests of the dry
land had great evolutionary results. It is hardly too much to say that
the invasion which the Amphibians led was the beginning of better
brains, more controlled activities, and higher expressions of family
life.
[Illustration: ALBATROSS: A CHARACTERISTIC PELAGIC BIRD OF THE SOUTHERN
SEA
It may have a spread of wing of over 11 feet from tip to tip. It is
famous for its extraordinary power of "sailing" round the ship without
any apparent strokes of its wings.]
VI. THE AIR
There are no animals thoroughly aerial, but many insects spend much of
their adult life in the free air, and the swift hardly pauses in its
flight from dawn to dusk of the long summer day, alighting only for
brief moments at the nest to deliver insects to the young. All the
active life of bats certainly deserves to be called aerial.
The air was the last haunt of life to be conquered, and it is
interesting to inquire what the conquest implied. (1) It meant
transcending the radical difficulty of terrestrial life which confines
the creatures of the dry land to moving on one plane, the surface of the
earth. But the power of flight brought its possessors back to the
universal freedom of movement which water animals enjoy. When we watch a
sparrow rise into the air just as the cat has completed her stealthy
stalking, we see that flight implies an enormous increase of safety. (2)
The power of flight also opened up new possibilities of following the
prey, of exploring new territories, of prospecting for water. (3) Of
great importance too was the practicability of placing the eggs and the
young, perhaps in a nest, in some place inaccessible to most enemies.
When one thinks of it, the rooks' nests swaying on the tree-tops express
the climax of a brilliant experiment. (4) The crowning advantage was the
possibility of migrating, of conquering time (by circumventing the arid
summer and the severe winter) and of conquering space (by passing
quickly from one country to another and sometimes almost girdling the
globe). There are not many acquisitions that have meant more to their
possessors than the power of flight. It was a key opening the doors of a
new freedom.
The problem of flight, as has been said in a previous chapter, has been
solved four times, and the solution has been different in each case. The
four solutions are those offered by insects, extinct Pterodactyls,
birds, and bats. Moreover, as has been pointed out, there have been
numerous attempts at flight which remain glorious failures, notably the
flying fishes, which take a great leap and hold their pectoral fins
taut; the Flying Tree-Toad, whose webbed fingers and toes form a
parachute; the Flying Lizard (_Draco volans_), which has its skin pushed
out on five or six greatly elongated mobile ribs; and various "flying"
mammals, e.g. Flying Phalangers and Flying Squirrels, which take great
swooping leaps from tree to tree.
The wings of an insect are hollow flattened sacs which grow out from the
upper parts of the sides of the second and third rings of the region
called the thorax. They are worked by powerful muscles, and are
supported, like a fan, by ribs of chitin, which may be accompanied by
air-tubes, blood-channels, and nerves. The insect's body is lightly
built and very perfectly aerated, and the principle of the insect's
flight is the extremely rapid striking of the air by means of the
lightly built elastic wings. Many an insect has over two hundred strokes
of its wings in one _second_. Hence, in many cases, the familiar hum,
comparable on a small scale to that produced by the rapidly revolving
blades of an aeroplane's propeller. For a short distance a bee can
outfly a pigeon, but few insects can fly far, and they are easily blown
away or blown back by the wind. Dragon-flies and bees may be cited as
examples of insects that often fly for two or three miles. But this is
exceptional, and the usual shortness of insect flight is an important
fact for man since it limits the range of insects like house-flies and
mosquitoes which are vehicles of typhoid fever and malaria respectively.
The most primitive insects (spring-tails and bristle-tails) show no
trace of wings, while fleas and lice have become secondarily wingless.
It is interesting to notice that some insects only fly once in their
lifetime, namely, in connection with mating. The evolution of the
insect's wing remains quite obscure, but it is probable that insects
could run, leap, and parachute before they could actually fly.
The extinct Flying Dragons or Pterodactyls had their golden age in the
Cretaceous era, after which they disappeared, leaving no descendants. A
fold of skin was spread out from the sides of the body by the enormously
elongated outermost finger (usually regarded as corresponding to our
little finger); it was continued to the hind-legs and thence to the
tail.
It is unlikely that the Pterodactyls could fly far, for they have at
most a weak keel on their breast-bone; on the other hand, some of them
show a marked fusion of dorsal vertebræ, which, as in flying birds, must
have served as a firm fulcrum for the stroke of the wings. The quaint
creatures varied from the size of a sparrow up to a magnificent spread
of 15-20 feet from tip to tip of the wings. They were the largest of all
flying creatures.
The bird's solution of the problem of flight, which will be discussed
separately, is centred in the feather, which forms a coherent vane for
striking the air. In Pterodactyl and bat the wing is a web-wing or
patagium, and a small web is to be seen on the front side of the bird's
wing. But the bird's patagium is unimportant, and the bird's wing is on
an evolutionary tack of its own--a fore-limb transformed for bearing the
feathers of flight. Feathers are in a general way comparable to the
scales of reptiles, but only in a general way, and no transition stage
is known between the two. Birds evolved from a bipedal Dinosaur stock,
as has been noticed already, and it is highly probable that they began
their ascent by taking running leaps along the ground, flapping their
scaly fore-limbs, and balancing themselves in kangaroo-like fashion with
an extended tail. A second chapter was probably an arboreal
apprenticeship, during which they made a fine art of parachuting--a
persistence of which is to be seen in the pigeon "gliding" from the
dovecot to the ground. It is in birds that the mastery of the air
reaches its climax, and the mysterious "sailing" of the albatross and
the vulture is surely the most remarkable locomotor triumph that has
ever been achieved. Without any apparent stroke of the wings, the bird
sails for half an hour at a time with the wind and against the wind,
around the ship and in majestic spirals in the sky, probably taking
advantage of currents of air of different velocities, and continually
changing energy of position into energy of motion as it sinks, and
energy of motion into energy of position as it rises. It is interesting
to know that some dragon-flies are also able to "sail."
The web-wing of bats involves much more than the fore-arm. The double
fold of skin begins on the side of the neck, passes along the front of
the arm, skips the thumb, and is continued over the elongated palm-bones
and fingers to the sides of the body again, and to the hind-legs, and to
the tail if there is a tail. It is interesting to find that the bones of
the bat's skeleton tend to be lightly built as in birds, that the
breast-bone has likewise a keel for the better insertion of the pectoral
muscles, and that there is a solidifying of the vertebræ of the back,
affording as in birds a firm basis for the wing action. Such similar
adaptations to similar needs, occurring in animals not nearly related to
one another, are called "convergences," and form a very interesting
study. In addition to adaptations which the bat shares with the flying
bird, it has many of its own. There are so many nerve-endings on the
wing, and often also on special skin-leaves about the ears and nose,
that the bat flying in the dusk does not knock against branches or other
obstacles. Some say that it is helped by the echoes of its high-pitched
voice, but there is no doubt as to its exquisite tactility. That it
usually produces only a single young one at a time is a clear adaptation
to flight, and similarly the sharp, mountain-top-like cusps on the back
teeth are adapted in insectivorous bats for crunching insects.
Whether we think of the triumphant flight of birds, reaching a climax in
migration, or of the marvel that a creature of the earth--as a mammal
essentially is--should evolve such a mastery of the air as we see in
bats, or even of the repeated but splendid failures which parachuting
animals illustrate, we gain an impression of the insurgence of living
creatures in their characteristic endeavour after fuller well-being.
We have said enough to show how well adapted many animals are to meet
the particular difficulties of the haunt which they tenant. But
difficulties and limitations are ever arising afresh, and so one fitness
follows on another. It is natural, therefore, to pass to the frequent
occurrence of protective resemblance, camouflage, and mimicry--the
subject of the next article.
BIBLIOGRAPHY
ELMHIRST, R., _Animals of the Shore_.
FLATTELY AND WALTON, _The Biology of the Shore_ (1921).
FURNEAUX, _Life of Ponds and Streams_.
HICKSON, S. J., _Story of Life in the Seas_ and _Fauna of the Deep Sea_.
JOHNSTONE, J., _Life in the Sea_ (Cambridge Manual of Science).
MIALL, L. C., _Aquatic Insects_.
MURRAY, SIR JOHN, _The Ocean_ (Home University Library).
MURRAY, SIR JOHN AND HJORT, DR. J., _The Depths of the Ocean_.
NEWBIGIN, M. I., _Life by the Sea Shore_.
PYCRAFT, W. P., _History of Birds_.
SCHARFF, R. F., _History of the European Fauna_ (Contemp. Sci. Series).
THOMSON, J. ARTHUR, _The Wonder of Life_ (1914) and
_The Haunts of Life_ (1921).
IV
THE STRUGGLE FOR EXISTENCE
ANIMAL AND BIRD MIMICRY AND DISGUISE
§ 1
For every animal one discovers when observing carefully, there must be
ten unseen. This is partly because many animals burrow in the ground or
get in underneath things and into dark corners, being what is called
cryptozoic or elusive. But it is partly because many animals put on
disguise or have in some way acquired a garment of invisibility. This is
very common among animals, and it occurs in many forms and degrees. The
reason why it is so common is because the struggle for existence is
often very keen, and the reasons why the struggle for existence is keen
are four. First, there is the tendency to over-population in many
animals, especially those of low degree. Second, there is the fact that
the scheme of nature involves nutritive chains or successive
incarnations, one animal depending upon another for food, and all in the
long run on plants; thirdly, every vigorous animal is a bit of a
hustler, given to insurgence and sticking out his elbows. There is a
fourth great reason for the struggle for existence, namely, the frequent
changefulness of the physical environment, which forces animals to
answer back or die; but the first three reasons have most to do with the
very common assumption of some sort of disguise. Even when an animal is
in no sense a weakling, it may be very advantageous for it to be
inconspicuous when it is resting or when it is taking care of its young.
Our problem is the evolution of elusiveness, so far at least as that
depends on likeness to surroundings, on protective resemblance to other
objects, and in its highest reaches on true mimicry.
Colour Permanently Like That of Surroundings
Many animals living on sandy places have a light-brown colour, as is
seen in some lizards and snakes. The green lizard is like the grass and
the green tree-snake is inconspicuous among the branches. The spotted
leopard is suited to the interrupted light of the forest, and it is
sometimes hard to tell where the jungle ends and the striped tiger
begins. There is no better case than the hare or the partridge sitting a
few yards off on the ploughed field. Even a donkey grazing in the dusk
is much more readily heard than seen.
The experiment has been made of tethering the green variety of Praying
Mantis on green herbage, fastening them with silk threads. They escape
the notice of birds. The same is true when the brown variety is tethered
on withered herbage. But if the green ones are put on brown plants, or
the brown ones on green plants, the birds pick them off. Similarly, out
of 300 chickens in a field, 240 white or black and therefore
conspicuous, 60 spotted and inconspicuous, 24 were soon picked off by
crows, but only one of these was spotted. This was not the proportion
that there should have been if the mortality had been fortuitous. There
is no doubt that it often pays an animal to be like its habitual
surroundings, like a little piece of scenery if the animal is not
moving. It is safe to say that in process of time wide departures from
the safest coloration will be wiped out in the course of Nature's
ceaseless sifting.
But we must not be credulous, and there are three cautions to be borne
in mind. (1) An animal may be very like its surroundings without there
being any protection implied. The arrow-worms in the sea are as clear as
glass, and so are many open-sea animals. But this is because their
tissues are so watery, with a specific gravity near that of the salt
water. And the invisibility does not save them, always or often, from
being swallowed by larger animals that gather the harvest of the sea.
(2) Among the cleverer animals it looks as if the creature sometimes
sought out a spot where it was most inconspicuous. A spider may place
itself in the middle of a little patch of lichen, where its
self-effacement is complete. Perhaps it is more comfortable as well as
safer to rest in surroundings the general colour of which is like that
of the animal's body. (3) The fishes that live among the coral-reefs are
startling in their brilliant coloration, and there are many different
patterns. To explain this it has been suggested that these fishes are so
safe among the mazy passages and endless nooks of the reefs, that they
can well afford to wear any colour that suits their constitution. In
some cases this may be true, but naturalists who have put on a diving
suit and walked about among the coral have told us that each kind of
fish is particularly suited to some particular place, and that some are
suited for midday work and others for evening work. Sometimes there is a
sort of Box and Cox arrangement by which two different fishes utilise
the same corner at different times.
[Illustration: THE PRAYING MANTIS (_Mantis Religiosa_)
A very voracious insect with a quiet, unobtrusive appearance. It holds
its formidable forelegs as if in the attitude of prayer; its movements
are very slow and stealthy; and there is a suggestion of a leaf in the
forewing. But there is no reason to credit the creature with conscious
guile!]
[Illustration: PROTECTIVE COLORATION: A WINTER SCENE IN NORTH
SCANDINAVIA
Showing Variable Hare, Willow Grouse, and Arctic Fox, all white in
winter and inconspicuous against the snow. But the white dress is also
the dress that is physiologically best, for it loses least of the animal
heat.]
[Illustration: THE VARIABLE MONITOR (_Varanus_)
The monitors are the largest of existing lizards, the Australian species
represented in the photograph attaining a length of four feet. It has a
brown colour with yellow spots, and in spite of its size it is not
conspicuous against certain backgrounds, such as the bark of a tree.]
§ 2
Gradual Change of Colour
The common shore-crab shows many different colours and mottlings,
especially when it is young. It may be green or grey, red or brown, and
so forth, and it is often in admirable adjustment to the colour of the
rock-pool where it is living. Experiments, which require extension, have
shown that when the crab has moulted, which it has to do very often when
it is young, the colour of the new shell tends to harmonise with the
general colour of the rocks and seaweed. How this is brought about, we
do not know. The colour does not seem to change till the next moult, and
not then unless there is some reason for it. A full-grown shore-crab is
well able to look after itself, and it is of interest to notice,
therefore, that the variety of coloration is mainly among the small
individuals, who have, of course, a much less secure position. It is
possible, moreover, that the resemblance to the surroundings admits of
more successful hunting, enabling the small crab to take its victim
unawares.
Professor Poulton's experiments with the caterpillars of the small
tortoise-shell butterfly showed that in black surroundings the pupæ tend
to be darker, in white surroundings lighter, in gilded boxes golden; and
the same is true in other cases. It appears that the surrounding colour
affects the caterpillars through the skin during a sensitive period--the
twenty hours immediately preceding the last twelve hours of the larval
state. The result will tend to make the quiescent pupæ less conspicuous
during the critical time of metamorphosis. The physiology of this
sympathetic colouring remains obscure.
Seasonal Change of Colouring
The ptarmigan moults three times in the year. Its summer plumage is
rather grouselike above, with a good deal of rufous brown; the back
becomes much more grey in autumn; almost all the feathers of the winter
plumage are white. That is to say, they develop without any pigment and
with numerous gas-bubbles in their cells. Now there can be no doubt that
this white winter plumage makes the ptarmigan very inconspicuous amidst
the snow. Sometimes one comes within a few feet of the crouching bird
without seeing it, and this garment of invisibility may save it from the
hungry eyes of golden eagles.
Similarly the brown stoat becomes the white ermine, mainly by the
growth, of a new suit of white fur, and the same is true of the mountain
hare. The ermine is all white except the black tip of its tail; the
mountain hare in its winter dress is all white save the black tips of
its ears. In some cases, especially in the mountain hare, it seems that
individual hairs may turn white, by a loss of pigment, as may occur in
man. According to Metchnikoff, the wandering amoeboid cells of the
body, called phagocytes, may creep up into the hairs and come back again
with microscopic burdens of pigment. The place of the pigment is taken
by gas-bubbles, and that is what causes the whiteness. In no animals is
there any white _pigment_; the white _colour_ is like that of snow or
foam, it is due to the complete reflection of the light from innumerable
minute surfaces of crystals or bubbles.
[Illustration: _Photo: W. S. Berridge, F.Z.S._
BANDED KRAIT: A VERY POISONOUS SNAKE WITH ALTERNATING YELLOW AND DARK
BANDS
It is very conspicuous and may serve as an illustration of warning
coloration. Perhaps, that is to say, its striking coloration serves as
an advertisement, impressing other creatures with the fact that the
Banded Krait should be left alone. It is very unprofitable for a snake
to waste its venom on creatures it does not want.]
[Illustration: _Photos: W. S. Berridge, F.Z.S._
THE WARTY CHAMELEON
The upper photograph shows the Warty Chameleon inflated and conspicuous.
At another time, however, with compressed body and adjusted coloration,
the animal is very inconspicuous. The lower photograph shows the sudden
protrusion of the very long tongue on a fly.]
[Illustration: SEASONAL COLOUR-CHANGE: A SUMMER SCENE IN NORTH
SCANDINAVIA
Showing a brown Variable Hare, Willow Grouse, and Arctic Fox, all
inconspicuous in their coloration when seen in their natural
surroundings.]
The mountain hare may escape the fox the more readily because its
whiteness makes it so inconspicuous against a background of snow; and
yet, at other times, we have seen the creature standing out like a
target on the dark moorland. So it cuts both ways. The ermine has almost
no enemies except the gamekeeper, but its winter whiteness may help it
to sneak upon its victims, such as grouse or rabbit, when there is snow
upon the ground. In both cases, however, the probability is that the
constitutional rhythm which leads to white hair in winter has been
fostered and fixed for a reason quite apart from protection. The fact is
that for a warm-blooded creature, whether bird or mammal, the
physiologically best dress is a white one, for there is less radiation
of the precious animal heat from white plumage or white pelage than from
any other colour. The quality of warm-bloodedness is a prerogative of
birds and mammals, and it means that the body keeps an almost constant
temperature, day and night, year in and year out. This is effected by
automatic internal adjustments which regulate the supply of heat,
chiefly from the muscles, to the loss of heat, chiefly through the skin
and from the lungs. The chief importance of this internal heat is that
it facilitates the smooth continuance of the chemical processes on which
life depends. If the temperature falls, as in hibernating mammals (whose
warm-bloodedness is imperfect), the rate of the vital process is slowed
down--sometimes dangerously. Thus we see how the white coat helps the
life of the creature.
§ 3
Rapid Colour-change
Bony flat-fishes, like plaice and sole, have a remarkable power of
adjusting their hue and pattern to the surrounding gravel and sand, so
that it is difficult to find them even when we know that they are there.
It must be admitted that they are also very quick to get a sprinkling
of sand over their upturned side, so that only the eyes are left
showing. But there is no doubt as to the exactness with which they often
adjust themselves to be like a little piece of the substratum on which
they lie; they will do this within limits in experimental conditions
when they are placed on a quite artificial floor. As these fishes are
very palatable and are much sought after by such enemies as cormorants
and otters, it is highly probably that their power of self-effacement
often saves their life. And it may be effected within a few minutes, in
some cases within a minute.
In these self-effacing flat-fishes we know with some precision what
happens. The adjustment of colour and pattern is due to changes in the
size, shape, and position of mobile pigment-cells (chromatophores) and
the skin. But what makes the pigment-cells change? The fact that a blind
flat-fish does not change its colour gives us the first part of the
answer. The colour and the pattern of the surroundings must affect the
eye. The message travels by the optic nerve to the brain; from the
brain, instead of passing down the spinal cord, the message travels down
the chain of sympathetic ganglia. From these it passes along the nerves
which comes out of the spinal cord and control the skin. Thus the
message reaches the colour-cells in the skin, and before you have
carefully read these lines the flat-fish has slipped on its Gyges ring
and become invisible.
The same power of rapid colour-change is seen in cuttlefishes, where it
is often an expression of nervous excitement, though it sometimes helps
to conceal. It occurs with much subtlety in the Æsop prawn, Hippolyte,
which may be brown on a brown seaweed, green on sea-lettuce or
sea-grass, red on red seaweed, and so on through an extensive repertory.
According to the nature of the background, [Professor Gamble writes]
so is the mixture of the pigments compounded so as to form a close
reproduction both of its colour and its pattern. A sweep of the
shrimp net detaches a battalion of these sleeping prawns, and if
we turn the motley into a dish and give a choice of seaweed, each
variety after its kind will select the one with which it agrees in
colour, and vanish. Both when young and when full-grown, the Æsop
prawn takes on the colour of its immediate surroundings. At
nightfall Hippolyte, of whatever colour, changes to a transparent
azure blue: its stolidity gives place to a nervous restlessness; at
the least tremor it leaps violently, and often swims actively from
one food-plant to another. This blue fit lasts till daybreak, and is
then succeeded by the prawn's diurnal tint.
Thus, Professor Gamble continues, the colour of an animal may express a
nervous rhythm.
[Illustration: _Photo: J. J. Ward, F.E.S._
PROTECTIVE RESEMBLANCE
Hawk Moth, settled down on a branch, and very difficult to detect as
long as it remains stationary. Note its remarkable sucking tongue, which
is about twice the length of its body. The tongue can be quickly coiled
up and put safely away beneath the lower part of the head.]
[Illustration: WHEN ONLY A FEW DAYS OLD, YOUNG BITTERN BEGIN TO STRIKE
THE SAME ATTITUDE AS THEIR PARENTS THRUSTING THEIR BILLS UPWARDS AND
DRAWING THEIR BODIES UP SO THAT THEY RESEMBLE A BUNCH OF REEDS
The soft browns and blue-greens harmonise with the dull sheaths of the
young reeds; the nestling bittern is thus completely camouflaged.]
The Case of Chameleons
The highest level at which rapid colour-change occurs is among lizards,
and the finest exhibition of it is among the chameleons. These quaint
creatures are characteristic of Africa; but they occur also in
Andalusia, Arabia, Ceylon, and Southern India. They are adapted for life
on trees, where they hunt insects with great deliberateness and success.
The protrusible tongue, ending in a sticky club, can be shot out for
about seven inches in the common chameleon. Their hands and feet are
split so that they grip the branches firmly, and the prehensile tail
rivals a monkey's. When they wish they can make themselves very slim,
contracting the body from side to side, so that they are not very
readily seen. In other circumstances, however, they do not practise
self-effacement, but the very reverse. They inflate their bodies, having
not only large lungs, but air-sacs in connection with them. The throat
bulges; the body sways from side to side; and the creature expresses its
sentiments in a hiss. The power of colour-change is very remarkable, and
depends partly on the contraction and expansion of the colour-cells
(chromatophores) in the under-skin (or dermis) and partly on
close-packed refractive granules and crystals of a waste-product called
guanin. The repertory of possible colours in the common chameleon is
greater than in any other animal except the Æsop prawn. There is a
legend of a chameleon which was brown in a brown box, green in a green
box, and blue in a blue box, and died when put into one lined with
tartan; and there is no doubt that one and the same animal has a wide
range of colours. The so-called "chameleon" (_Anolis_) of North America
is so sensitive that a passing cloud makes it change its emerald hue.
There is no doubt that a chameleon may make itself more inconspicuous by
changing its colour, being affected by the play of light on its eyes. A
bright-green hue is often seen on those that are sitting among strongly
illumined green leaves. But the colour also changes with the time of day
and with the animal's moods. A sudden irritation may bring about a rapid
change; in other cases the transformation comes about very gradually.
When the colour-change expresses the chameleon's feelings it might be
compared to blushing, but that is due to an expansion of the arteries of
the face, allowing more blood to get into the capillaries of the
under-skin. The case of the chameleon is peculiarly interesting because
the animal has two kinds of tactics--self-effacement on the one hand and
bluffing on the other. There can be little doubt that the power of
colour-change sometimes justifies itself by driving off intruders. Dr.
Cyril Crossland observed that a chameleon attacked by a fox-terrier
"turned round and opened its great pink mouth in the face of the
advancing dog, at the same time rapidly changing colour, becoming almost
black. This ruse succeeded every time, the dog turning off at once." In
natural leafy surroundings the startling effect would be much greater--a
sudden throwing off of the mantle of invisibility and the exposure of a
conspicuous black body with a large red mouth.
§ 4
Likeness to Other Things
Dr. H. O. Forbes tells of a flat spider which presents a striking
resemblance to a bird's dropping on a leaf. Years after he first
found it he was watching in a forest in the Far East when his eye fell
on a leaf before him which had been blotched by a bird. He wondered idly
why he had not seen for so long another specimen of the bird-dropping
spider (_Ornithoscatoides decipiens_), and drew the leaf towards him.
Instantaneously he got a characteristic sharp nip; it was the spider
after all! Here the colour-resemblance was enhanced by a
form-resemblance.
[Illustration: A. PROTECTIVE COLORATION OR CAMOUFLAGING, GIVING ANIMALS
A GARMENT OF INVISIBILITY
At the foot of the plate is a Nightjar, with plumage like bark and
withering leaves; to the right, resting on a branch, is shown a
Chameleon in a green phase amid green surroundings; the insects on the
reeds are Locusts; while a green Frog, merged into its surroundings,
rests on a leaf near the centre at the top of the picture.
B. ANOTHER EXAMPLE OF PROTECTIVE COLORATION OR CAMOUFLAGE
A shore scene showing Trout in the pool almost invisible against their
background. The Stone Curlews, both adult and young, are very
inconspicuous among the stones on the beach.]
But why should it profit a spider to be like a bird-dropping? Perhaps
because it thereby escapes attention; but there is another possibility.
It seems that some butterflies, allied to our Blues, are often attracted
to excrementitious material, and the spider Dr. Forbes observed had
actually caught its victim. This is borne out by a recent observation by
Dr. D. G. H. Carpenter, who found a Uganda bug closely resembling a
bird-dropping on sand. The bug actually settled down on a bird-dropping
on sand, and caught a blue butterfly which came to feed there!
Some of the walking-stick insects, belonging to the order of crickets
and grasshoppers (Orthoptera), have their body elongated and narrow,
like a thin dry branch, and they have a way of sticking out their limbs
at abrupt and diverse angles, which makes the resemblance to twigs very
close indeed. Some of these quaint insects rest through the day and have
the remarkable habit of putting themselves into a sort of kataleptic
state. Many creatures turn stiff when they get a shock, or pass suddenly
into new surroundings, like some of the sand-hoppers when we lay them on
the palm of our hand; but these twig-insects put themselves into this
strange state. The body is rocked from side to side for a short time,
and then it stiffens. An advantage may be that even if they were
surprised by a bird or a lizard, they will not be able to betray
themselves by even a tremor. Disguise is perfected by a remarkable
habit, a habit which leads us to think of a whole series of different
ways of lying low and saying nothing which are often of life-preserving
value. The top end of the series is seen when a fox plays 'possum.
The leaf-butterfly _Kallima_, conspicuously coloured on its upper
surface, is like a withered leaf when it settles down and shows the
under side of its wings. Here, again, there is precise form-resemblance,
for the nervures on the wings are like the mid-rib and side veins on a
leaf, and the touch of perfection is given in the presence of whitish
spots which look exactly like the discolorations produced by lichens on
leaves. An old entomologist, Mr. Jenner Weir, confessed that he
repeatedly pruned off a caterpillar on a bush in mistake for a
superfluous twig, for many brownish caterpillars fasten themselves by
their posterior claspers and by an invisible thread of silk from their
mouth, and project from the branch at a twig-like angle. An insect may
be the very image of a sharp prickle or a piece of soft moss; a spider
may look precisely like a tiny knob on a branch or a fragment of lichen;
one of the sea-horses (_Phyllopteryx_) has frond-like tassels on various
parts of its body, so that it looks extraordinarily like the seaweeds
among which it lives. In a few cases, e.g. among spiders, it has been
shown that animals with a special protective resemblance to something
else seek out a position where this resemblance tells, and there is
urgent need for observations bearing on this selection of environment.
§ 5
Mimicry in the True Sense
It sometimes happens that in one and the same place there are two groups
of animals not very nearly related which are "doubles" of one another.
Investigation shows that the members of the one group, _always in the
majority_, are in some way specially protected, e.g. by being
unpalatable. They are the "mimicked." The members of the other group,
_always in the minority_, have not got the special protection possessed
by the others. They are the "mimickers," though the resemblance is not,
of course, associated with any conscious imitation. The theory is that
the mimickers live on the reputation of the mimicked. If the mimicked
are left alone by birds because they have a reputation for
unpalatability, or because they are able to sting, the mimickers
survive--although they are palatable and stingless. They succeed, not
through any virtue of their own, but because of their resemblance to the
mimicked, for whom they are mistaken. There are many cases of mimetic
resemblance so striking and so subtle that it seems impossible to doubt
that the thing works; there are other cases which are rather
far-fetched, and may be somewhat of the nature of coincidences. Thus
although Mr. Bates tells us that he repeatedly shot humming-bird moths
in mistake for humming-birds, we cannot think that this is a good
illustration of mimicry. What is needed for many cases is what is
forthcoming for some, namely, experimental evidence, e.g. that the
unpalatable mimicked butterflies are left in relative peace while
similar palatable butterflies are persecuted. It is also necessary to
show that the mimickers do actually consort with the mimicked. Some
beetles and moths are curiously wasplike, which may be a great
advantage; the common drone-fly is superficially like a small bee; some
harmless snakes are very like poisonous species; and Mr. Wallace
maintained that the powerful "friar-birds" of the Far East are mimicked
by the weak and timid orioles. When the model is unpalatable or
repulsive or dangerous, and the mimic the reverse, the mimicry is called
"Batesian" (after Mr. Bates), but there is another kind of mimicry
called Müllerian (after Fritz Müller) where the mimic is also
unpalatable. The theory in this case is that the mimicry serves as
mutual assurance, the members of the ring getting on better by
consistently presenting the same appearance, which has come to mean to
possible enemies a signal, _Noli me tangere_ ("Leave me alone"). There
is nothing out of the question in this theory, but it requires to be
taken in a critical spirit. It leads us to think of "warning colours,"
which are the very opposite of the disguises which we are now studying.
Some creatures like skunks, magpies, coral-snakes, cobras, brightly
coloured tree-frogs are obtrusive rather than elusive, and the theory
of Alfred Russel Wallace was that the flaunting conspicuousness serves
as a useful advertisement, impressing itself on the memories of
inexperienced enemies, who soon learn to leave creatures with "warning
colours" alone. In any case it is plain that an animal which is as safe
as a wasp or a coral-snake can afford to wear any suit of clothes it
likes.
[Illustration: DEAD-LEAF BUTTERFLY (_Kallima Inachis_) FROM INDIA
It is conspicuous on its upper surface, but when it settles down on a
twig and shows the underside of its wings it is practically invisible.
The colouring of the under surface of the wings is like that of the
withering leaf; there are spots like fungas spots; and the venation of
the wings suggests the mid-rib and veins of the leaf. A, showing upper
surface; B, showing under surface; C, a leaf.]
[Illustration: PROTECTIVE RESEMBLANCE BETWEEN A SMALL SPIDER (_to the
left_) AND AN ANT (_to the right_)
As ants are much dreaded, it is probably profitable to the spider to be
like an ant. It will be noted that the spider has four pairs of legs and
no feelers, whereas the ant has three pairs of legs and a pair of
feelers.]
[Illustration: _Photo: J. J. Ward, F.E.S._
THE WASP BEETLE, WHICH, WHEN MOVING AMONGST THE BRANCHES GIVES A
WASP-LIKE IMPRESSION]
[Illustration: HERMIT-CRAB WITH PARTNER SEA-ANEMONES
Hermit-crabs hide their soft tail in the shell of a whelk or some other
sea-snail. But some hermit-crabs place sea-anemones on the back of their
borrowed shell. The sea-anemones mask the hermit-crab and their
tentacles can sting. As for the sea-anemones, they are carried about by
the hermit-crab and they get crumbs from its table. This kind of
mutually beneficial external partnership is called commensalism, i.e.
eating at the same table.]
[Illustration: _Photo: G. P. Duffus._
CUCKOO-SPIT
The white mass in the centre of the picture is a soapy froth which the
young frog-hopper makes, and within which it lies safe both from the
heat of the sun and almost all enemies. After sojourning for a time in
the cuckoo-spit, the frog-hopper becomes a winged insect.]
Masking
The episode in Scottish history called "The Walking Wood of Birnam,"
when the advancing troop masked their approach by cutting down branches
of the trees, has had its counterpart in many countries. But it is also
enacted on the seashore. There are many kinds of crabs that put on
disguise with what looks like deliberateness. The sand-crab takes a
piece of seaweed, nibbles at the end of it, and then rubs it on the back
of the carapace or on the legs so that it fixes to the bristles. As the
seaweed continues to live, the crab soon has a little garden on its back
which masks the crab's real nature. It is most effective camouflaging,
but if the crab continues to grow it has to moult, and that means losing
the disguise. It is then necessary to make a new one. The crab must have
on the shore something corresponding to a reputation; that is to say,
other animals are clearly or dimly aware that the crab is a voracious
and combative creature. How useful to the crab, then, to have its
appearance cloaked by a growth of innocent seaweed, or sponge, or
zoophyte. It will enable the creature to sneak upon its victims or to
escape the attention of its own enemies.
If a narrow-beaked crab is cleaned artificially it will proceed to
clothe itself again, the habit has become instinctive; and it must be
admitted that while a particular crab prefers a particular kind of
seaweed for its dress, it will cover itself with unsuitable and even
conspicuous material, such as pieces of coloured cloth, if nothing
better is available. The disguise differs greatly, for one crab is
masked by a brightly coloured and unpalatable sponge densely packed
with flinty needles; another cuts off the tunic of a sea-squirt and
throws it over its shoulders; another trundles about a bivalve shell.
The facts recall the familiar case of the hermit-crab, which protects
its soft tail by tucking it into the empty shell of a periwinkle or a
whelk or some other sea-snail, and that case leads on to the elaboration
known as commensalism, where the hermit-crab fixes sea-anemones on the
back of its borrowed house. The advantage here is beyond that of
masking, for the sea-anemone can sting, which is a useful quality in a
partner. That this second advantage may become the main one is evident
in several cases where the sea-anemone is borne, just like a weapon, on
each of the crustacean's great claws. Moreover, as the term commensalism
(eating at the same table) suggests, the partnership is _mutually_
beneficial. For the sea-anemone is carried about by the hermit-crab, and
it doubtless gets its share of crumbs from its partner's frequent meals.
There is a very interesting sidelight on the mutual benefit in the case
of a dislodged sea-anemone which sulked for a while and then waited in a
state of preparedness until a hermit-crab passed by and touched it.
Whereupon the sea-anemone gripped and slowly worked itself up on to the
back of the shell.
§ 6
Other Kinds of Elusiveness
There are various kinds of disguise which are not readily classified. A
troop of cuttlefish swimming in the sea is a beautiful sight. They keep
time with one another in their movements and they show the same change
of colour almost at the same moment. They are suddenly attacked,
however, by a small shark, and then comes a simultaneous discharge of
sepia from their ink-bags. There are clouds of ink in the clear water,
for, as Professor Hickson puts it, the cuttlefishes have thrown dust in
the eyes of their enemies. One can see a newborn cuttlefish do this a
minute after it escapes from the egg.
Very beautiful is the way in which many birds, like our common
chaffinch, disguise the outside of their nest with moss and lichen and
other trifles felted together, so that the cradle is as inconspicuous as
possible. There seems to be a touch of art in fastening pieces of
spider's web on the outside of a nest!
How curious is the case of the tree-sloth of South American forests,
that walks slowly, back downwards, along the undersides of the branches,
hanging on by its long, curved fingers and toes. It is a nocturnal
animal, and therefore not in special danger, but when resting during the
day it is almost invisible because its shaggy hair is so like certain
lichens and other growths on the branches. But the protective
resemblance is enhanced by the presence of a green alga, which actually
lives on the surface of the sloth's hairs--an alga like the one that
makes tree-stems and gate-posts green in damp weather.
There is no commoner sight in the early summer than the cuckoo-spit on
the grasses and herbage by the wayside. It is conspicuous and yet it is
said to be left severely alone by almost all creatures. In some way it
must be a disguise. It is a sort of soap made by the activity of small
frog-hoppers while they are still in the wingless larval stage, before
they begin to hop. The insect pierces with its sharp mouth-parts the
skin of the plant and sucks in sweet sap which by and by overflows over
its body. It works its body up and down many times, whipping in air,
which mixes with the sugary sap, reminding one of how "whipped egg" is
made. But along with the sugary sap and the air, there is a little
ferment from the food-canal and a little wax from glands on the skin,
and the four things mixed together make a kind of soap which lasts
through the heat of the day.
There are many other modes of disguise besides those which we have been
able to illustrate. Indeed, the biggest fact is that there are so many,
for it brings us back to the idea that life is not an easy business. It
is true, as Walt Whitman says, that animals do not sweat and whine about
their condition; perhaps it is true, as he says, that not one is
unhappy over the whole earth. But there is another truth, that this
world is not a place for the unlit lamp and the ungirt loin, and that
when a creature has not armour or weapons or cleverness it must find
some path of safety or go back. One of these paths of safety is
disguise, and we have illustrated its evolution.
V
THE ASCENT OF MAN
THE ASCENT OF MAN
§ 1
No one thinks less of Sir Isaac Newton because he was born as a very
puny infant, and no one should think less of the human race because it
sprang from a stock of arboreal mammals. There is no doubt as to man's
apartness from the rest of creation when he is seen at his best--"a
little lower than the angels, crowned with glory and honour." "What a
piece of work is a man! How noble in reason! How infinite in faculty! in
form and moving how express and admirable! in action how like an angel!
in apprehension so like a God." Nevertheless, all the facts point to his
affiliation to the stock to which monkeys and apes also belong. Not,
indeed, that man is descended from any living ape or monkey; it is
rather that he and they have sprung from a common ancestry--are branches
of the same stem. This conclusion is so momentous that the reasons for
accepting it must be carefully considered. They were expounded with
masterly skill in Darwin's _Descent of Man_ in 1871--a book which was
but an expansion of a chapter in _The Origin of Species_ (1859).
Anatomical Proof of Man's Relationship with a Simian Stock
The anatomical structure of man is closely similar to that of the
anthropoid apes--the gorilla, the orang, the chimpanzee, and the gibbon.
Bone for bone, muscle for muscle, blood-vessel for blood-vessel, nerve
for nerve, man and ape agree. As the conservative anatomist, Sir
Richard Owen, said, there is between them "an all-pervading similitude
of structure." Differences, of course, there are, but they are not
momentous except man's big brain, which may be three times as heavy as
that of a gorilla. The average human brain weighs about 48 ounces; the
gorilla brain does not exceed 20 ounces at its best. The capacity of the
human skull is never less than 55 cubic inches; in the orang and the
chimpanzee the figures are 26 and 27-1/2 respectively. We are not
suggesting that the most distinctive features of man are such as can be
measured and weighed, but it is important to notice that the main seat
of his mental powers is physically far ahead of that of the highest of
the anthropoid apes.
Man alone is thoroughly erect after his infancy is past; his head
weighted with the heavy brain does not droop forward as the ape's does;
with his erect attitude there is perhaps to be associated his more
highly developed vocal organs. Compared with an anthropoid ape, man has
a bigger and more upright forehead, a less protrusive face region,
smaller cheek-bones and eyebrow ridges, and more uniform teeth. He is
almost unique in having a chin. Man plants the sole of his foot flat on
the ground, his big toe is usually in a line with the other toes, and he
has a better heel than any monkey has. The change in the shape of the
head is to be thought of in connection with the enlargement of the
brain, and also in connection with the natural reduction of the muzzle
region when the hand was freed from being an organ of support and became
suited for grasping the food and conveying it to the mouth.
Everyone is familiar in man's clothing with traces of the past
persisting in the present, though their use has long since disappeared.
There are buttons on the back of the waist of the morning coat to which
the tails of the coat used to be fastened up, and there are buttons,
occasionally with buttonholes, at the wrist which were once useful in
turning up the sleeve. The same is true of man's body, which is a
veritable museum of relics. Some anatomists have made out a list of
over a hundred of these _vestigial_ structures, and though this number
is perhaps too high, there is no doubt that the list is long. In the
inner upper corner of the eye there is a minute tag--but larger in some
races than in others--which is the last dwindling relic of the third
eyelid, used in cleaning the front of the eye, which most mammals
possess in a large and well-developed form. It can be easily seen, for
instance, in ox and rabbit. In man and in monkeys it has become a
useless vestige, and the dwindling must be associated with the fact that
the upper eyelid is much more mobile in man and monkeys than in the
other mammals. The vestigial third eyelid in man is enough of itself to
prove his relationship with the mammals, but it is only one example out
of many. Some of these are discussed in the article dealing with the
human body, but we may mention the vestigial muscles going to the
ear-trumpet, man's dwindling counterpart of the skin-twitching muscle
which we see a horse use when he jerks a fly off his flanks, and the
short tail which in the seven-weeks-old human embryo is actually longer
than the leg. Without committing ourselves to a belief in the entire
uselessness of the vermiform appendix, which grows out as a blind alley
at the junction of the small intestine with the large, we are safe in
saying that it is a dwindling structure--the remains of a blind gut
which must have been capacious and useful in ancestral forms. In some
mammals, like the rabbit, the blind gut is the bulkiest structure in the
body, and bears the vermiform appendix at its far end. In man the
appendix alone is left, and it tells its tale. It is interesting to
notice that it is usually longer in the orang than in man, and that it
is very variable, as dwindling structures tend to be. One of the
unpleasant expressions of this variability is the liability to go wrong:
hence appendicitis. Now these vestigial structures are, as Darwin said,
like the unsounded, i.e. functionless, letters in words, such as the _o_
in "leopard," the _b_ in "doubt," the _g_ in "reign." They are of no
use, but they tell us something of the history of the words. So do man's
vestigial structures reveal his pedigree. They must have an historical
or evolutionary significance. No other interpretation is possible.
[Illustration: _Photo: New York Zoological Park._
CHIMPANZEE, SITTING
The head shows certain facial characteristics, e.g. the beetling eyebrow
ridges, which were marked in the Neanderthal race of men. Note the
shortening of the thumb and the enlargement of the big toe.]
[Illustration: _Photo: New York Zoological Park._
CHIMPANZEE, ILLUSTRATING WALKING POWERS
Note the great length of the arms and the relative shortness of the
legs.]
[Illustration: SURFACE VIEW OF THE BRAINS OF MAN (1) AND CHIMPANZEE (2)
The human brain is much larger and heavier, more dome-like, and with
much more numerous and complicated convolutions.]
[Illustration: _Photo: New York Zoological Park._
SIDE-VIEW OF CHIMPANZEE'S HEAD.
(Compare with opposite picture.)]
[Illustration: _After a model by J. H. McGregor._
PROFILE VIEW OF HEAD OF PITHECANTHROPUS, THE JAVA APE MAN, RECONSTRUCTED
FROM THE SKULL-CAP.]
[Illustration: THE FLIPPER OF A WHALE AND THE HAND OF A MAN
In the bones and in their arrangement there is a close resemblance in
the two cases, yet the outcome is very different. The multiplication of
finger joints in the whale is a striking feature.]
Some men, oftener than women, show on the inturned margin of the
ear-trumpet or pinna, a little conical projection of great interest. It
is a vestige of the tip of the pointed ear of lower mammals, and it is
well named _Darwin's point_. It was he who described it as a "surviving
symbol of the stirring times and dangerous days of man's animal youth."
§ 2
Physiological Proof of Man's Relationship with a Simian Stock
The everyday functions of the human body are practically the same as
those of the anthropoid ape, and similar disorders are common to both.
Monkeys may be infected with certain microbes to which man is peculiarly
liable, such as the bacillus of tuberculosis. Darwin showed that various
human gestures and facial expressions have their counterparts in
monkeys. The sneering curl of the upper lip, which tends to expose the
canine tooth, is a case in point, though it may be seen in many other
mammals besides monkeys--in dogs, for instance, which are at some
considerable distance from the simian branch to which man's ancestors
belonged.
When human blood is transfused into a dog or even a monkey, it behaves
in a hostile way to the other blood, bringing about a destruction of the
red blood corpuscles. But when it is transfused into a chimpanzee there
is an harmonious mingling of the two. This is a very literal
demonstration of man's blood-relationship with the higher apes. But
there is a finer form of the same experiment. When the blood-fluid (or
serum) of a rabbit, which has had human blood injected into it, is
mingled with human blood, it forms a cloudy precipitate. It forms almost
as marked a precipitate when it is mingled with the blood of an
anthropoid ape. But when it is mingled with the blood of an American
monkey there is only a slight clouding after a considerable time and
no actual precipitate. When it is added to the blood of one of the
distantly related "half-monkeys" or lemurs there is no reaction or only
a very weak one. With the blood of mammals off the simian line
altogether there is no reaction at all. Thus, as a distinguished
anthropologist, Professor Schwalbe, has said: "We have in this not only
a proof of the literal blood-relationship between man and apes, but the
degree of relationship with the different main groups of apes can be
determined beyond possibility of mistake." We can imagine how this
modern line of experiment would have delighted Darwin.
[Illustration: THE GORILLA, INHABITING THE FOREST TRACT OF THE GABOON IN
AFRICA
A full-grown individual stands about 5 feet high. The gait is shuffling,
the strength enormous, the diet mainly vegetarian, the temper rather
ferocious.]
Embryological Proof of Man's Relationship with a Simian Stock
In his individual development, man does in some measure climb up his own
genealogical tree. Stages in the development of the body during its nine
months of ante-natal life are closely similar to stages in the
development of the anthropoid embryo. Babies born in times of famine or
siege are sometimes, as it were, imperfectly finished, and sometimes
have what may be described as monkeyish features and ways. A visit to an
institution for the care of children who show arrested, defective, or
disturbed development leaves one sadly impressed with the risk of
slipping down the rungs of the steep ladder of evolution; and even in
adults the occurrence of serious nervous disturbance, such as
"shell-shock," is sometimes marked by relapses to animal ways. It is a
familiar fact that a normal baby reveals the past in its surprising
power of grip, and the careful experiments of Dr. Louis Robinson showed
that an infant three weeks old could support its own weight for over two
minutes, holding on to a horizontal bar. "In many cases no sign of
distress is evinced and no cry uttered, until the grasp begins to give
way." This persistent grasp probably points back to the time when the
baby had to cling to its arboreal mother. The human tail is represented
in the adult by a fusion of four or five vertebræ forming the "coccyx"
at the end of the backbone, and is normally concealed beneath the
flesh, but in the embryo the tail projects freely and is movable. Up to
the sixth month of the ante-natal sleep the body is covered, all but the
palms and soles, with longish hair (the lanugo), which usually
disappears before birth. This is a stage in the normal development,
which is reasonably interpreted as a recapitulation of a stage in the
racial evolution. We draw this inference when we find that the unborn
offspring of an almost hairless whale has an abundant representation of
hairs; we must draw a similar inference in the case of man.
It must be noticed that there are two serious errors in the careless
statement often made that man in his development is at one time like a
little fish, at a later stage like a little reptile, at a later stage
like a little primitive mammal, and eventually like a little monkey. The
first error here is that the comparison should be made with
_embryo_-fish, _embryo_-reptile, _embryo_-mammal, and so on. It is in
the making of the embryos that the great resemblance lies. When the
human embryo shows the laying down of the essential vertebrate
characters, such as brain and spinal cord, then it is closely comparable
to the embryo of a lower vertebrate at a similar stage. When, at a
subsequent stage, its heart, for instance, is about to become a
four-chambered mammalian heart, it is closely comparable to the heart
of, let us say, a turtle, which never becomes more than three-chambered.
The point is that in the making of the organs of the body, say brain and
kidneys, the embryo of man pursues a path closely corresponding to the
path followed by the embryos of other backboned animals lower in the
scale, but at successive stages it parts company with these, with the
lowest first and so on in succession. A human embryo is never like a
little reptile, but the developing organs pass through stages which very
closely resemble the corresponding stages in lower types which are in a
general way ancestral.
The second error is that every kind of animal, man included, has from
the first a certain individuality, with peculiar characteristics which
are all its own. This is expressed by the somewhat difficult word
_specificity_, which just means that every species is itself and no
other. So in the development of the human embryo, while there are close
resemblances to the embryos of apes, monkeys, other mammals, and even,
at earlier stages still, to the embryos of reptile and fish, it has to
be admitted that we are dealing from first to last with a human embryo
with peculiarities of its own.
[Illustration: "DARWIN'S POINT" ON HUMAN EAR (MARKED D.P.)
It corresponds to the tip (T) of the ear of an ordinary mammal, as shown
in the hare's ear below. In the young orang the part corresponding to
Darwin's point is still at the tip of the ear.]
[Illustration: _Photo: J. Russell & Sons._
PROFESSOR SIR ARTHUR KEITH, M.D., LL.D., F.R.S.
Conservator of the Museum and Hunterian Professor, Royal College of
Surgeons of England. One of the foremost living anthropologists and a
leading authority on the antiquity of man.]
[Illustration: _After T. H. Huxley (by permission of Messrs.
Macmillan)._
SKELETONS OF THE GIBBON, ORANG, CHIMPANZEE, GORILLA, MAN
Photographically reduced from diagrams of the natural size (except that
of the gibbon, which was twice as large as nature) drawn by Mr.
Waterhouse Hawkins from specimens in the Museum of the Royal College of
Surgeons.]
Every human being begins his or her life as a single cell--a fertilised
egg-cell, a treasure-house of all the ages. For in this living
microcosm, only a small fraction (1/125) of an inch in diameter, there
is condensed--who can imagine how?--all the natural inheritance of man,
all the legacy of his parentage, of his ancestry, of his long pre-human
pedigree. Darwin called the pinhead brain of the ant the most marvellous
atom of matter in the world, but the human ovum is more marvellous
still. It has more possibilities in it than any other thing, yet without
fertilisation it will die. The fertilised ovum divides and redivides;
there results a ball of cells and a sack of cells; gradually division of
labour becomes the rule; there is a laying down of nervous system and
food-canal, muscular system and skeleton, and so proceeds what is
learnedly called differentiation. Out of the apparently simple there
emerges the obviously complex. As Aristotle observed more than two
thousand years ago, in the developing egg of the hen there soon appears
the beating heart! There is nothing like this in the non-living world.
But to return to the developing human embryo, there is formed from and
above the embryonic food-canal a skeletal rod, which is called the
notochord. It thrills the imagination to learn that this is the only
supporting axis that the lower orders of the backboned race possess. The
curious thing is that it does not become the backbone, which is
certainly one of the essential features of the vertebrate race. The
notochord is the supporting axis of the pioneer backboned animals,
namely the Lancelets and the Round-mouths (Cyclostomes), such as the
Lamprey. They have no backbone in the strict sense, but they have this
notochord. It can easily be dissected out in the lamprey--a long gristly
rod. It is surrounded by a sheath which becomes the backbone of most
fishes and of all higher animals. The interesting point is that although
the notochord is only a vestige in the adults of these types, it is
never absent from the embryo. It occurs even in man, a short-lived relic
of the primeval supporting axis of the body. It comes and then it goes,
leaving only minute traces in the adult. We cannot say that it is of any
use, unless it serves as a stimulus to the development of its
substitute, the backbone. It is only a piece of preliminary scaffolding,
but there is no more eloquent instance of the living hand of the past.
One other instance must suffice of what Professor Lull calls the
wonderful changes wrought in the dark of the ante-natal period, which
recapitulate in rapid abbreviation the great evolutionary steps which
were taken by man's ancestors "during the long night of the geological
past." On the sides of the neck of the human embryo there are four pairs
of slits, the "visceral clefts," openings from the beginning of the
food-canals to the surface. There is no doubt as to their significance.
They correspond to the gill-slits of fishes and tadpoles. Yet in
reptiles, birds, and mammals they have no connection with breathing,
which is their function in fishes and amphibians. Indeed, they are not
of any use at all, except that the first becomes the Eustachian tube
bringing the ear-passage into connection with the back of the mouth, and
that the second and third have to do with the development of a curious
organ called the thymus gland. Persistent, nevertheless, these
gill-slits are, recalling even in man an aquatic ancestry of many
millions of years ago.
When all these lines of evidence are considered, they are seen to
converge in the conclusion that man is derived from a simian stock of
mammals. He is solidary with the rest of creation. To quote the closing
words of Darwin's _Descent of Man_:
We must, however, acknowledge, as it seems to me, that man with all
his noble qualities, with sympathy which feels for the most debased,
with benevolence which extends not only to other men but to the
humblest living creature, with his God-like intellect, which has
penetrated into the movements and constitution of the solar
system--with all these exalted powers--man still bears in his bodily
frame the indelible stamp of his lowly origin.
We should be clear that this view does not say more than that man sprang
from a stock common to him and to the higher apes. Those who are
repelled by the idea of man's derivation from a simian type should
remember that the theory implies rather more than this, namely, that man
is the outcome of a genealogy which has implied many millions of years
of experimenting and sifting--the groaning and travailing of a whole
creation. Speaking of man's mental qualities, Sir Ray Lankester says:
"They justify the view that man forms a new departure in the gradual
unfolding of Nature's predestined plan." In any case, we have to try to
square our views with the facts, not the facts with our views, and while
one of the facts is that man stands unique and apart, the other is that
man is a scion of a progressive simian stock. Naturalists have exposed
the pit whence man has been digged and the rock whence he has been hewn,
but it is surely a heartening encouragement to know that it is an
ascent, not a descent, that we have behind us. There is wisdom in
Pascal's maxim:
It is dangerous to show man too plainly how like he is to the
animals, without, at the same time, reminding him of his greatness.
It is equally unwise to impress him with his greatness and not with
his lowliness. It is worse to leave him in ignorance of both. But it
is very profitable to recognise the two facts.
§ 3
Man's Pedigree
The facts of anatomy, physiology, and embryology, of which we have given
illustrations, all point to man's affiliation with the order of monkeys
and apes. To this order is given the name Primates, and our first and
second question must be when and whence the Primates began. The rock
record answers the first question: the Primates emerged about the dawn
of the Eocene era, when grass was beginning to cover the earth with a
garment. Their ancestral home was in the north in both hemispheres, and
then they migrated to Africa, India, Malay, and South America. In North
America the Primates soon became extinct, and the same thing happened
later on in Europe. In this case, however, there was a repeopling from
the South (in the Lower Miocene) and then a second extinction (in the
Upper Pliocene) before man appeared. There is considerable evidence in
support of Professor R. S. Lull's conclusion, that in Southern Asia,
Africa, and South America the evolution of Primates was continuous since
the first great southward migration, and there is, of course, an
abundant modern representation of Primates in these regions to-day.
As to the second question: Whence the Primates sprang, the answer must
be more conjectural. But it is a reasonable view that Carnivores and
Primates sprang from a common Insectivore stock, the one order diverging
towards flesh-eating and hunting on the ground, the other order
diverging towards fruit-eating and arboreal habits. There is no doubt
that the Insectivores (including shrews, tree-shrews, hedgehog, mole,
and the like) were very plastic and progressive mammals.
What followed in the course of ages was the divergence of branch after
branch from the main Primate stem. First there diverged the South
American monkeys on a line of their own, and then the Old World monkeys,
such as the macaques and baboons. Ages passed and the main stems gave
off (in the Oligocene period) the branch now represented by the small
anthropoid apes--the gibbon and the siamang. Distinctly later there
diverged the branch of the large anthropoid apes--the gorilla, the
chimpanzee, and the orang. That left a generalised humanoid stock
separated off from all monkeys and apes, and including the immediate
precursors of man. When this sifting out of a generalised humanoid stock
took place remains very uncertain, some authorities referring it to the
Miocene, others to the early Pliocene. Some would estimate its date at
half a million years ago, others at two millions! The fact is that
questions of chronology do not as yet admit of scientific statement.
[Illustration: SIDE-VIEW OF SKULL OF MAN (M) AND GORILLA (G)
Notice in the gorilla's skull the protrusive face region, the big
eyebrow ridges, the much less domed cranial cavity, the massive lower
jaw, the big canine teeth. Notice in man's skull the well-developed
forehead, the domed and spacious cranial cavity, the absence of any
snout, the chin process, and many other marked differences separating
the human skull from the ape's.]
[Illustration: THE SKULL AND BRAIN-CASE OF PITHECANTHROPUS, THE JAVA
APE-MAN, AS RESTORED. BY J. H. McGREGOR FROM THE SCANTY REMAINS
The restoration shows the low, retreating forehead and the prominent
eyebrow ridges.]
[Illustration: SUGGESTED GENEALOGICAL TREE OF MAN AND ANTHROPOID APES
From Sir Arthur Keith; the lettering to the right has been slightly
simplified.]
We are on firmer, though still uncertain, ground when we state the
probability that it was in Asia that the precursors of man were
separated off from monkeys and apes, and began to be terrestrial rather
than arboreal. Professor Lull points out that Asia is nearest to the
oldest known human remains (in Java), and that Asia was the seat of the
most ancient civilisations and the original home of many domesticated
animals and cultivated plants. The probability is that the cradle of the
human race was in Asia.
Man's Arboreal Apprenticeship
At this point it will be useful to consider man's arboreal
apprenticeship and how he became a terrestrial journeyman. Professor
Wood Jones has worked out very convincingly the thesis that man had no
direct four-footed ancestry, but that the Primate stock to which he
belongs was from its first divergence arboreal. He maintains that the
leading peculiarities of the immediate precursors of man were wrought
out during a long arboreal apprenticeship. The first great gain of
arboreal life on bipedal erect lines (not after the quadrupedal fashion
of tree-sloths, for instance) was the emancipation of the hand. The
foot became the supporting and branch-gripping member, and the hand was
set free to reach upward, to hang on by, to seize the fruit, to lift it
and hold it to the mouth, and to hug the young one close to the breast.
The hand thus set free has remained plastic--a generalised, not a
specialised member. Much has followed from man's "handiness."
The arboreal life had many other consequences. It led to an increased
freedom of movement of the thigh on the hip joint, to muscular
arrangements for balancing the body on the leg, to making the backbone a
supple yet stable curved pillar, to a strongly developed collar-bone
which is only found well-formed when the fore-limb is used for more than
support, and to a power of "opposing" the thumb and the big toe to the
other digits of the hand and foot--an obvious advantage for
branch-gripping. But the evolution of a free hand made it possible to
dispense with protrusive lips and gripping teeth. Thus began the
recession of the snout region, the associated enlargement of the
brain-box, and the bringing of the eyes to the front. The overcrowding
of the teeth that followed the shortening of the snout was one of the
taxes on progress of which modern man is often reminded in his dental
troubles.
Another acquisition associated with arboreal life was a greatly
increased power of turning the head from side to side--a mobility very
important in locating sounds and in exploring with the eyes.
Furthermore, there came about a flattening of the chest and of the back,
and the movements of the midriff (or diaphragm) came to count for more
in respiration than the movements of the ribs. The sense of touch came
to be of more importance and the sense of smell of less; the part of the
brain receiving tidings from hand and eye and ear came to predominate
over the part for receiving olfactory messages. Finally, the need for
carrying the infant about among the branches must surely have implied an
intensification of family relations, and favoured the evolution of
gentleness.
[Illustration: _Photo: New York Zoological Park._
THE GIBBON IS LOWER THAN THE OTHER APES AS REGARDS ITS SKULL AND
DENTITION, BUT IT IS HIGHLY SPECIALIZED IN THE ADAPTATION OF ITS LIMBS
TO ARBOREAL LIFE]
[Illustration: _Photo: New York Zoological Park._
THE ORANG HAS A HIGH ROUNDED SKULL AND A LONG FACE]
[Illustration: _Photo: British Museum (Natural History)._
COMPARISONS OF THE SKELETONS OF HORSE AND MAN
Bone for bone, the two skeletons are like one another, though man is a
biped and the horse a quadruped. The backbone in man is mainly vertical;
the backbone in the horse is horizontal except in the neck and the tail.
Man's skull is mainly in a line with the backbone; the horse's at an
angle to it. Both man and horse have seven neck vertebræ. Man has five
digits on each limb; the horse has only one digit well developed on each
limb.]
It may be urged that we are attaching too much importance to the
arboreal apprenticeship, since many tree-loving animals remain to-day
very innocent creatures. To this reasonable objection there are two
answers, first that in its many acquisitions the arboreal evolution of
the _humanoid_ precursors of man prepared the way for the survival of a
_human_ type marked by a great step in brain-development; and second
that the passage from the humanoid to the human was probably associated
with _a return to mother earth_.
According to Professor Lull, to whose fine textbook, _Organic Evolution_
(1917), we are much indebted, "climatic conditions in Asia in the
Miocene or early Pliocene were such as to compel the descent of the
pre-human ancestor from the trees, a step which was absolutely essential
to further human development." Continental elevation and consequent
aridity led to a dwindling of the forests, and forced the ape-man to
come to earth. "And at the last arose the man."
According to Lull, the descent from the trees was associated with the
assumption of a more erect posture, with increased liberation and
plasticity of the hand, with becoming a hunter, with experiments towards
clothing and shelter, with an exploring habit, and with the beginning of
communal life.
It is a plausible view that the transition from the humanoid to the
human was effected by a discontinuous variation of considerable
magnitude, what is nowadays called a _mutation_, and that it had mainly
to do with the brain and the vocal organs. But given the gains of the
arboreal apprenticeship, the stimulus of an enforced descent to terra
firma, and an evolving brain and voice, we can recognise accessory
factors which helped success to succeed. Perhaps the absence of great
physical strength prompted reliance on wits; the prolongation of infancy
would help to educate the parents in gentleness; the strengthening of
the feeling of kinship would favour the evolution of family and social
life--of which there are many anticipations at lower levels. There is
much truth in the saying: "Man did not make society, society made man."
A continuation of the story will deal with the emergence of the
primitive types of man and the gradual ascent of the modern species.
§ 4
Tentative Men
So far the story has been that of the sifting out of a humanoid stock
and of the transition to human kind, from the ancestors of apes and men
to the man-ape, and from the man-ape to man. It looks as if the
sifting-out process had proceeded further, for there were several human
branches that did not lead on to the modern type of man.
1. The first of these is represented by the scanty fossil remains known
as _Pithecanthropus erectus_, found in Java in fossiliferous beds which
date from the end of the Pliocene or the beginning of the Pleistocene
era. Perhaps this means half a million years ago, and the remains
occurred along with those of some mammals which are now extinct.
Unfortunately the remains of Pithecanthropus the Erect consisted only of
a skull-cap, a thigh-bone, and two back teeth, so it is not surprising
that experts should differ considerably in their interpretation of what
was found. Some have regarded the remains as those of a large gibbon,
others as those of a pre-human ape-man, and others as those of a
primitive man off the main line of ascent. According to Sir Arthur
Keith, Pithecanthropus was "a being human in stature, human in gait,
human in all its parts, save its brain." The thigh-bone indicates a
height of about 5 feet 7 inches, one inch less than the average height
of the men of to-day. The skull-cap indicates a low, flat forehead,
beetling brows, and a capacity about two-thirds of the modern size. The
remains were found by Dubois, in 1894, in Trinil in Central Java.
2. The next offshoot is represented by the Heidelberg man (_Homo
heidelbergensis_), discovered near Heidelberg in 1907 by Dr.
Schoetensack. But the remains consisted only of a lower jaw and its
teeth. Along with this relic were bones of various mammals, including
some long since extinct in Europe, such as elephant, rhinoceros, bison,
and lion. The circumstances indicate an age of perhaps 300,000 years
ago. There were also very crude flint implements (or eoliths). But the
teeth are human teeth, and the jaw seems transitional between that of an
anthropoid ape and that of man. Thus there was no chin. According to
most authorities the lower jaw from the Heidelberg sand-pit must be
regarded as a relic of a primitive type off the main line of human
ascent.
[Illustration: A RECONSTRUCTION OF THE JAVA MAN
(_Pithecanthropus erectus._)]
3. It was in all probability in the Pliocene that there took origin the
Neanderthal species of man, _Homo neanderthalensis_, first known from
remains found in 1856 in the Neanderthal ravine near Düsseldorf.
According to some authorities Neanderthal man was living in Europe a
quarter of a million years ago. Other specimens were afterwards found
elsewhere, e.g. in Belgium ("the men of Spy"), in France, in Croatia,
and at Gibraltar, so that a good deal is known of Neanderthal man. He
was a loose-limbed fellow, short of stature and of slouching gait, but a
skilful artificer, fashioning beautifully worked flints with a
characteristic style. He used fire; he buried his dead reverently and
furnished them with an outfit for a long journey; and he had a big
brain. But he had great beetling, ape-like eyebrow ridges and massive
jaws, and he showed "simian characters swarming in the details of his
structure." In most of the points in which he differs from modern man he
approaches the anthropoid apes, and he must be regarded as a low type of
man off the main line. Huxley regarded the Neanderthal man as a low form
of the modern type, but expert opinion seems to agree rather with the
view maintained in 1864 by Professor William King of Galway, that the
Neanderthal man represents a distinct species off the main line of
ascent. He disappeared with apparent suddenness (like some aboriginal
races to-day) about the end of the Fourth Great Ice Age; but there is
evidence that before he ceased to be there had emerged a successor
rather than a descendant--the modern man.
4. Another offshoot from the main line is probably represented by the
Piltdown man, found in Sussex in 1912. The remains consisted of the
walls of the skull, which indicate a large brain, and a high forehead
without the beetling eyebrows of the Neanderthal man and
Pithecanthropus. The "find" included a tooth and part of a lower jaw,
but these perhaps belong to some ape, for they are very discrepant. The
Piltdown skull represents the most ancient human remains as yet found in
Britain, and Dr. Smith Woodward's establishment of a separate genus
Eoanthropus expresses his conviction that the Piltdown man was off the
line of the evolution of the modern type. If the tooth and piece of
lower jaw belong to the Piltdown skull, then there was a remarkable
combination of ape-like and human characters. As regards the brain,
_inferred_ from the skull-walls, Sir Arthur Keith says:
All the essential features of the brain of modern man are to be seen
in the brain cast. There are some which must be regarded as
primitive. There can be no doubt that it is built on exactly the
same lines as our modern brains. A few minor alterations would make
it in all respects a modern brain.... Although our knowledge of the
human brain is limited--there are large areas to which we can assign
no definite function--we may rest assured that a brain which was
shaped in a mould so similar to our own was one which responded to
the outside world as ours does. Piltdown man saw, heard, felt,
thought, and dreamt much as we do still.
And this was 150,000 years ago at a modern estimate, and some would say
half a million.
There is neither agreement nor certainty as to the antiquity of man,
except that the modern type was distinguishable from its collaterals
hundreds of thousands of years ago. The general impression left is very
grand. In remote antiquity the Primate stem diverged from the other
orders of mammals; it sent forth its tentative branches, and the result
was a tangle of monkeys; ages passed and the monkeys were left behind,
while the main stem, still probing its way, gave off the Anthropoid
apes, both small and large. But they too were left behind, and the main
line gave off other experiments--indications of which we know in Java,
at Heidelberg, in the Neanderthal, and at Piltdown. None of these lasted
or was made perfect. They represent _tentative_ men who had their day
and ceased to be, our predecessors rather than our ancestors. Still, the
main stem goes on evolving, and who will be bold enough to say what
fruit it has yet to bear!
[Illustration: _After a model by J. H. McGregor._
PROFILE VIEW OF THE HEAD OF PITHECANTHROPUS, THE JAVA APE-MAN--AN EARLY
OFFSHOOT FROM THE MAIN LINE OF MAN'S ASCENT
The animal remains found along with the skull-cap, thigh-bone, and two
teeth of Pithecanthropus seem to indicate the lowest Pleistocene period,
perhaps 500,000 years ago.]
[Illustration: _From the reconstruction by J. H. McGregor._
PILTDOWN SKULL. THE DARK PARTS ONLY ARE PRESERVED, NAMELY PORTIONS OF
THE CRANIAL WALLS AND THE NASAL BONES
Some authorities include a canine tooth and part of the lower jaw which
were found close by. The remains were found in 1912 in Thames gravels in
Sussex, and are usually regarded as vastly more ancient than those of
Neanderthal Man. It has been suggested that Piltdown Man lived 100,000
to 150,000 years ago, in the Third Interglacial period.]
[Illustration: _Reproduced by permission from Osborn's "Men of the Old
Stone Age."_
SAND-PIT AT MAUER, NEAR HEIDELBERG: DISCOVERY SITE OF THE JAW OF
HEIDELBERG MAN
_a-b._ "Newer loess," either of Third Interglacial or of Postglacial
times.
_b-c._ "Older loess" (sandy loess), of the close of Second Interglacial
times.
_c-f._ The "sands of Mauer."
_d-e._ An intermediate layer of clay.
The white cross (X) indicates the spot at the base of the "sands of
Mauer" at which the jaw of Heidelberg was discovered.]
Primitive Men
Ancient skeletons of men of the modern type have been found in many
places, e.g. Combe Capelle in Dordogne, Galley Hill in Kent, Cro-Magnon
in Périgord, Mentone on the Riviera; and they are often referred to as
"Cave-men" or "men of the Early Stone Age." They had large skulls, high
foreheads, well-marked chins, and other features such as modern man
possesses. They were true men at last--that is to say, like ourselves!
The spirited pictures they made on the walls of caves in France and
Spain show artistic sense and skill. Well-finished statuettes
representing nude female figures are also known. The elaborate burial
customs point to a belief in life after death. They made stone
implements--knives, scrapers, gravers, and the like, of the type known
as Palæolithic, and these show interesting gradations of skill and
peculiarities of style. The "Cave-men" lived between the third and
fourth Ice Ages, along with cave-bear, cave-lion, cave-hyæna, mammoth,
woolly rhinoceros, Irish elk, and other mammals now extinct--taking us
back to 30,000-50,000 years ago, and many would say much more. Some of
the big-brained skulls of these Palæolithic cave-men show not a single
feature that could be called primitive. They show teeth which in size
and form are exactly the same as those of a thousand generations
afterwards--and suffering from gumboil too! There seems little doubt
that these vigorous Palæolithic Cave-men of Europe were living for a
while contemporaneously with the men of Neanderthal, and it is possible
that they directly or indirectly hastened the disappearance of their
more primitive collaterals. Curiously enough, however, they had not
themselves adequate lasting power in Europe, for they seem for the most
part to have dwindled away, leaving perhaps stray present-day survivors
in isolated districts. The probability is that after their decline
Europe was repeopled by immigrants from Asia. It cannot be said that
there is any inherent biological necessity for the decline of a vigorous
race--many animal races go back for millions of years--but in mankind
the historical fact is that a period of great racial vigour and success
is often followed by a period of decline, sometimes leading to practical
disappearance as a definite race. The causes of this waning remain very
obscure--sometimes environmental, sometimes constitutional, sometimes
competitive. Sometimes the introduction of a new parasite, like the
malaria organism, may have been to blame.
After the Ice Ages had passed, perhaps 25,000 years ago, the Palæolithic
culture gave place to the Neolithic. The men who made rudely dressed but
often beautiful stone implements were succeeded or replaced by men who
made polished stone implements. The earliest inhabitants of Scotland
were of this Neolithic culture, migrating from the Continent when the
ice-fields of the Great Glaciation had disappeared. Their remains are
often associated with the "Fifty-foot Beach" which, though now high and
dry, was the seashore in early Neolithic days. Much is known about these
men of the polished stones. They were hunters, fowlers, and fishermen;
without domesticated animals or agriculture; short folk, two or three
inches below the present standard; living an active strenuous life.
Similarly, for the south, Sir Arthur Keith pictures for us a Neolithic
community at Coldrum in Kent, dating from about 4,000 years ago--a few
ticks of the geological clock. It consisted, in this case, of
agricultural pioneers, men with large heads and big brains, about two
inches shorter in stature than the modern British average (5 ft. 8 in.),
with better teeth and broader palates than men have in these days of
soft food, with beliefs concerning life and death similar to those that
swayed their contemporaries in Western and Southern Europe. Very
interesting is the manipulative skill they showed on a large scale in
erecting standing stones (probably connected with calendar-keeping and
with worship), and on a small scale in making daring operations on the
skull. Four thousand years ago is given as a probable date for that
early community in Kent, but evidences of Neolithic man occur in
situations which demand a much greater antiquity--perhaps 30,000 years.
And man was not young then!
[Illustration: PAINTINGS ON THE ROOF OF THE ALTAMIRA CAVE IN NORTHERN
SPAIN, SHOWING A BISON ABOVE AND A GALLOPING BOAR BELOW
The artistic drawings, over 2 feet in length, were made by the Reindeer
Men or "Cromagnards" in the time of the Upper or Post-Glacial
Pleistocene, before the appearance of the Neolithic men.]
We must open one more chapter in the thrilling story of the Ascent of
Man--the Metal Ages, which are in a sense still continuing. Metals began
to be used in the late Polished Stone (Neolithic) times, for there were
always overlappings. Copper came first, Bronze second, and Iron last.
The working of copper in the East has been traced back to the fourth
millennium B.C., and there was also a very ancient Copper Age in the New
World. It need hardly be said that where copper is scarce, as in
Britain, we cannot expect to find much trace of a Copper Age.
The ores of different metals seem to have been smelted together in an
experimental way by many prehistoric metallurgists, and bronze was the
alloy that rewarded the combination of tin with copper. There is
evidence of a more or less definite Bronze Age in Egypt and Babylonia,
Greece and Europe.
It is not clear why iron should not have been the earliest metal to be
used by man, but the Iron Age dates from about the middle of the second
millennium B.C. From Egypt the usage spread through the Mediterranean
region to North Europe, or it may have been that discoveries made in
Central Europe, so rich in iron-mines, saturated southwards, following
for instance, the route of the amber trade from the Baltic. Compared
with stone, the metals afforded much greater possibilities of
implements, instruments, and weapons, and their discovery and usage had
undoubtedly great influence on the Ascent of Man. Occasionally, however,
on his descent.
Retrospect
Looking backwards, we discern the following stages: (1) The setting
apart of a Primate stock, marked off from other mammals by a tendency to
big brains, a free hand, gregariousness, and good-humoured
talkativeness. (2) The divergence of marmosets and New World monkeys and
Old World monkeys, leaving a stock--an anthropoid stock--common to the
present-day and extinct apes and to mankind. (3) From this common stock
the Anthropoid apes diverged, far from ignoble creatures, and a humanoid
stock was set apart. (4) From the latter (we follow Sir Arthur Keith and
other authorities) there arose what may be called, without
disparagement, tentative or experimental men, indicated by
Pithecanthropus "the Erect," the Heidelberg man, the Neanderthalers,
and, best of all, the early men of the Sussex Weald--hinted at by the
Piltdown skull. It matters little whether particular items are
corroborated or disproved--e.g. whether the Heidelberg man came before
or after the Neanderthalers--the general trend of evolution remains
clear. (5) In any case, the result was the evolution of _Homo sapiens,
the man we are_--a quite different fellow from the Neanderthaler. (6)
Then arose various stocks of primitive men, proving everything and
holding fast to that which is good. There were the Palæolithic peoples,
with rude stone implements, a strong vigorous race, but probably, in
most cases, supplanted by fresh experiments. These may have arisen as
shoots from the growing point of the old race, or as a fresh offshoot
from more generalised members at a lower level. This is the eternal
possible victory alike of aristocracy and democracy. (7) Palæolithic men
were involved in the succession of four Great Ice Ages or
Glaciations, and it may be that the human race owes much to the
alternation of hard times and easy times--glacial and interglacial. When
the ice-fields cleared off Neolithic man had his innings. (8) And we
have closed the story, in the meantime, with the Metal Ages.
[Illustration: _After the restoration modelled by J. H. McGregor._
PILTDOWN MAN, PRECEDING NEANDERTHAL MAN, PERHAPS 100,000 TO 150,000
YEARS AGO]
[Illustration: _After the restoration modelled by J. H. McGregor._
THE NEANDERTHAL MAN OF LA CHAPELLE-AUX-SAINTS
The men of this race lived in Europe from the Third Interglacial period
through the Fourth Glacial. They disappeared somewhat suddenly, being
replaced by the Modern Man type, such as the Cromagnards. Many regard
the Neanderthal Men as a distinct species.]
It seems not unfitting that we should at this point sound another
note--that of the man of feeling. It is clear in William James's words:
Bone of our bone, and flesh of our flesh, are these half-brutish
prehistoric brothers. Girdled about with the immense darkness of
this mysterious universe even as we are, they were born and died,
suffered and struggled. Given over to fearful crime and passion,
plunged in the blackest ignorance, preyed upon by hideous and
grotesque delusions, yet steadfastly serving the profoundest of
ideals in their fixed faith that existence in any form is better
than non-existence, they ever rescued triumphantly from the jaws of
ever imminent destruction the torch of life which, thanks to them,
now lights the world for us.
Races of Mankind
Given a variable stock spreading over diverse territory, we expect to
find it splitting up into varieties which may become steadied into races
or incipient species. Thus we have races of hive-bees, "Italians,"
"Punics," and so forth; and thus there arose races of men. Certain types
suited certain areas, and periods of in-breeding tended to make the
distinctive peculiarities of each incipient race well-defined and
stable. When the original peculiarities, say, of negro and Mongol,
Australian and Caucasian, arose as brusque variations or "mutations,"
then they would have great staying power from generation to generation.
They would not be readily swamped by intercrossing or averaged off.
Peculiarities and changes of climate and surroundings, not to speak of
other change-producing factors, would provoke new departures from age to
age, and so fresh racial ventures were made. Moreover, the occurrence
of out-breeding when two races met, in peace or in war, would certainly
serve to induce fresh starts. Very important in the evolution of human
races must have been the alternating occurrence of periods of
in-breeding (endogamy), tending to stability and sameness, and periods
of out-breeding (exogamy), tending to changefulness and diversity.
Thus we may distinguish several more or less clearly defined primitive
races of mankind--notably the African, the Australian, the Mongolian,
and the Caucasian. The woolly-haired African race includes the negroes
and the very primitive bushmen. The wavy-to curly-haired Australian race
includes the Jungle Tribes of the Deccan, the Vedda of Ceylon, the
Jungle Folk or Semang, and the natives of unsettled parts of
Australia--all sometimes slumped together as "Pre-Dravidians." The
straight-haired Mongols include those of Tibet, Indo-China, China, and
Formosa, those of many oceanic islands, and of the north from Japan to
Lapland. The Caucasians include Mediterraneans, Semites, Nordics,
Afghans, Alpines, and many more.
There are very few corners of knowledge more difficult than that of the
Races of Men, the chief reason being that there has been so much
movement and migration in the course of the ages. One physical type has
mingled with another, inducing strange amalgams and novelties. If we
start with what might be called "zoological" races or strains differing,
for instance, in their hair (woolly-haired Africans, straight-haired
Mongols, curly-or wavy-haired Pre-Dravidians and Caucasians), we find
these replaced by _peoples_ who are mixtures of various races, "brethren
by civilisation more than by blood." As Professor Flinders Petrie has
said, the only meaning the term "race" now can have is that of a group
of human beings whose type has been unified by their rate of
assimilation exceeding the rate of change produced by the infiltration
of foreign elements. It is probable, however, that the progress of
precise anthropology will make it possible to distinguish the various
racial "strains" that make up any people. For the human sense of race
is so strong that it convinces us of reality even when scientific
definition is impossible. It was this the British sailor expressed in
his answer to the question "What is a Dago?" "Dagoes," he replied, "is
anything wot isn't our sort of chaps."
[Illustration: RESTORATION BY A. FORESTIER OF THE RHODESIAN MAN WHOSE
SKULL WAS DISCOVERED IN 1921
Attention may be drawn to the beetling eyebrow ridges, the projecting
upper lip, the large eye-sockets, the well-poised head, the strong
shoulders.
The squatting figure is crushing seeds with a stone, and a crusher is
lying on the rock to his right.]
[Illustration: RESTORATION BY A. FORESTIER OF THE RHODESIAN MAN WHOSE
SKULL WAS DISCOVERED IN 1921
The figure in the foreground, holding a staff, shows the erect attitude
and the straight legs. His left hand holds a flint implement.
On the left, behind the sitting figure, is seen the entrance to the
cave. This new Rhodesian cave-man may be regarded as a southern
representative of a Neanderthal race, or as an extinct type intermediate
between the Neanderthal Men and the Modern Man type.]
Steps in Human Evolution
Real men arose, we believe, by variational uplifts of considerable
magnitude which led to big and complex brains and to the power of
reasoned discourse. In some other lines of mammalian evolution there
were from time to time great advances in the size and complexity of the
brain, as is clear, for instance, in the case of horses and elephants.
The same is true of birds as compared with reptiles, and everyone
recognises the high level of excellence that has been attained by their
vocal powers. How these great cerebral advances came about we do not
know, but it has been one of the main trends of animal evolution to
improve the nervous system. Two suggestions may be made. First, the
prolongation of the period of ante-natal life, in intimate physiological
partnership with the mother, may have made it practicable to start the
higher mammal with a much better brain than in the lower orders, like
Insectivores and Rodents, and still more Marsupials, where the period
before birth (gestation) is short. Second, we know that the individual
development of the brain is profoundly influenced by the internal
secretions of certain ductless glands notably the thyroid. When this
organ is not functioning properly the child's brain development is
arrested. It may be that increased production of certain
hormones--itself, of course, to be accounted for--may have stimulated
brain development in man's remote ancestors.
Given variability along the line of better brains and given a process of
discriminate sifting which would consistently offer rewards to alertness
and foresight, to kin-sympathy and parental care, there seems no great
difficulty in imagining how Man would evolve. We must not think of an
Aristotle or a Newton except as fine results which justify all the
groaning and travailing; we must think of average men, of primitive
peoples to-day, and of our forbears long ago. We must remember how much
of man's advance is dependent on the external registration of the social
heritage, not on the slowly changing natural inheritance.
Looking backwards it is impossible, we think, to fail to recognise
progress. There is a ring of truth in the fine description Æschylus gave
of primitive men that--
first, beholding they beheld in vain, and, hearing, heard not, but,
like shapes in dreams, mixed all things wildly down the tedious
time, nor knew to build a house against the sun with wicketed sides,
nor any woodwork knew, but lived like silly ants, beneath the
ground, in hollow caves unsunned. There came to them no steadfast
sign of winter, nor of spring flower-perfumed, nor of summer full of
fruit, but blindly and lawlessly they did all things.
Contrast this picture with the position of man to-day. He has mastered
the forces of Nature and is learning to use their resources more and
more economically; he has harnessed electricity to his chariot and he
has made the ether carry his messages. He tapped supplies of material
which seemed for centuries unavailable, having learned, for instance,
how to capture and utilise the free nitrogen of the air. With his
telegraph and "wireless" he has annihilated distance, and he has added
to his navigable kingdom the depths of the sea and the heights of the
air. He has conquered one disease after another, and the young science
of heredity is showing him how to control in his domesticated animals
and cultivated plants the nature of the generations yet unborn. With all
his faults he has his ethical face set in the right direction. The main
line of movement is towards the fuller embodiment of the true, the
beautiful, and the good in healthy lives which are increasingly a
satisfaction in themselves.
[Illustration: _Photo: British Museum (Natural History)._
SIDE-VIEW OF A PREHISTORIC HUMAN SKULL DISCOVERED IN 1921 IN BROKEN HILL
CAVE, NORTHERN RHODESIA
Very striking are the prominent eyebrow ridges and the broad massive
face. The skull looks less domed than that of modern man, but its
cranial capacity is far above the lowest human limit. The teeth are
interesting in showing marked rotting or "caries," hitherto unknown in
prehistoric skulls. In all probability the Rhodesian man was an African
representative of the extinct Neanderthal species hitherto known only
from Europe.]
[Illustration: _After the restoration modelled by J. H. McGregor._
A CROMAGNON MAN OR CROMAGNARD, REPRESENTATIVE OF A STRONG ARTISTIC RACE
LIVING IN THE SOUTH OF FRANCE IN THE UPPER PLEISTOCENE, PERHAPS 25,000
YEARS AGO
They seemed to have lived for a while contemporaneously with the
Neanderthal Men, and there may have been interbreeding. Some Cromagnards
probably survive, but the race as a whole declined, and there was
repopulation of Europe from the East.]
[Illustration: _Reproduced by permission from Osborn's "Men of the Old
Stone Age."_
PHOTOGRAPH SHOWING A NARROW PASSAGE IN THE CAVERN OF FONT-DE-GAUME ON
THE BEUNE
Throughout the cavern the walls are crowded with engravings; on the left
wall, shown in the photograph, are two painted bison. In the great
gallery there may be found not less than eighty figures--bison,
reindeer, and mammoths. A specimen of the last is reproduced below.]
[Illustration: A MAMMOTH DRAWN ON THE WALL OF THE FONT-DE-GAUME CAVERN
The mammoth age was in the Middle Pleistocene, while Neanderthal Men
still flourished, probably far over 30,000 years ago.]
[Illustration: A GRAZING BISON, DELICATELY AND CAREFULLY DRAWN, ENGRAVED
ON A WALL OF THE ALTAMIRA CAVE, NORTHERN SPAIN
This was the work of a Reindeer Man or Cromagnard, in the Upper or
Post-Glacial Pleistocene, perhaps 25,000 years ago. Firelight must have
been used in making these cave drawings and engravings.]
Factors in Human Progress
Many, we believe, were the gains that rewarded the arboreal
apprenticeship of man's ancestors. Many, likewise, were the results of
leaving the trees and coming down to the solid earth--a transition which
marked the emergence of more than tentative men. What great steps
followed?
Some of the greatest were--the working out of a spoken language and of
external methods of registration; the invention of tools; the discovery
of the use of fire; the utilisation of iron and other metals; the taming
of wild animals such as dog and sheep, horses and cattle; the
cultivation of wild plants such as wheat and rice; and the irrigation of
fields. All through the ages necessity has been the mother of invention
and curiosity its father; but perhaps we miss the heart of the matter if
we forget the importance of some leisure time--wherein to observe and
think. If our earth had been so clouded that the stars were hidden from
men's eyes the whole history of our race would have been different. For
it was through his leisure-time observations of the stars that early man
discovered the regularity of the year and got his fundamental
impressions of the order of Nature--on which all his science is founded.
If we are to think clearly of the factors of human progress we must
recall the three great biological ideas--the living organism, its
environment, and its functioning. For man these mean (1) the living
creature, the outcome of parents and ancestors, a fresh expression of a
bodily and mental inheritance; (2) the surroundings, including climate
and soil, the plants and animals these allow; and (3) the activities of
all sorts, occupations and habits, all the actions and reactions between
man and his milieu. In short, we have to deal with FOLK, PLACE, WORK;
the _Famille_, _Lieu_, _Travail_ of the LePlay school.
As to FOLK, human progress depends on intrinsic racial
qualities--notably health and vigour of body, clearness and alertness of
mind, and an indispensable sociality. The most powerful factors in the
world are clear ideas in the minds of energetic men of good will. The
differences in bodily and mental health which mark races, and stocks
within a people, just as they mark individuals, are themselves traceable
back to germinal variations or mutations, and to the kind of sifting to
which the race or stock has been subjected. Easygoing conditions are not
only without stimulus to new departures, they are without the sifting
which progress demands.
As to PLACE, it is plain that different areas differ greatly in their
material resources and in the availability of these. Moreover, even when
abundant material resources are present, they will not make for much
progress unless the climate is such that they can be readily utilised.
Indeed, climate has been one of the great factors in civilisation, here
stimulating and there depressing energy, in one place favouring certain
plants and animals important to man, in another place preventing their
presence. Moreover, climate has slowly changed from age to age.
As to WORK, the form of a civilisation is in some measure dependent on
the primary occupations, whether hunting or fishing, farming or
shepherding; and on the industries of later ages which have a profound
moulding effect on the individual at least. We cannot, however, say more
than that the factors of human progress have always had these three
aspects, Folk, Place, Work, and that if progress is to continue on
stable lines it must always recognise the essential correlation of
fitter folk in body and mind: improved habits and functions, alike in
work and leisure; and bettered surroundings in the widest and deepest
sense.
BIBLIOGRAPHY
DARWIN, CHARLES, _Descent of Man_.
HADDON, A. C., _Races of Men_.
HADDON, A. C., _History of Anthropology_.
KEANE, A. H., _Man Past and Present_.
KEITH, ARTHUR, _Antiquity of Man_.
LULL, R. S., _Organic Evolution_.
MCCABE, JOSEPH, _Evolution of Civilization_.
MARETT, R. R., _Anthropology_ (Home University Library).
OSBORN, H. F., _Men of the Early Stone Age_.
SOLLAS, W. J., _Ancient Hunters and their Modern Representatives_.
TYLOR, E. B., _Anthropology and Primitive Culture_.
VI
EVOLUTION GOING ON
EVOLUTION GOING ON
Evolution, as we have seen in a previous chapter, is another word for
race-history. It means the ceaseless process of Becoming, linking
generation to generation of living creatures. The Doctrine of Evolution
states the fact that the present is the child of the past and the parent
of the future. It comes to this, that the living plants and animals we
know are descended from ancestors on the whole simpler, and these from
others likewise simpler, and so on, back and back--till we reach the
first living creatures, of which, unfortunately, we know nothing.
Evolution is a process of racial change in a definite direction, whereby
new forms arise, take root, and flourish, alongside of or in the place
of their ancestors, which were in most cases rather simpler in structure
and behaviour.
The rock-record, which cannot be wrong, though we may read it wrongly,
shows clearly that there was once a time in the history of the Earth
when the only backboned animals were Fishes. Ages passed, and there
evolved Amphibians, with fingers and toes, scrambling on to dry land.
Ages passed, and there evolved Reptiles, in bewildering profusion. There
were fish-lizards and sea-serpents, terrestrial dragons and flying
dragons, a prolific and varied stock. From the terrestrial Dinosaurs it
seems that Birds and Mammals arose. In succeeding ages there evolved all
the variety of Birds and all the variety of Mammals. Until at last arose
the Man. The question is whether similar processes of evolution are
still going on.
We are so keenly aware of rapid changes in mankind, though these
concern the social heritage much more than the flesh-and-blood natural
inheritance, that we find no difficulty in the idea that evolution is
going on in mankind. We know the contrast between modern man and
primitive man, and we are convinced that in the past, at least, progress
has been a reality. That degeneration may set in is an awful
possibility--involution rather than evolution--but even if going back
became for a time the rule, we cannot give up the hope that the race
would recover itself and begin afresh to go forward. For although there
have been retrogressions in the history of life, continued through
unthinkably long ages, and although great races, the Flying Dragons for
instance, have become utterly extinct, leaving no successors whatsoever,
we feel sure that there has been on the whole a progress towards nobler,
more masterful, more emancipated, more intelligent, and _better_ forms
of life--a progress towards what mankind at its best has always regarded
as best, i.e. affording most enduring satisfaction. So we think of
evolution going on in mankind, evolution chequered by involution, but on
the whole _progressive evolution_.
Evolutionary Prospect for Man
It is not likely that man's body will admit of _great_ change, but there
is room for some improvement, e.g. in the superfluous length of the
food-canal and the overcrowding of the teeth. It is likely, however,
that there will be constitutional changes, e.g. of prolonged
youthfulness, a higher standard of healthfulness, and a greater
resistance to disease. It is justifiable to look forward to great
improvements in intelligence and in control. The potentialities of the
human brain, as it is, are far from being utilised to the full, and new
departures of promise are of continual occurrence. What is of great
importance is that the new departures or variations which emerge in fine
children should be fostered, not nipped in the bud, by the social
environment, education included. The evolutionary prospect for man is
promising.
[Illustration: PHOTOGRAPH OF A MEDIAN SECTION THROUGH THE SHELL OF THE
PEARLY NAUTILUS
It is only the large terminal chamber that is occupied by the animal.]
[Illustration: PHOTOGRAPH OF THE ENTIRE SHELL OF THE PEARLY NAUTILUS
The headquarters of the Nautilus are in the Indian and Pacific Oceans.
They sometimes swim at the surface of the sea, but they usually creep
slowly about on the floor of comparatively shallow water.]
[Illustration: NAUTILUS
A section through the Pearly Nautilus, _Nautilus pompilius_, common from
Malay to Fiji. The shell is often about 9 inches long. The animal lives
in the last chamber only, but a tube (S) runs through the empty
chambers, perforating the partitions (SE). The bulk of the animal is
marked VM; the eye is shown at E; a hood is marked H; round the mouth
there are numerous lobes (L) bearing protrusible tentacles, some of
which are shown. When the animal is swimming near the surface the
tentacles radiate out in all directions, and it has been described as "a
shell with something like a cauliflower sticking out of it." The Pearly
Nautilus is a good example of a conservative type, for it began in the
Triassic Era. But the family of Nautiloids to which it belongs
illustrates very vividly what is meant by a dwindling race. The
Nautiloids began in the Cambrian, reached their golden age in the
Silurian, and began to decline markedly in the Carboniferous. There are
2,500 extinct or fossil species of Nautiloids, and only 4 living
to-day.]
[Illustration: _Photo: W. S. Berridge._
SHOEBILL
A bird of a savage nature, never mixing with other marsh birds.
According to Dr. Chalmers Mitchell, it shows affinities to herons,
storks, pelicans, and gannets, and is a representative of a type equal
to both herons and storks and falling between the two.]
But it is very important to realise that among plant and animals
likewise, _Evolution is going on_.
The Fountain of Change: Variability
On an ordinary big clock we do not readily see that even the minute hand
is moving, and if the clock struck only once in a hundred years we can
conceive of people arguing whether the hands did really move at all. So
it often is with the changes that go on from generation to generation in
living creatures. The flux is so slow, like the flowing of a glacier,
that some people fail to be convinced of its reality. And it must, of
course, be admitted that some kinds of living creatures, like the
Lamp-shell _Ligula_ or the Pearly Nautilus, hardly change from age to
age, whereas others, like some of the birds and butterflies, are always
giving rise to something new. The Evening Primrose among plants, and the
Fruit-fly, Drosophila, among animals, are well-known examples of
organisms which are at present in a sporting or mutating mood.
Certain dark varieties of moth, e.g. of the Peppered Moth, are taking
the place of the paler type in some parts of England, and the same is
true of some dark forms of Sugar-bird in the West Indian islands. Very
important is the piece of statistics worked out by Professor R. C.
Punnett, that "if a population contains .001 per cent of a new variety,
and if that variety has even a 5 per cent selection advantage over the
original form, the latter will almost completely disappear in less than
a hundred generations." This sort of thing has been going on all over
the world for untold ages, and the face of animate nature has
consequently changed.
We are impressed by striking novelties that crop up--a clever dwarf, a
musical genius, a calculating boy, a cock with a 10 ft. tail, a
"wonder-horse" with a mane reaching to the ground, a tailless cat, a
white blackbird, a copper beech, a Greater Celandine with much cut up
leaves; but this sort of mutation is common, and smaller, less brusque
variations are commoner still. _They form the raw materials of possible
evolution._ We are actually standing before an apparently inexhaustible
fountain of change. This is evolution going on.
The Sporting Jellyfish
It is of interest to consider a common animal like the jellyfish
Aurelia. It is admirably suited for a leisurely life in the open sea,
where it swims about by contracting its saucer-shaped body, thus driving
water out from its concavity. By means of millions of stinging cells on
its four frilled lips and on its marginal tentacles it is able to
paralyse and lasso minute crustaceans and the like, which it then wafts
into its mouth. It has a very eventful life-history, for it has in its
early youth to pass through a fixed stage, fastened to rock or seaweed,
but it is a successful animal, well suited for its habitat, and
practically cosmopolitan in its distribution. It is certainly an
old-established creature. Yet it is very variable in colour and in size,
and even in internal structure. Very often it is the size of a saucer or
a soup-plate, but giants over two feet in diameter are well known. Much
more important, however, than variation in colour and size are the
inborn changes in structure. Normally a jellyfish has its parts in four
or multiples of four. Thus it has four frilled lips, four tufts of
digestive filaments in its stomach, and four brightly coloured
reproductive organs. It has eight sense-organs round the margin of its
disc, eight branched and eight unbranched radial canals running from the
central stomach to a canal round the circumference. The point of giving
these details is just this, that every now and then we find a jellyfish
with its parts in sixes, fives, or threes, and with a multitude of minor
idiosyncrasies. _Even in the well-established jellyfish there is a
fountain of change._
§ 1
Evolution of Plants
It is instructive to look at the various kinds of cabbages, such as
cauliflower and Brussels sprouts, kale and curly greens, and remember
that they are all scions of the not very promising wild cabbage found on
our shores. And are not all the aristocrat apple-trees of our orchards
descended from the plebeian crab-apple of the roadside? We know far too
little about the precise origin of our cultivated plants, but there is
no doubt that after man got a hold of them he took advantage of their
variability to establish race after race, say, of rose and
chrysanthemum, of potato and cereal. The evolution of cultivated plants
is continuing before our eyes, and the creations of Mr. Luther Burbank,
such as the stoneless plum and the primus berry, the spineless cactus
and the Shasta daisy, are merely striking instances of what is always
going on.
There is reason to believe that the domestic dog has risen three times,
from three distinct ancestors--a wolf, a jackal, and a coyote. So a
multiple pedigree must be allowed for in the case of the dog, and the
same is true in regard to some other domesticated animals. But the big
fact is the great variety of breeds that man has been able to fix, after
he once got started with a domesticated type. There are over 200
well-marked breeds of domestic pigeons, and there is very strong
evidence that all are descended from the wild rock-dove, just as the
numerous kinds of poultry are descended from the jungle-fowl of some
parts of India and the Malay Islands. Even more familiar is the way in
which man has, so to speak, unpacked the complex fur of the wild rabbit,
and established all the numerous colour-varieties which we see among
domestic rabbits. And apart from colour-varieties there are long-haired
Angoras and quaint lop-eared forms, and many more besides. All this
points to evolution going on.
The Romance of the Wheat
It is well-known that Neolithic man grew wheat, and some authorities
have put the date of the first wheat harvest at between fifteen thousand
and ten thousand years ago. The ancient civilisations of Babylonia,
Egypt, Crete, Greece, and Rome were largely based on wheat, and it is
highly probable that the first great wheatfields were in the fertile
land between the Tigris and the Euphrates. The oldest Egyptian tombs
that contain wheat, which, by the way, never germinates after its
millennia of rest, belong to the First Dynasty, and are about six
thousand years old. But there must have been a long history of wheat
before that.
Now it is a very interesting fact that the almost certain ancestor of
the cultivated wheat is at present living on the arid and rocky slopes
of Mount Hermon. It is called _Triticum hermonis_, and it is varying
notably to-day, as it did long ago when it gave rise to the emmer, which
was cultivated in the Neolithic Age and is the ancestor of all our
ordinary wheats. We must think of Neolithic man noticing the big seeds
of this Hermon grass, gathering some of the heads, breaking the brittle
spikelet-bearing axis in his fingers, knocking off the rough awns or
bruising the spikelets in his hand till the glumes or chaff separated
off and could be blown away, chewing a mouthful of the seeds--and
resolving to sow and sow again.
That was the beginning of a long story, in the course of which man took
advantage of the numerous variations that cropped up in this sporting
stock and established one successful race after another on his fields.
Virgil refers in the "Georgics" to the gathering of the largest and
fullest ears of wheat in order to get good seed for another sowing, but
it was not till the first quarter of the nineteenth century that the
great step was taken, by men like Patrick Sheriff of Haddington, of
deliberately selecting individual ears of great excellence and
segregating their progeny from mingling with mediocre stock. This is the
method which has been followed with remarkable success in modern times.
One of the factors that assisted the Allies in overcoming the food
crisis in the darkest period of the war was the virtue of Marquis Wheat,
a very prolific, early ripening, hard red spring wheat with excellent
milling and baking qualities. It is now the dominant spring wheat in
Canada and the United States, and it has enormously increased the real
wealth of the world in the last ten years (1921). Now our point is
simply that this Marquis Wheat is a fine example of evolution going on.
In 1917 upwards of 250,000,000 bushels of this wheat were raised in
North America, and in 1918 upwards of 300,000,000 bushels; yet the whole
originated from a single grain planted in an experimental plot at Ottawa
by Dr. Charles E. Saunders so recently as the spring of 1903.
[Illustration: THE WALKING-FISH OR MUD-SKIPPER (PERIOPHTHALMUS), COMMON
AT THE MOUTHS OF RIVERS IN TROPICAL AFRICA, ASIA, AND NORTH-WEST
AUSTRALIA
It skips about by means of its strong pectoral fins on the mud-flats; it
jumps from stone to stone hunting small shore-animals; it climbs up the
roots of the mangrove-trees. The close-set eyes protrude greatly and are
very mobile. The tail seems to help in respiration.]
[Illustration: _Photo: "The Times."_
THE AUSTRALIAN MORE-PORK OR PODARGUS
A bird with a frog-like mouth, allied to the British Nightjar. Now in
the London Zoological Gardens.
The capacious mouth is well suited for engulfing large insects such as
locusts and mantises, which are mostly caught on the trees. During the
day the More-pork or Frog-mouth sleeps upright on a branch, and its
mottled brown plumage makes it almost invisible.]
[Illustration: PELICAN'S BILL, ADAPTED FOR CATCHING AND STORING FISHES
There is an enormous dilatable sac beneath the lower jaw.]
[Illustration: HORNBILL'S BILL, ADAPTED FOR EXCAVATING A NEST IN A TREE,
AND ALSO FOR SEIZING AND BREAKING DIVERSE FORMS OF FOOD, FROM MAMMALS TO
TORTOISES, FROM ROOTS TO FRUITS
The use of the helmet or casque is obscure.]
[Illustration: SPOONBILL'S BILL, ADAPTED FOR SIFTING THE MUD AND
CATCHING THE SMALL ANIMALS, E.G. FISHES, CRUSTACEANS, INSECT LARVÆ,
WHICH LIVE THERE]
[Illustration: FALCON'S BILL, ADAPTED FOR SEIZING, KILLING, AND TEARING
SMALL MAMMALS AND BIRDS]
[Illustration: AVOCET'S BILL, ADAPTED FOR A CURIOUS SIDEWAYS SCOOPING IN
THE SHORE-POOLS AND CATCHING SMALL ANIMALS]
[Illustration: PUFFIN'S BILL, ADAPTED FOR CATCHING SMALL FISHES NEAR THE
SURFACE OF THE SEA, AND FOR HOLDING THEM WHEN CAUGHT AND CARRYING THEM
TO THE NEST
The scaly covering is moulted in the autumn.]
We must not dwell too long on this particular instance of evolution,
though it has meant much to our race. We wish, however, following
Professor Buller's _Essays on Wheat_ (1919), to explain the method by
which this good seed was discovered. From one we may learn all. The
parent of Marquis Wheat on the male side was the mid-Europe Red Fife--a
first-class cereal. The parent on the female side was less promising, a
rather nondescript, not pure-bred wheat, called Red Calcutta, which was
imported from India into Canada about thirty years ago. The father was
part of a cargo that came from the Baltic to Glasgow, and was happily
included in a sample sent on to David Fife in Ontario about 1842. From
one kernel of this sample David Fife started his stock of Red Fife,
which was crossed by Dr. Saunders with Hard Red Calcutta. The result of
the cross was a medley of types, nearly a hundred varieties altogether,
and it was in scrutinising these that Dr. Saunders hit upon Marquis. He
worked steadily through the material, studying head after head of what
resulted from sowing, and selecting out those that gave most promise.
Each of the heads selected was propagated; most of the results were
rejected; the elect were sifted again and yet again, and finally Marquis
Wheat emerged, rich in constructive possibilities, probably the most
valuable food-plant in the world. It is like a romance to read that "the
first crop of the wheat that was destined within a dozen years to
overtax the mightiest elevators in the land was stored away in the
winter of 1904-5 in a paper packet no larger than an envelope."
Thus from the Wild Wheat of Mount Hermon there evolved one of the most
important food-plants of the world. This surely is _Evolution going on_.
§ 2
Changes in the Animal Life of a Country
Nothing gives us a more convincing impression of evolution in being than
a succession of pictures of the animal life of a country in different
ages. Dr. James Ritchie, a naturalist of distinction, has written a
masterly book, _The Influence of Man on Animal Life in Scotland_ (1920),
in which we get this succession of pictures. "Within itself," he says,
"a fauna is in a constant state of uneasy restlessness, an assemblage of
creatures which in its parts ebbs and flows as one local influence or
another plays upon it." There are temporary and local changes, endless
disturbances and readjustments of the "balance of nature." One year
there is a plague of field-voles, perhaps next year "grouse disease" is
rife; in one place there is huge increase of starlings, in another place
of rabbits; here cockchafers are in the ascendant, and there the moles
are spoiling the pasture. "But while the parts fluctuate, the fauna as a
whole follows a path of its own. As well as internal tides which swing
to and fro about an average level, there is a drift which carries the
fauna bodily along an 'irretraceable course.'" This is partly due to
considerable changes of climate, for climate calls the tune to which
living creatures dance, but it is also due to new departures among the
animals themselves. We need not go back to the extinct animals and lost
faunas of past ages--for Britain has plenty of relics of these--which
"illustrate the reality of the faunal drift," but it may be very useful,
in illustration of evolution in being, to notice what has happened in
Scotland since the end of the Great Ice Age.
Some nine thousand years ago or more, certain long-headed,
square-jawed, short-limbed, but agile hunters and fishermen, whom we
call Neolithic Man, established themselves in Scotland. What was the
state of the country then?
It was a country of swamps, low forests of birch, alder, and willow,
fertile meadows, and snow-capped mountains. Its estuaries penetrated
further inland than they now do, and the sea stood at the level of
the Fifty-Foot Beach. On its plains and in its forests roamed many
creatures which are strange to the fauna of to-day--the Elk and the
Reindeer, Wild Cattle, the Wild Boar and perhaps Wild Horses, a
fauna of large animals which paid toll to the European Lynx, the
Brown Bear and the Wolf. In all likelihood, the marshes resounded to
the boom of the Bittern and the plains to the breeding calls of the
Crane and the Great Bustard.
Such is Dr. Ritchie's initial picture.
[Illustration: LIFE-HISTORY OF A FROG
1, Before hatching; 2, newly hatched larvæ hanging on to water-weed; 3,
with external gills; 4, external gills are covered over and are
absorbed; 5, limbless larva about a month old with internal gills; 6,
tadpole with hind-legs, about two months old; 7, with the fore-limbs
emerging; 8, with all four legs free; 9, a young frog, about three
months old, showing the almost complete absorption of the tail and the
change of the tadpole mouth into a frog mouth.]
[Illustration: _Photo: J. J. Ward. F.E.S._
HIND-LEG OF WHIRLIGIG BEETLE WHICH HAS BECOME BEAUTIFULLY MODIFIED FOR
AQUATIC LOCOMOTION
The flattened tips form an expanding "fan" or paddle, which opens and
closes with astonishing rapidity. The closing of the "fan," like the
"feathering" of an oar, reduces friction when the leg is being moved
forwards for the next stroke.]
[Illustration: THE BIG ROBBER-CRAB (_Birgus Latro_), THAT CLIMBS THE
COCO-NUT PALM AND BREAKS OFF THE NUTS
It occurs on islands in the Indian Ocean and Pacific, and is often found
far above sea-level. It is able to breathe dry air. One is seen emerging
from its burrow, which is often lined with coco-nut fibre. The empty
coco-nut shell is sometimes used by the Robber-Crab for the protection
of its tail.]
Now what happened in this kingdom of Caledonia which Neolithic Man had
found? He began to introduce domesticated animals, and that meant a
thinning of the ranks of predacious creatures. "Safety first" was the
dangerous motto in obedience to which man exterminated the lynx, the
brown bear, and the wolf. Other creatures, such as the great auk, were
destroyed for food, and others like the marten for their furs. Small
pests were destroyed to protect the beginnings of agriculture; larger
animals like the boar were hunted out of existence; others, like the
pearl-bearing river-mussels, yielded to subtler demands. No doubt there
was protection also--protection for sport, for utility, for æsthetic
reasons, and because of humane sentiments; even wholesome superstitions
have safeguarded the robin redbreast and the wren. There were
introductions too--the rabbit for utility, the pheasant for sport, and
the peacock for amenity. And every introduction, every protection, every
killing out had its far-reaching influences.
But if we are to picture the evolution going on, we must think also of
man's indirect interference with animal life. He destroyed the forests,
he cultivated the wild, he made bridges, he allowed aliens, like rats
and cockroaches, to get in unawares. Of course, he often did good, as
when he drained swamps and got rid of the mosquitoes which once made
malaria rife in Scotland.
What has been the net result? Not, as one might think for a moment, a
reduction in the _number_ of different kinds of animals. Fourteen or so
species of birds and beasts have been banished from Scotland since man
interfered, but as far as numbers go they have been more than replaced
by deliberate introductions like fallow deer, rabbit, squirrel, and
pheasant, and by accidental introductions like rats and cockroaches. But
the change is rather in _quality_ than in quantity; the smaller have
taken the place of the larger, rather paltry pigmies of noble giants.
Thus we get a vivid idea that evolution, especially when man interferes,
is not necessarily progressive. That depends on the nature of the sieves
with which the living materials are sifted. As Dr. Ritchie well says,
the standard of the wild fauna as regards size has fallen and is
falling, and it is not in size only that there is loss, there is a
deterioration of quality. "For how can the increase of Rabbits and
Sparrows and Earthworms and Caterpillars, and the addition of millions
of Rats and Cochroaches and Crickets and Bugs, ever take the place of
those fine creatures round the memories of which the glamour of
Scotland's past still plays--the Reindeer and the Elk, the Wolf, the
Brown Bear, the Lynx, and the Beaver, the Bustard, the Crane, the
Bumbling Bittern, and many another, lost or disappearing." Thus we see
again that evolution is going on.
§ 3
The Adventurers
All through the millions of years during which animals have tenanted the
earth and the waters under the earth, there has been a search for new
kingdoms to conquer, for new corners in which to make a home. And this
still goes on. _It has been and is one of the methods of evolution to
fill every niche of opportunity._ There is a spider that lives inside a
pitcher-plant, catching some of the inquisitive insects which slip down
the treacherous internal surface of the trap. There is another that
makes its home in crevices among the rocks on the shore of the
Mediterranean, or even in empty tubular shells, keeping the water out,
more or less successfully, by spinning threads of silk across the
entrance to its retreat. The beautiful brine-shrimp, _Artemia salina_,
that used to occur in British salterns has found a home in the dense
waters of the Great Salt Lake of Utah. Several kinds of earthworms have
been found up trees, and there is a fish, Arges, that climbs on the
stones of steep mountain torrents of the Andes. The intrepid explorers
of the _Scotia_ voyage found quite a number of Arctic terns spending our
winter within the summer of the Antarctic Circle--which means girdling
the globe from pole to pole; and every now and then there are incursions
of rare birds, like Pallas's Sand-grouse, into Britain, just as if they
were prospecting in search of a promised land. Twice or thrice the
distinctively North American Killdeer Plover has been found in Britain,
having somehow or other got across the Atlantic. We miss part of the
meaning of evolution if we do not catch this note of insurgence and
adventure, which some animal or other never ceases to sound, though many
establish themselves in a security not easily disturbed, and though a
small minority give up the struggle against the stream and are content
to acquiesce, as parasites or rottenness eaters, in a drifting life of
ease.
More important than very peculiar cases is the broad fact that over and
over again in different groups of animals there have been attempts to
master different kinds of haunts--such as the underground world, the
trees, the freshwaters, and the air. There are burrowing amphibians,
burrowing reptiles, burrowing birds, and burrowing mammals; there are
tree-toads, tree-snakes, tree-lizards, tree-kangaroos, tree-sloths,
tree-shrews, tree-mice, tree-porcupines, and so on; enough of a list to
show, without mentioning birds, how many different kinds of animals
have entered upon an arboreal apprenticeship--an apprenticeship often
with far-reaching consequences. What the freeing of the hand from being
an organ of terrestrial support has meant in the evolution of monkeys is
a question that gives a spur to our imagination.
The Case of the Robber Crab
On some of the coral islands of the Indian and Pacific Oceans there
lives a land-crab, Birgus, which has learned to breathe on land. It
breathes dry air by means of curious blood-containing tufts in the upper
part of its gill-cavity, and it has also rudimentary gills. It is often
about a foot long, and it has very heavy great claws, especially on the
left-hand side. With this great claw it hammers on the "eye-hole" of a
coconut, from which it has torn off the fibrous husk. It hammers until a
hole is made by which it can get at the pulp. Part of the shell is
sometimes used as a protection for the soft abdomen--for the
robber-crab, as it is called, is an offshoot from the hermit-crab stock.
Every year this quaint explorer, which may go far up the hills and climb
the coco-palms, has to go back to the sea to spawn. The young ones are
hatched in the same state as in our common shore-crab. That is to say,
they are free-swimming larvæ which pass through an open-water period
before they settle down on the shore, and eventually creep up on to dry
land. Just as open-water turtles lay their eggs on sandy shores, going
back to their old terrestrial haunt, so the robber-crab, which has
almost conquered the dry land, has to return to the seashore to breed.
There is a peculiar interest in the association of the robber-crab with
the coco-palm, for that tree is not a native of these coral islands, but
has been introduced, perhaps from Mexico, by the Polynesian mariners
before the discovery of America by Columbus. So the learning to deal
with coconuts is a recent achievement, and we are face to face with a
very good example of evolution going on.
[Illustration: EARLY LIFE-HISTORY OF THE SALMON
1. The fertilised egg, shed in the gravelly bed of the river.
2. The embryo within the egg, just before hatching. The embryo has been
constricted off from the yolk-laden portion of the egg.
3. The newly hatched salmon, or alevin, encumbered with its legacy of
yolk (Y.S.).
4 and 5. The larval salmon, still being nourished from the yolk-sac
(Y.S.), which is diminishing in size as the fish grows larger.
6. The salmon fry about six weeks old, with the yolk fully absorbed, so
that the young fish has now to feed for itself. The fry become parr,
which go to the sea as smolts, and return as grilse.
In all cases the small figures to the right indicate the natural size.]
[Illustration: THE SALMON LEAPING AT THE FALL IS A MOST FASCINATING
SPECTACLE
Again and again we see them jumping out of the seething foam beneath the
fall, casting themselves into the curtain of the down-rushing water,
only to be carried back by it into the depths whence they have risen.
One here and another there makes its effort good, touches the upper lip
of the cataract, gives a swift stroke of its tail, and rushes on towards
those upper reaches which are the immemorial spawning beds of its
race.]
The Story of the Salmon
In late autumn or in winter the salmon spawn in the rivers. The female
makes a shallow trough in the gravel by moving her tail from side to
side, and therein lays many eggs. The male, who is in attendance,
fertilises these with the milt, and then the female covers them deeply
with gravel. The process is repeated over and over again for a week or
more till all the eggs are shed. For three to four months the eggs
develop, and eventually there emerge the larvæ or _alevins_, which lurk
among the pebbles. They cannot swim much, for they are encumbered by a
big legacy of yolk. In a few weeks, perhaps eight, the protruding bag of
yolk has disappeared and the _fry_, about an inch long, begin to move
about more actively and to fend for themselves. By the end of the year
they have grown to be rather trout-like _parr_, about four inches long.
In two years these are double that length. Usually in the second year,
but it may be earlier or later, the parr become silvery _smolts_, which
go out to sea, usually about the month of May. They feed on young
herring and the like and grow large and strong. When they are about
three and a half years old they come up the rivers as _grilse_ and may
spawn. Or they may pass through the whole grilse stage in the sea and
come up the rivers with all the characters of the full-grown fish. In
many cases the salmon spawn only once, and some (they are called _kelts_
after spawning) are so much exhausted by starting a new generation that
they die or fall a victim to otters and other enemies. In the case of
the salmon of the North Pacific (in the genus _Oncorhynchus_, not
_Salmo_) all the individuals die after spawning, none being able to
return to the sea. It must be remembered that full-grown salmon do not
as a rule feed in fresh water, though they may be unable to resist
snapping at the angler's strange creations. A very interesting fact is
that the salmon keeps as it were a diary of its movements, which vary a
good deal in different rivers. This diary is written in the scales, and
a careful reading of the concentric lines on the scales shows the age of
the fish, and when it went out to sea, and whether it has spawned or
not, and more besides.
Interpretation of the Salmon's Story
When an animal frequents two different haunts, in one of which it
breeds, it is very often safe to say that the breeding-place represents
the original home. The flounder is quite comfortable far up the rivers,
but it has to go to the shore-waters to spawn, and there is no doubt
that the flounder is a marine fish which has recently learned to
colonise the fresh waters. Its relatives, like plaice and sole, are
strictly marine. But it is impossible to make a dogma of the rule that
the breeding-place corresponds to the original home. Thus some kinds of
bass, which belong to the marine family of sea-perches, live in the sea
or in estuaries, while two have become permanent residents in fresh
water. Or, again, the members of the herring family are very
distinctively marine, but the shad, which belong to this family, spawn
in rivers and may spend their lives there.
So there are two different ways of interpreting the life-history of the
salmon. Some authorities regard the salmon as a marine fish which is
establishing itself in fresh water. But others read the story the other
way and regard the salmon as a member of a freshwater race, that has
taken to the sea for feeding purposes. In regard to trout, we know that
the ranks of those in rivers and lakes are continually being reinforced
by migrants from the sea, and that some trout go down to the sea while
others remain in the freshwater. We know also in regard to a related
fish, the char, that while the great majority of kinds are now permanent
residents in cold and deep, isolated northern lakes, there are Arctic
forms which live in the sea but enter the rivers to spawn. These facts
favour the view that the salmon was originally a marine fish. But there
are arguments on both sides, and, for our present purpose, the important
fact is that the salmon is conquering _two_ haunts. Its evolution is
going on.
The Romance of the Eel
Early in summer, at dates varying with the distance of the rivers from
the open Atlantic, crowds of young eels or elvers come up-stream.
Sometimes the procession or eel-fare includes thousands of individuals,
each about the length of our first finger, and as thick as a stout
knitting needle. They obey an inborn impulse to swim against the stream,
seeking automatically to have both sides of their body equally
stimulated by the current. So they go straight ahead. The obligation
works only during the day, for when the sun goes down behind the hills
the elvers snuggle under stones or beneath the bank and rest till dawn.
In the course of time they reach the quiet upper reaches of the river or
go up rivulets and drainpipes to the isolated ponds. Their impulse to go
on must be very imperious, for they may wriggle up the wet moss by the
side of a waterfall or even make a short excursion in a damp meadow.
In the quiet-flowing stretches of the river or in the ponds they feed
and grow for years and years. They account for a good many young fishes.
Eventually, after five or six years in the case of the males, six to
eight years in the case of the females, the well-grown fishes, perhaps a
foot and a half to two feet long, are seized by a novel restlessness.
They are beginning to be mature. They put on a silvery jacket and become
large of eye, and they return to the sea. In getting away from the pond
it may be necessary to wriggle through the damp meadow-grass before
reaching the river. They travel by night and rather excitedly. The
Arctic Ocean is too cold for them and the North Sea too shallow. They
must go far out to sea, to where the old margin of the once larger
continent of Europe slopes down to the great abysses, from the Hebrides
southwards. Eels seem to spawn in the deep dark water; but the just
liberated eggs have not yet been found. The young fry rises to near the
surface and becomes a knife-blade-like larva, transparent all but its
eye. It lives for many months in this state, growing to be about three
inches long, rising and sinking in the water, and swimming gently.
These open-sea young eels are known as Leptocephali, a name given to
them before their real nature was proved. They gradually become shorter,
and the shape changes from knife-blade-like to cylindrical. During this
change they fast, and the weight of their delicate body decreases. They
turn into glass-eels, about 2-1/2 inches long, like a knitting-needle in
girth. They begin to move towards the distant shores and rivers, and
they may be a year and a half old before they reach their destination
and go up-stream as elvers. Those that ascend the rivers of the Eastern
Baltic must have journeyed three thousand miles. It is certain that no
eel ever matures or spawns in fresh water. It is practically certain
that all the young eels ascending the rivers of North Europe have come
in from the Atlantic, some of them perhaps from the Azores or further
out still. It is interesting to inquire how the young eels circumvent
the Falls of the Rhine and get into Lake Constance, or how their kindred
on the other side of the Atlantic overcome the obstacle of Niagara; but
it is more important to lay emphasis on the variety of habitats which
this fish is trying--the deep waters, the open sea, the shore, the
river, the pond, and even, it may be, a little taste of solid earth. It
seems highly probable that the common eel is a deep-water marine fish
which has learned to colonise the freshwaters. It has been adventurous
and it has succeeded. The only shadow on the story of achievement is
that there seems to be no return from the spawning. There is little
doubt that death is the nemesis of their reproduction. In any case, no
adult eel ever comes back from the deep sea. We are minded of Goethe's
hard saying: "Death is Nature's expert advice to get plenty of life."
§ 4
Forming New Habits
There is a well-known mudfish of Australia, Neoceratodus by name, which
has turned its swim-bladder into a lung and comes to the surface to
spout. It expels vitiated air with considerable force and takes fresh
gulps. At the same time, like an ordinary fish, it has gills which allow
the usual interchange of gases between the blood and the water. Now this
Australian mudfish or double-breather (Dipnoan), which may be a long way
over a yard in length, is a direct and little-changed descendant of an
ancient extinct fish, Ceratodus, which lived in Mesozoic times, as far
back as the Jurassic, which probably means over five millions of years
ago. The Queensland mudfish is an antiquity, and there has not been much
change in its lineage for millions of years. We might take it as an
illustration of the inertia of evolution. And yet, though its structure
has changed but little, the fish probably illustrates evolution in
process, for it is a fish that is learning to breathe dry air. It cannot
leave the water; but it can live comfortably in pools which are foul
with decomposing animal and vegetable matter. In partially dried-up and
foul waterholes, full of dead fishes of various kinds, Neoceratodus has
been found vigorous and lively. Unless we take the view, which is
_possible_, that the swim-bladder of fishes was originally a lung, the
mud-fishes are learning to breathe dry air. They illustrate evolution
agoing.
[Illustration: DIAGRAM OF THE LIFE HISTORY OF THE COMMON EEL (_Anguilla
Vulgalis_)
1. The transparent open-sea knife-blade-like larva called a
Leptocephalus.
2 and 3. The gradual change of shape from knife-blade-like to
cylindrical. The body becomes shorter and loses weight.
4. The young elver, at least a year old, which makes its way from the
open sea to the estuaries and rivers. It is 2/3 inches long and almost
cylindrical.
5. The fully-formed eel.]
[Illustration: _Photo: Gambier Bolton._
CASSOWARY
Its bare head is capped with a helmet. Unlike the plumage of most birds
its feathers are loose and hair-like, whilst its wings are merely
represented by a few black quills. It is flightless and entirely
dependent on its short powerful legs to carry it out of danger.]
[Illustration: _Photo: Gambier Bolton._
THE KIWI, ANOTHER FLIGHTLESS BIRD, OF REMARKABLE APPEARANCE, HABITS, AND
STRUCTURE]
The herring-gull is by nature a fish-eater; but of recent years, in some
parts of Britain, it has been becoming in the summer months more and
more of a vegetarian, scooping out the turnips, devouring potatoes,
settling on the sheaves in the harvest field and gorging itself with
grain. Similar experiments, usually less striking, are known in many
birds; but the most signal illustration is that of the kea or Nestor
parrot of New Zealand, which has taken to lighting on the loins of the
sheep, tearing away the fleece, cutting at the skin, and gouging out
fat. Now the parrot belongs to a vegetarian or frugivorous stock, and
this change of diet in the relatively short time since sheep-ranches
were established in New Zealand is very striking. Here, since we know
the dates, we may speak of evolution going on under our eyes. It must be
remembered that variations in habit may give an animal a new
opportunity to test variations in structure which arise mysteriously
from within, as expressions of germinal changefulness rather than as
imprints from without. For of the transmissibility of the latter there
is little secure evidence.
Experiments in Locomotion
It is very interesting to think of the numerous types of locomotion
which animals have discovered--pulling and punting, sculling and rowing,
and of the changes that are rung on these four main methods. How
striking is the case of the frilled lizard (Chlamydosaurus) of
Australia, which at the present time is, as it were, experimenting in
bipedal progression--always a rather eventful thing to do. It gets up on
its hind-legs and runs totteringly for a few feet, just like a baby
learning to walk.
How beautiful is the adventure which has led our dipper or
water-ouzel--a bird allied to the wrens--to try walking and flying under
water! How admirable is the volplaning of numerous parachutists--"flying
fish," "flying frog," "flying dragon," "flying phalanger," "flying
squirrel," and more besides, which take great leaps through the air. For
are these not the splendid failures that might have succeeded in
starting new modes of flight?
Most daring of all, perhaps, are the aerial journeys undertaken by many
small spiders. On a breezy morning, especially in the autumn, they mount
on gate-posts and palings and herbage, and, standing with their head to
the wind, pay out three or four long threads of silk. When the wind tugs
at these threads, the spinners let go, and are borne, usually back
downwards, on the wings of the wind from one parish to another. It is
said that if the wind falls they can unfurl more sail, or furl if it
rises. In any case, these wingless creatures make aerial journeys. When
tens of thousands of the used threads sink to earth, there is a "shower
of gossamer." On his _Beagle_ voyage Darwin observed that vast numbers
of small gossamer spiders were borne on to the ship when it was sixty
miles distant from the land.
[Illustration: THE AUSTRALIAN FRILLED LIZARD, WHICH IS AT PRESENT TRYING
TO BECOME A BIPED
When it gets up on its hind-legs and runs for a short distance it folds
its big collar round its neck.]
[Illustration: A CARPET OF GOSSAMER
The silken threads used by thousands of gossamer spiders in their
migrations are here seen entangled in the grass, forming what is called
a shower of gossamer. At the edge of the grass the gossamer forms a
curtain, floating out and looking extraordinarily like waves breaking on
a seashore.]
[Illustration: THE WATER-SPIDER
The spider is seen just leaving its diving-bell to ascend to the surface
to capture air.
The spider jerks its body and legs out at the surface and then dives--
--carrying with it what looks like a silvery air-bubble--air entangled
in the hair.
The spider reaches its air-dome. Note how the touch of its legs indents
the inflated balloon.
Running down the side of the nest, the spider
--brushes off the air at the entrance, and the bubble ascends into the
silken balloon.
_Photos: J. J. Ward, F.E.S._]
New Devices
It is impossible, we must admit, to fix dates, except in a few cases,
relatively recent; but there is a smack of modernity in some striking
devices which we can observe in operation to-day. Thus no one will
dispute the statement that spiders are thoroughly terrestrial animals
breathing dry air, but we have the fact of the water-spider conquering
the under-water world. There are a few spiders about the seashore, and a
few that can survive douching with freshwater, but the particular case
of the true water-spider, _Argyroneta natans_, stands by itself because
the creature, as regards the female at least, has _conquered_ the
sub-aquatic environment. A flattish web is woven, somehow, underneath
the water, and pegged down by threads of silk. Along a special vertical
line the mother spider ascends to the surface and descends again, having
entangled air in the hairs of her body. She brushes off this air
underneath her web, which is thereby buoyed up into a sort of dome. She
does this over and over again, never getting wet all the time, until the
domed web has become like a diving-bell, full of dry air. In this
eloquent anticipation of man's rational device, this creature--far from
being endowed with reason--lays her eggs and looks after her young. The
general significance of the facts is that when competition is keen, a
new area of exploitation is a promised land. Thus spiders have spread
over all the earth except the polar areas. But here is a spider with
some spirit of adventure, which has endeavoured, instead of trekking, to
find a new corner near at home. It has tackled a problem surely
difficult for a terrestrial animal, the problem of living in great part
under water, and it has solved it in a manner at once effective and
beautiful.
In Conclusion
We have given but a few representative illustrations of a great theme.
When we consider the changefulness of living creatures, the
transformations of cultivated plants and domesticated animals, the
gradual alterations in the fauna of a country, the search after new
haunts, the forming of new habits, and the discovery of many inventions,
are we not convinced that Evolution is going on? And why should it
stop?
VII
THE DAWN OF MIND
THE DAWN OF MIND
In the story of evolution there is no chapter more interesting than the
emergence of mind in the animal kingdom. But it is a difficult chapter
to read, partly because "mind" cannot be seen or measured, only
_inferred_ from the outward behaviour of the creature, and partly
because it is almost impossible to avoid reading ourselves into the much
simpler animals.
§ 1
Two Extremes to be Avoided
The one extreme is that of uncritical generosity which credits every
animal, like Brer Rabbit--who, by the way, was the hare--with human
qualities. The other extreme is that of thinking of the animal as if it
were an automatic machine, in the working of which there is no place or
use for mind. Both these extremes are to be avoided.
When Professor Whitman took the eggs of the Passenger Pigeon (which
became extinct not long ago with startling rapidity) and placed them a
few inches to one side of the nest, the bird looked a little uneasy and
put her beak under her body as if to feel for something that was not
there. But she did not try to retrieve her eggs, close at hand as they
were. In a short time she flew away altogether. This shows that the mind
of the pigeon is in some respects very different from the mind of man.
On the other hand, when a certain clever dog, carrying a basket of eggs,
with the handle in his mouth, came to a stile which had to be
negotiated, he laid the basket on the ground, pushed it gently through a
low gap to the other side, and then took a running leap over. We dare
not talk of this dog as an automatic machine.
A Caution in Regard to Instinct
In studying the behaviour of animals, which is the only way of getting
at their mind, for it is only of our own mind that we have direct
knowledge, it is essential to give prominence to the fact that there has
been throughout the evolution of living creatures a strong tendency to
enregister or engrain capacities of doing things effectively. Thus
certain abilities come to be inborn; they are parts of the inheritance,
which will express themselves whenever the appropriate trigger is
pulled. The newly born child does not require to learn its breathing
movements, as it afterwards requires to learn its walking movements. The
ability to go through the breathing movements is inborn, engrained,
enregistered.
In other words, there are hereditary pre-arrangements of nerve-cells and
muscle-cells which come into activity almost as easily as the beating of
the heart. In a minute or two the newborn pigling creeps close to its
mother and sucks milk. It has not to learn how to do this any more than
we have to learn to cough or sneeze. Thus animals have many useful
ready-made, or almost ready-made, capacities of doing apparently clever
things. In simple cases of these inborn pre-arrangements we speak of
reflex actions; in more complicated cases, of instinctive behaviour. Now
the caution is this, that while these inborn capacities usually work
well in natural conditions, they sometimes work badly when the ordinary
routine is disturbed. We see this when a pigeon continues sitting for
many days on an empty nest, or when it fails to retrieve its eggs only
two inches away. But it would be a mistake to call the pigeon, because
of this, an unutterably stupid bird. We have only to think of the
achievements of homing pigeons to know that this cannot be true. We must
not judge animals in regard to those kinds of behaviour which have been
handed over to instinct, and go badly agee when the normal routine is
disturbed. In ninety-nine cases out of a hundred the enregistered
instinctive capacities work well, and the advantage of their becoming
stereotyped was to leave the animal more free for adventures at a higher
level. Being "a slave of instinct" may give the animal a security that
enables it to discover some new home or new food or new joy. Somewhat in
the same way, a man of methodical habits, which he has himself
established, may gain leisure to make some new departure of racial
profit.
[Illustration: _Photo: O. J. Wilkinson._
JACKDAW BALANCING ON A GATEPOST
The jackdaw is a big-brained, extremely alert, very educable, loquacious
bird.]
[Illustration: _From Ingersoll's "The Wit of the Wild."_
TWO OPOSSUMS FEIGNING DEATH
The Opossums are mainly arboreal marsupials, insectivorous and
carnivorous, confined to the American Continent from the United States
to Patagonia. Many have no pouch and carry their numerous young ones on
their back, the tail of the young twined round that of the mother. The
opossums are agile, clever creatures, and famous for "playing 'possum,"
lying inert just as if they were dead.]
[Illustration: MALE OF THREE-SPINED STICKLEBACK, MAKING A NEST OF
WATER-WEED, GLUED TOGETHER BY VISCID THREADS SECRETED FROM THE KIDNEYS
AT THE BREEDING SEASON]
[Illustration: A FEMALE STICKLEBACK ENTERS THE NEST WHICH THE MALE HAS
MADE, LAYS THE EGGS INSIDE, AND THEN DEPARTS
In many cases two or three females use the same nest, the stickleback
being polygamous. Above the nest the male, who mounts guard, is seen
driving away an intruder.]
When we draw back our finger from something very hot, or shut our eye to
avoid a blow from a rebounding branch, we do not will the action; and
this is more or less the case, probably, when a young mammal sucks its
mother for the first time. Some Mound-birds of Celebes lay their eggs in
warm volcanic ash by the shore of the sea, others in a great mass of
fermenting vegetation; it is inborn in the newly hatched bird to
struggle out as quickly as it can from such a strange nest, else it will
suffocate. If it stops struggling too soon, it perishes, for it seems
that the trigger of the instinct cannot be pulled twice. Similarly, when
the eggs of the turtle, that have been laid in the sand of the shore,
hatch out, the young ones make _instinctively_ for the sea. Some of the
crocodiles bury their eggs two feet or so below the surface among sand
and decaying vegetation--an awkward situation for a birthplace. When the
young crocodile is ready to break out of the egg-shell, just as a chick
does at the end of the three weeks of brooding, it utters
_instinctively_ a piping cry. On hearing this, the watchful mother digs
away the heavy blankets, otherwise the young crocodile would be buried
alive at birth. Now there is no warrant for believing that the young
Mound-birds, young crocodiles, and young turtles have an intelligent
appreciation of what they do when they are hatched. They act
instinctively, "as to the manner born." But this is not to say that
their activity is not backed by endeavour or even suffused with a
certain amount of awareness. Of course, it is necessarily difficult for
man, who is so much a creature of intelligence, to get even an inkling
of the mental side of instinctive behaviour.
In many of the higher reaches of animal instinct, as in courtship or
nest-building, in hunting or preparing the food, it looks as if the
starting of the routine activity also "rang up" the higher centres of
the brain and put the intelligence on the _qui vive_, ready to interpose
when needed. So the twofold caution is this: (1) We must not depreciate
the creature too much if, in unusual circumstances, it acts in an
ineffective way along lines of behaviour which are normally handed over
to instinct; and (2) we must leave open the possibility that even
routine instinctive behaviour may be suffused with awareness and backed
by endeavour.
§ 2
A Useful Law
But how are we to know when to credit the animal with intelligence and
when with something less spontaneous? Above all, how are we to know when
the effective action, like opening the mouth the very instant it is
touched by food in the mother's beak, is just a physiological action
like coughing or sneezing, and when there is behind it--a mind at work?
The answer to this question is no doubt that given by Prof. Lloyd
Morgan, who may be called the founder of comparative psychology, that we
must describe the piece of behaviour very carefully, just as it
occurred, without reading anything into it, and that we must not ascribe
it to a higher faculty if it can be satisfactorily accounted for in
terms of a lower one. In following this principle we may be sometimes
niggardly, for the behaviour may have a mental subtlety that we have
missed; but in nine cases out of ten our conclusions are likely to be
sound. It is the critical, scientific way.
Bearing this law in mind, let us take a survey of the emergence of mind
among backboned animals.
Senses of Fishes
Fishes cannot shut their eyes, having no true lids; but the eyes
themselves are very well developed and the vision is acute, especially
for moving objects. Except in gristly fishes, the external opening to
the ear has been lost, so that sound-waves and coarser vibrations must
influence the inner ear, which is well developed, through the
surrounding flesh and bones. It seems that the main use of the ear in
fishes is in connection with balancing, not with hearing. In many cases,
however, the sense of hearing has been demonstrated; thus fishes will
come to the side of a pond to be fed when a bell is rung or when a
whistle is blown by someone not visible from the water. The fact that
many fishes pay no attention at all to loud noises does not prove that
they are deaf, for an animal may hear a sound and yet remain quite
indifferent or irresponsive. This merely means that the sound has no
vital interest for the animal. Some fishes, such as bullhead and
dogfish, have a true sense of smell, detecting by their nostrils very
dilute substances permeating the water from a distance. Others, such as
members of the cod family, perceive their food in part at least by the
sense of taste, which is susceptible to substances near at hand and
present in considerable quantity. This sense of taste may be located on
the fins as well as about the mouth. At this low level the senses of
smell and taste do not seem to be very readily separated. The chief use
of the sensitive line or lateral line seen on each side of a bony fish
is to make the animal aware of slow vibrations and changes of pressure
in the water. The skin responds to pressures, the ear to vibrations of
high frequency; the lateral line is between the two in its function.
Interesting Ways of Fishes
The brain of the ordinary bony fish is at a very low level. Thus the
cerebral hemispheres, destined to become more and more the seat of
intelligence, are poorly developed. In gristly fishes, like skates and
sharks, the brain is much more promising. But although the state of the
brain does not lead one to expect very much from a bony fish like trout
or eel, haddock or herring, illustrations are not wanting of what might
be called pretty pieces of behaviour. Let us select a few cases.
The Stickleback's Nest
The three-spined and two-spined sticklebacks live equally well in fresh
or salt water; the larger fifteen-spined stickleback is entirely marine.
In all three species the male fish makes a nest, in fresh or brackish
water in the first two cases, in shore-pools in the third case. The
little species use the leaves and stems of water-plants; the larger
species use seaweed and zoophyte. The leaves or fronds are entangled
together and fastened by glue-like threads, secreted, strange to say, by
the kidneys. It is just as if a temporary diseased condition had been
regularised and turned to good purpose. Going through the nest several
times, the male makes a little room in the middle. Partly by coercion
and partly by coaxing he induces a female--first one and then
another--to pass through the nest with two doors, depositing eggs during
her short sojourn. The females go their way, and the male mounts guard
over the nest. He drives off intruding fishes much bigger than himself.
When the young are hatched, the male has for a time much to do, keeping
his charges within bounds until they are able to move about with
agility. It seems that sticklebacks are short-lived fishes, probably
breeding only once; and it is reasonable to suppose that their success
as a race depends to some extent on the paternal care. Now if we could
believe that the nesting behaviour had appeared suddenly in its present
form, we should be inclined to credit the fish with considerable mental
ability. But we are less likely to be so generous if we reflect that the
routine has been in all likelihood the outcome of a long racial process
of slight improvements and critical testings. The secretion of the glue
probably came about as a pathological variation; its utilisation was
perhaps discovered by accident; the types that had wit enough to take
advantage of this were most successful; the routine became enregistered
hereditarily. The stickleback is not so clever as it looks.
[Illustration: _Photo: Imperial War Museum._
HOMING PIGEON
A blue chequer hen, which during the War (in September of 1918) flew 22
miles in as many minutes, saving the crew of an aeroplane in
difficulties.]
[Illustration: _Photo: Imperial War Museum._
CARRIER PIGEON
Carrier pigeons were much used in the War to carry messages. The
photograph shows how the message is fixed to the carrier pigeon's leg,
in the form of light rings.]
[Illustration: _Photo: James's Press Agency._
YELLOW-CROWNED PENGUIN
Notice the flightless wings turned into flippers, which are often
flapped very vigorously. The very strong feet are also noteworthy.
Penguins are mostly confined to the Far South.]
[Illustration: _Photo: Cagcombe & Co._
PENGUINS ARE "A PECULIAR PEOPLE"
Their wings have been turned into flippers for swimming in the sea and
tobogganing on snow. The penguins come back over hundreds of miles of
trackless waste to their birthplace, where they breed. When they reach
the Antarctic shore they walk with determination to a suitable site,
often at the top of a steep cliff. Some species waddle 130 steps per
minute, 6 inches per step, two-thirds of a mile per hour.]
The Mind of a Minnow
To find solid ground on which to base an appreciation of the behaviour
of fishes, it is necessary to experiment, and we may refer to Miss
Gertrude White's interesting work on American minnows and sticklebacks.
After the fishes had become quite at home in their artificial
surroundings, their lessons began. Cloth packets, one of which contained
meat and the other cotton, were suspended at opposite ends of the
aquarium. The mud-minnows did not show that they perceived either
packet, though they swam close by them; the sticklebacks were intrigued
at once. Those that went towards the packet containing meat darted
furiously upon it and pulled at it with great excitement. Those that
went towards the cotton packet turned sharply away when they were within
about two inches off. They then perceived what those at the other end
were after and joined them--a common habit amongst fishes. Although the
minnows were not interested in the tiny "bags of mystery," they were
even more alert than the sticklebacks in perceiving moving objects in or
on the water, and there is no doubt that both these shallow-water
species discover their food largely by sense of sight.
The next set of lessons had to do with colour-associations. The fishes
were fed on minced snail, chopped earthworm, fragments of liver, and the
like, and the food was given to them from the end of forceps held above
the surface of the water, so that the fishes could not be influenced by
smell. They had to leap out of the water to take the food from the
forceps. Discs of coloured cardboard were slipped over the end of the
forceps, so that what the fishes saw was a morsel of food in the centre
of a coloured disc. After a week or so of preliminary training, they
were so well accustomed to the coloured discs that the presentation of
one served as a signal for the fishes to dart to the surface and spring
out of the water. When baits of paper were substituted for the food, the
fishes continued to jump at the discs. When, however, a blue disc was
persistently used for the paper bait and a red disc for the real food,
or _vice versa_, some of the minnows learned to discriminate infallibly
between shadow and substance, both when these were presented alternately
and when they were presented simultaneously. This is not far from the
dawn of mind.
In the course of a few lessons, both minnows and sticklebacks learned to
associate particular colours with food, and other associations were also
formed. A kind of larva that a minnow could make nothing of after
repeated trials was subsequently ignored. The approach of the
experimenter or anyone else soon began to serve as a food-signal. There
can be no doubt that in the ordinary life of fishes there is a process
of forming useful associations and suppressing useless responses. Given
an inborn repertory of profitable movements that require no training,
given the power of forming associations such as those we have
illustrated, and given a considerable degree of sensory alertness along
certain lines, fishes do not require much more. And in truth they have
not got it. Moving with great freedom in three dimensions in a medium
that supports them and is very uniform and constant, able in most cases
to get plenty of food without fatiguing exertions and to dispense with
it for considerable periods if it is scarce, multiplying usually in
great abundance so that the huge infantile mortality hardly counts,
rarely dying a natural death but usually coming with their strength
unabated to a violent end, fishes hold their own in the struggle for
existence without much in the way of mental endowment. Their brain has
more to do with motion than with mentality, and they have remained at a
low psychical level.
Yet just as we should greatly misjudge our own race if we confined our
attention to everyday routine, so in our total, as distinguished from
our average, estimate of fishes, we must remember the salmon surmounting
the falls, the wary trout eluding the angler's skill, the common
mud-skipper (Periophthalmus) of many tropical shores which climbs on the
rocks and the roots of the mangrove-trees, or actively hunts small
shore-animals. We must remember the adventurous life-history of the eel
and the quaint ways in which some fishes, males especially, look after
their family. The male sea-horse puts the eggs in his breast-pocket; the
male Kurtus carries them on the top of his head; the cock-paidle or
lumpsucker guards them and aerates them in a corner of a shore-pool.
§ 3
The Mind of Amphibians
Towards the end of the age of the Old Red Sandstone or Devonian, a great
step in evolution was taken--the emergence of Amphibians. The earliest
representatives had fish-like characters even more marked than those
which may be discerned in the tadpoles of our frogs and toads, and there
is no doubt that amphibians sprang from a fish stock. But they made
great strides, associated in part with their attempts to get out of the
water on to dry land. From fossil forms we cannot say much in regard to
soft parts; but if we consider the living representatives of the class,
we may credit amphibians with such important acquisitions as fingers and
toes, a three-chambered heart, true ventral lungs, a drum to the ear, a
mobile tongue, and vocal cords. When animals began to be able to grasp
an object and when they began to be able to utter sufficient sounds, two
new doors were opened. Apart from insects, whose instrumental music had
probably begun before the end of the Devonian age, amphibians were the
first animals to have a voice. The primary meaning of this voice was
doubtless, as it is to-day in our frogs, a sex-call; but it was the
beginning of what was destined to play a very important part in the
evolution of the mind. In the course of ages the significance of the
voice broadened out; it became a parental call; it became an infant's
cry. Broadening still, it became a very useful means of recognition
among kindred, especially in the dark and in the intricacies of the
forest. Ages passed, and the voice rose on another turn of the
evolutionary spiral to be expressive of particular emotions beyond the
immediate circle of sex--emotions of joy and of fear, of jealousy and of
contentment. Finally, we judge, the animal--perhaps the bird was
first--began to give utterance to particular "words," indicative not
merely of emotions, but of particular things with an emotional halo,
such as "food," "enemy," "home." Long afterwards, words became _in man_
the medium of reasoned discourse. Sentences were made and judgments
expressed. But was not the beginning in the croaking of Amphibia?
Senses of Amphibians
Frogs have good eyes, and the toad's eyes are "jewels." There is
evidence of precise vision in the neat way in which a frog catches a
fly, flicking out its tongue, which is fixed in front and loose behind.
There is also experimental proof that a frog discriminates between red
and blue, or between red and white, and an interesting point is that
while our skin is sensitive to heat rays but not to light, the skin of
the frog answers back to light rays as well. Professor Yerkes
experimented with a frog which had to go through a simple labyrinth if
it wished to reach a tank of water. At the first alternative between two
paths, a red card was placed on the wrong side and a white one on the
other. When the frog had learned to take the correct path, marked by the
white card, Prof. Yerkes changed the cards. The confusion of the frog
showed how thoroughly it had learned its lesson.
We know very little in regard to sense of smell or taste in amphibians;
but the sense of hearing is well developed, more developed than might be
inferred from the indifference that frogs show to almost all sounds
except the croaking of their kindred and splashes in the water.
The toad looks almost sagacious when it is climbing up a bank, and some
of the tree-frogs are very alert; but there is very little that we dare
say about the amphibian mind. We have mentioned that frogs may learn the
secret of a simple maze, and toads sometimes make for a particular
spawning-pond from a considerable distance. But an examination of their
brains, occupying a relatively small part of the broad, flat skull,
warns us not to expect much intelligence. On the other hand, when we
take frogs along a line that is very vital to them, namely, the
discrimination of palatable and unpalatable insects, we find, by
experiment, that they are quick to learn and that they remember their
lessons for many days. Frogs sometimes deposit their eggs in very
unsuitable pools of water; but perhaps that is not quite so stupid as it
looks. The egg-laying is a matter that has been, as it were, handed over
to instinctive registration.
[Illustration: _Photo: W. S. Berridge._
HARPY-EAGLE
"Clean and dainty and proud as a Spanish Don."
It is an arboreal and cliff-loving bird, feeding chiefly on mammals,
very fierce and strong. The under parts are mostly white, with a greyish
zone on the chest. The upper parts are blackish-grey. The harpy occurs
from Mexico to Paraguay and Bolivia.]
[Illustration: _Photo: W. S. Berridge, F.Z.S._
THE DINGO OR WILD DOG OF AUSTRALIA, PERHAPS AN INDIGENOUS WILD SPECIES,
PERHAPS A DOMESTICATED DOG THAT HAS GONE WILD OR FERAL
It does much harm in destroying sheep. It is famous for its persistent
"death-feigning," for an individual has been known to allow part of its
skin to be removed, in the belief that it was dead, before betraying its
vitality.]
[Illustration: WOODPECKER, HAMMERING AT A COTTON-REEL, ATTACHED TO A
TREE
Notice how the stiff tail-feathers braced against the stem help the bird
to cling on with its toes. The original hole, in which this woodpecker
inserted nuts for the purposes of cracking the shell and extracting the
kernel, is seen towards the top of the tree. But the taker of the
photograph tied on a hollowed-out cotton-reel as a receptacle for a nut,
and it was promptly discovered and used by the bird.]
Experiments in Parental Care
It must be put to the credit of amphibians that they have made many
experiments in methods of parental care, as if they were feeling their
way to new devices. A common frog lays her clumps of eggs in the cradle
of the water, sometimes far over a thousand together; the toad winds two
long strings round and between water-weeds; and in both cases that is
all. There is no parental care, and the prolific multiplication covers
the enormous infantile mortality. This is the spawning solution of the
problem of securing the continuance of the race. But there is another
solution, that of parental care associated with an economical reduction
of the number of eggs. Thus the male of the Nurse-Frog (Alytes), not
uncommon on the Continent, fixes a string of twenty to fifty eggs to the
upper part of his hind-legs, and retires to his hole, only coming out at
night to get some food and to keep up the moisture about the eggs. In
three weeks, when the tadpoles are ready to come out, he plunges into
the pond and is freed from his living burden and his family cares. In
the case of the thoroughly aquatic Surinam Toad (Pipa), the male helps
to press the eggs, perhaps a hundred in number, on to the back of the
female, where each sinks into a pocket of skin with a little lid. By and
by fully formed young toads jump out of the pockets.
In the South American tree-frogs called Nototrema there is a pouch on
the back of the female in which the eggs develop, and it is interesting
to find that in some species what come out are ordinary tadpoles, while
in other species the young emerge as miniatures of their parents.
Strangest of all, perhaps, is the case of Darwin's Frog (Rhinoderma of
Chili), where the young, about ten to fifteen in number, develop in the
male's croaking-sacs, which become in consequence enormously distended.
Eventually the strange spectacle is seen of miniature frogs jumping out
of their father's mouth. Needless to say we are not citing these methods
of parental care as examples of intelligence; but perhaps they correct
the impression of amphibians as a rather humdrum race. Whatever be the
mental aspect of the facts, there has certainly been some kind of
experimenting, and the increase of parental care, so marked in many
amphibians, with associated reduction of the number of offspring is a
finger-post on the path of progress.
§ 4
The Reptilian Mind
We speak of the wisdom of the serpent; but it is not very easy to
justify the phrase. Among all the multitude of reptiles--snakes,
lizards, turtles, and crocodiles, a motley crowd--we cannot see much
more than occasional traces of intelligence. The inner life remains a
tiny rill.
No doubt many reptiles are very effective; but it is an instinctive
rather than an intelligent efficiency. The well-known "soft-shell"
tortoise of the United States swims with powerful strokes and runs so
quickly that it can hardly be overtaken. It hunts vigorously for
crayfish and insect larvæ in the rivers. It buries itself in the mud
when cold weather comes. It may lie on a floating log ready to slip into
the water at a moment's notice; it may bask on a sunny bank or in the
warm shallows. Great wariness is shown in choosing times and places for
egg-laying. The mother tramps the earth down upon the buried eggs. All
is effective. Similar statements might be made in regard to scores of
other reptiles; but what we see is almost wholly of the nature of
instinctive routine, and we get little glimpse of more than efficiency
and endeavour.
In a few cases there is proof of reptiles finding their way back to
their homes from a considerable distance, and recognition of persons is
indubitable. Gilbert White remarks of his tortoise: "Whenever the good
old lady came in sight who had waited on it for more than thirty years,
it always hobbled with awkward alacrity towards its benefactress, while
to strangers it was altogether inattentive." Of definite learning there
are a few records. Thus Professor Yerkes studied a sluggish turtle of
retiring disposition, taking advantage of its strong desire to efface
itself. On the path of the darkened nest of damp grass he interposed a
simple maze in the form of a partitioned box. After wandering about
constantly for thirty-five minutes the turtle found its way through the
maze by chance. Two hours afterwards it reached the nest in fifteen
minutes; and after another interval of two hours it only required five
minutes. After the third trial, the routes became more direct, there was
less aimless wandering. The time of the twentieth trial was forty-five
seconds; that of the thirtieth, forty seconds. In the thirtieth case,
the path followed was quite direct, and so it was on the fiftieth trip,
which only required thirty-five seconds. Of course, the whole thing did
not amount to very much; but there was a definite learning, _a learning
from experience_, which has played an important part in the evolution of
animal behaviour.
Comparing reptiles with amphibians, we may recognise an increased
masterliness of behaviour and a hint of greater plasticity. The records
of observers who have made pets of reptiles suggest that the life of
feeling or emotion is growing stronger, and so do stories, if they can
be accepted, which suggest the beginning of conjugal affection.
The error must be guarded against of interpreting in terms of
intelligence what is merely the outcome of long-continued structure
adaptation. When the limbless lizard called the Slow-worm is suddenly
seized by the tail, it escapes by surrendering the appendage, which
breaks across a preformed weak plane. But this is a reflex action, not a
reflective one. It is comparable to our sudden withdrawal of our finger
from a very hot cinder. The Egg-eating African snake Dasypeltis gets the
egg of a bird into its gullet unbroken, and cuts the shell against
downward-projecting sharp points of the vertebræ. None of the precious
contents is lost and the broken "empties" are returned. It is admirable,
indeed unsurpassable; but it is not intelligent.
§ 5
Mind in Birds
Sight and hearing are highly developed in birds, and the senses, besides
pulling the triggers of inborn efficiencies, supply the raw materials
for intelligence. There is some truth, though not the whole truth, in
the old philosophical dictum, that there is nothing in the intellect
which was not previously in the senses. Many people have admired the
certainty and alacrity with which gulls pick up a fragment of biscuit
from the white wake of a steamer, and the incident is characteristic. In
their power of rapidly altering the focus of the eye, birds are
unsurpassed.
To the sense of sight in birds, the sense of hearing comes a good
second. A twig breaks under our feet, and out sounds the danger-call of
the bird we were trying to watch. Many young birds, like partridges,
respond when two or three hours old to the anxious warning note of the
parents, and squat motionless on the ground, though other sounds, such
as the excited clucking of a foster-mother hen, leave them indifferent.
They do not know what they are doing when they squat; they are obeying
the living hand of the past which is within them. Their behaviour is
instinctive. But the present point is the discriminating quality of the
sense of hearing; and that is corroborated by the singing of birds.
It is emotional art, expressing feelings in the medium of sound. On the
part of the females, who are supposed to listen, it betokens a
cultivated ear.
[Illustration: THE BEAVER
The beaver will gnaw through trees a foot in diameter; to save itself
more trouble than is necessary, it will stop when it has gnawed the
trunk till there is only a narrow core left, having the wit to know that
the autumn gales will do the rest.]
[Illustration: _Photo: F. R. Hinkins & Son._
THE THRUSH AT ITS ANVIL
The song-thrush takes the snail's shell in its bill, and knocks it
against a stone until it breaks, making the palatable flesh available.
Many broken shells are often found around the anvil.]
As to the other senses, touch is not highly developed except about the
bill, where it reaches a climax in birds like the wood-cock, which probe
for unseen earthworms in the soft soil. Taste seems to be poorly
developed, for most birds bolt their food, but there is sometimes an
emphatic rejection of unpalatable things, like toads and caterpillars.
Of smell in birds little is known, but it has been proved to be present
in certain cases, e.g. in some nocturnal birds of prey. It seems certain
that it is by sight, not by smell, that the eagles gather to the
carcass; but perhaps there is more smell in birds than they are usually
credited with. One would like to experiment with the oil from the preen
gland of birds to see whether the scent of this does not help in the
recognition of kin by kin at night or amid the darkness of the forest.
There may be other senses in birds, such as a sense of temperature and a
sense of balance; but no success has attended the attempts made to
demonstrate a magnetic sense, which has been impatiently postulated by
students of bird migration in order to "explain" how the birds find
their way. The big fact is that in birds there are two widely open
gateways of knowledge, the sense of sight and the sense of hearing.
Instinctive Aptitudes
Many a young water-bird, such as a coot, swims right away when it is
tumbled into water for the first time. So chicks peck without any
learning or teaching, very young ducklings catch small moths that flit
by, and young plovers lie low when the danger-signal sounds. But birds
seem strangely limited as regards many of these instinctive
capacities--limited when compared with the "little-brained" ants and
bees, which have from the first such a rich repertory of ready-made
cleverness. The limitation in birds is of great interest, for it means
that intelligence is coming to its own and is going to take up the
reins at many corners of the daily round. Professor Lloyd Morgan
observed that his chickens incubated in the laboratory had no
instinctive awareness of the significance of their mother's cluck when
she was brought outside the door. Although thirsty and willing to drink
from a moistened finger-tip, they did not instinctively recognize water,
even when they walked through a saucerful. Only when they happened to
peck their toes as they stood in the water did they appreciate water as
the stuff they wanted, and raise their bills up to the sky. Once or
twice they actually stuffed their crops with "worms" of red worsted!
Instinctive aptitudes, then, the young birds have, but these are more
limited than in ants, bees, and wasps; and the reason is to be found in
the fact that the brain is now evolving on the tack of what Sir Ray
Lankester has called "educability." Young birds _learn_ with prodigious
rapidity; the emancipation of the mind from the tyranny of hereditary
obligations has begun. Young birds make mistakes, like the red worsted
mistake, but they do not make the same mistakes often. They are able to
profit by experience in a very rapid way. We do not mean that creatures
of the little-brain type, like ants, bees, and wasps, are unable to
profit by experience or are without intelligence. There are no such
hard-and-fast lines. We mean that in the ordinary life of insects the
enregistered instinctive capacities are on the whole sufficient for the
occasion, and that intelligent educability is very slightly developed.
Nor do we mean that birds are quite emancipated from the tyranny of
engrained instinctive obligations, and can always "ring up" intelligence
in a way that is impossible for the stereotyped bee. The sight of a
pigeon brooding on an empty nest, while her two eggs lie disregarded
only a couple of inches away, is enough to show that along certain lines
birds may find it impossible to get free from the trammels of instinct.
The peculiar interest of birds is that they have many instincts and yet
a notable power of learning intelligently.
Intelligence co-operating with Instinct
Professor Lloyd Morgan was foster-parent to two moorhens which grew up
in isolation from their kindred. They swam instinctively, but they would
not dive, neither in a large bath nor in a current. But it happened one
day when one of these moorhens was swimming in a pool on a Yorkshire
stream, that a puppy came barking down the bank and made an awkward
feint towards the young bird. In a moment the moorhen dived, disappeared
from view, and soon partially reappeared, his head just peeping above
the water beneath the overhanging bank. This was the first time the bird
had dived, and the performance was absolutely true to type.
There can be little doubt as to the meaning of this observation. The
moorhen has an hereditary or instinctive capacity for swimming and
diving, but the latter is not so easily called into activity as the
former. The particular moorhen in question had enjoyed about two months
of swimming experience, which probably counted for something, but in the
course of that experience nothing had pulled the trigger of the diving
capacity. On an eventful day the young moorhen saw and heard the dog; it
was emotionally excited; it probably did to some extent intelligently
appreciate a novel and meaningful situation. Intelligence cooperated
with instinct, and the bird dived appropriately.
Birds have inborn predispositions to certain effective ways of pecking,
scratching, swimming, diving, flying, crouching, lying low,
nest-building, and so on; but they are marked off from the much more
purely instinctive ants and bees by the extent to which individual
"nurture" seems to mingle with the inherited "nature." The two together
result in the fine product which we call the bird's behaviour. After
Lloyd Morgan's chicks had tried a few conspicuous and unpalatable
caterpillars, they had no use for any more. They learned in their early
days with prodigious rapidity, illustrating the deep difference between
the "big-brain" type, relatively poor in its endowment of instinctive
capacities, but eminently "educable," and the "little-brain" type, say,
of ants and bees, richly endowed with instinctive capacities, but very
far from being quick or glad to learn. We owe it to Sir Ray Lankester to
have made it clear that these two types of brain are, as it were, on
different tacks of evolution, and should not be directly pitted against
one another. The "little-brain" type makes for a climax in the ant,
where instinctive behaviour reaches a high degree of perfection; the
"big-brain" type reaches its climax in horse and dog, in elephant and
monkey. The particular interest that attaches to the behaviour of birds
is in the combination of a good deal of instinct with a great deal of
intelligent learning. This is well illustrated when birds make a nest
out of new materials or in some quite novel situation. It is clearly
seen when birds turn to some new kind of food, like the Kea parrot,
which attacks the sheep in New Zealand.
Some young woodpeckers are quite clever in opening fir cones to get at
the seeds, and this might be hastily referred to a well-defined
hereditary capacity. But the facts are that the parents bring their
young ones first the seeds themselves, then partly opened cones, and
then intact ones. There is an educative process, and so it is in scores
of cases.
Using their Wits
When the Greek eagle lifts the Greek tortoise in its talons, and lets it
fall from a height so that the strong carapace is broken and the flesh
exposed, it is making intelligent use of an expedient. Whether it
discovered the expedient by experimenting, as is possible, or by chance,
as is more likely, it uses it intelligently. In the same way
herring-gulls lift sea-urchins and clams in their bills, and let them
fall on the rocks so that the shells are broken. In the same way rooks
deal with freshwater mussels.
The Thrush's Anvil
A very instructive case is the behaviour of the song-thrush when it
takes a wood-snail in its beak and hammers it against a stone, its
so-called anvil. To a young thrush, which she had brought up by hand,
Miss Frances Pitt offered some wood-snails, but it took no interest in
them until one put out its head and began to move about. The bird then
pecked at the snail's horns, but was evidently puzzled when the creature
retreated within the shelter of the shell. This happened over and over
again, the thrush's inquisitive interest increasing day by day. It
pecked at the shell and even picked it up by the lip, but no real
progress was made till the sixth day, when the thrush seized the snail
and beat it on the ground as it would a big worm. On the same day it
picked up a shell and knocked it repeatedly against a stone, trying
first one snail and then another. After fifteen minutes' hard work, the
thrush managed to break one, and after that it was all easy. A certain
predisposition to beat things on the ground was doubtless present, but
the experiment showed that the use of an anvil could be arrived at by an
untutored bird. After prolonged trying it found out how to deal with a
difficult situation. It may be said that in more natural conditions this
might be picked up by imitation, but while this is quite possible, it is
useful to notice that experiments with animals lead us to doubt whether
imitation counts for nearly so much as used to be believed.
§ 6
The Mind of the Mammal
When we watch a collie at a sheep-driving competition, or an elephant
helping the forester, or a horse shunting waggons at a railway siding,
we are apt to be too generous to the mammal mind. For in the cases we
have just mentioned, part of man's mind has, so to speak, got into the
animal's. On the other hand, when we study rabbits and guinea-pigs, we
are apt to be too stingy, for these rodents are under the average of
mammals, and those that live in domestication illustrate the stupefying
effect of a too sheltered life. The same applies to domesticated sheep
contrasted with wild sheep, or even with their own lambs. If we are to
form a sound judgment on the intelligence of mammals we must not attend
too much to those that have profited by man's training, nor to those
whose mental life has been dulled by domestication.
Instinctive Aptitudes
What is to be said of the behaviour of beavers who gnaw the base of a
tree with their chisel-edged teeth till only a narrow core is left--to
snap in the first gale, bringing the useful branches down to the ground?
What is to be said of the harvest-mouse constructing its nest, or of the
squirrel making cache after cache of nuts? These and many similar pieces
of behaviour are fundamentally instinctive, due to inborn
predispositions of nerve-cells and muscle-cells. But in mammals they
seem to be often attended by a certain amount of intelligent attention,
saving the creature from the tyranny of routine so marked in the ways of
ants and bees.
Sheer Dexterity
Besides instinctive aptitudes, which are exhibited in almost equal
perfection by all the members of the same species, there are acquired
dexterities which depend on individual opportunities. They are also
marked by being outside and beyond ordinary routine--not that any
rigorous boundary line can be drawn. We read that at Mathura on the
Jumna doles of food are provided by the piety of pilgrims for the sacred
river-tortoises, which are so crowded when there is food going that
their smooth carapaces form a more or less continuous raft across the
river. On that unsteady slippery bridge the Langur monkeys
(_Semnopithecus entellus_) venture out and in spite of vicious snaps
secure a share of the booty. This picture of the monkeys securing a
footing on the moving mass of turtle-backs is almost a diagram of sheer
dexterity. It illustrates the spirit of adventure, the will to
experiment, which is, we believe, the main motive-force in new
departures in behaviour.
[Illustration: _Photo: Lafayette_
ALSATIAN WOLF-DOG
An animal of acute senses and great intelligence. It was of great
service in the war.
(The dog shown, Arno von Indetal, is a trained police dog and did
service abroad during the war.)]
[Illustration: _Photo: W. S. Berridge._
THE POLAR BEAR OF THE FAR NORTH
An animal of extraordinary strength, able with a stroke of its paw to
lift a big seal right out of the water and send it crashing along the
ice. The food consists chiefly of seals. The sexes wander separately. A
hole is often dug as a winter retreat, but there is no hibernation. A
polar bear in captivity has been seen making a current with its paw in
the water of its pool in order to secure floating buns without
trouble--an instance of sheer intelligence.]
[Illustration: _From the Smithsonian Report_, 1914
AN ALLIGATOR "YAWNING" IN EXPECTATION OF FOOD
Note the large number of sharp conical teeth fixed in sockets along the
jaws.]
Power of Association
A bull-terrier called Jasper, studied by Prof. J. B. Watson, showed
great power of associating certain words with certain actions. From a
position invisible to the dog the owner would give certain commands,
such as "Go into the next room and bring me a paper lying on the floor."
Jasper did this at once, and a score of similar things.
Lord Avebury's dog Van was accustomed to go to a box containing a small
number of printed cards and select the card TEA or OUT, as the occasion
suggested. It had established an association between certain black marks
on a white background and the gratification of certain desires. It is
probable that some of the extraordinary things horses and dogs have been
known to do in the way of stamping a certain number of times in supposed
indication of an answer to an arithmetical question (in the case of
horses), or of the name of an object drawn (in the case of dogs), are
dependent on clever associations established by the teacher between
minute signs and a number of stampings. What is certain is that mammals
have in varying degrees a strong power of establishing associations.
There is often some delicacy in the association established. Everyone
knows of cases where a dog, a cat, or a horse will remain quite
uninterested, to all appearance, in its owner's movements until some
little detail, such as taking a key from its peg, pulls the trigger. Now
the importance of this in the wild life of the fox or the hare, the
otter or the squirrel, is obviously that the young animals learn to
associate certain sounds in their environment with definite
possibilities. They have to learn an alphabet of woodcraft, the letters
of which are chiefly sounds and scents.
The Dancing Mouse as a Pupil
The dancing or waltzing mouse is a Japanese variety with many
peculiarities, such as having only one of the three semicircular canals
of the ear well developed. It has a strong tendency to waltz round and
round in circles without sufficient cause and to trip sideways towards
its dormitory instead of proceeding in the orthodox head-on fashion. But
this freak is a very educable creature, as Professor Yerkes has shown.
In a careful way he confronted his mouse-pupil with alternative pathways
marked by different degrees of illumination, or by different colours. If
the mouse chose compartment A, it found a clear passage direct to its
nest; if it chose compartment B, it was punished by a mild electric
shock and it had to take a roundabout road home. Needless to say, the A
compartment was sometimes to the right hand, sometimes to the left, else
mere position would have been a guide. The experiments showed that the
dancing mice learn to discriminate the right path from the wrong, and
similar results have been got from other mammals, such as rats and
squirrels. There is no proof of learning by ideas, but there is proof of
learning by experience. And the same must be true in wild life.
Many mammals, such as cats and rats, learn how to manipulate
puzzle-boxes and how to get at the treasure at the heart of a Hampton
Court maze. Some of the puzzle-boxes, with a reward of food inside, are
quite difficult, for the various bolts and bars have to be dealt with in
a particular order, and yet many mammals master the problem. What is
plain is that they gradually eliminate useless movements, that they make
fewer and fewer mistakes, that they eventually succeed, and that they
register the solution within themselves so that it remains with them for
a time. It looks a little like the behaviour of a man who learns a game
of skill without thinking. It is a learning by experience, not by ideas
or reflection. Thus it is very difficult to suppose that a rat or a cat
could form any idea or even picture of the Hampton Court maze--which
they nevertheless master.
Learning Tricks
Given sufficient inducement many of the cleverer mammals will learn to
do very sensible things, and no one is wise enough to say that they
never understand what they are doing. Yet it is certain that trained
animals often exhibit pieces of behaviour which are not nearly so clever
as they look. The elephant at the Belle Vue Gardens in Manchester used
to collect pennies from benevolent visitors. When it got a penny in its
trunk it put it in the slot of an automatic machine which delivered up a
biscuit. When a visitor gave the elephant a halfpenny it used to throw
it back with disgust. At first sight this seemed almost wise, and there
was no doubt some intelligent appreciation of the situation. But it was
largely a matter of habituation, the outcome of careful and prolonged
training. The elephant was laboriously taught to put the penny in the
slot and to discriminate between the useful pennies and the useless
halfpennies. It was not nearly so clever as it looked.
Using their Wits
In the beautiful Zoological Park in Edinburgh the Polar Bear was wont to
sit on a rocky peninsula of a water-filled quarry. The visitors threw in
buns, some of which floated on the surface. It was often easy for the
Polar Bear to collect half a dozen by plunging into the pool. But it had
discovered a more interesting way. At the edge of the peninsula it
scooped the water gently with its huge paw and made a current which
brought the buns ashore. This was a simple piece of behaviour, but it
has the smack of intelligence--of putting two and two together in a
novel way. It suggests the power of making what is called a "perceptual
inference."
On the occasion of a great flood in a meadow it was observed that a
number of mares brought their foals to the top of a knoll, and stood
round about them protecting them against the rising water. A dog has
been known to show what was at any rate a plastic appreciation of a
varying situation in swimming across a tidal river. It changed its
starting-point, they say, according to the flow or ebb of the tide.
Arctic foxes and some other wild mammals show great cleverness in
dealing with traps, and the manipulative intelligence of elephants is
worthy of all our admiration.
§ 7
Why is there not more Intelligence?
When we allow for dexterity and power of association, when we recognise
a certain amount of instinctive capacity and a capacity for profiting by
experience in an intelligent way, we must admit a certain degree of
disappointment when we take a survey of the behaviour of mammals,
especially of those with very fine brains, from which we should
naturally expect great things. Why is there not more frequent exhibition
of intelligence in the stricter sense?
The answer is that most mammals have become in the course of time very
well adapted to the ordinary conditions of their life, and tend to leave
well alone. They have got their repertory of efficient answers to the
ordinary questions of everyday life, and why should they experiment? In
the course of the struggle for existence what has been established is
efficiency in normal circumstances, and therefore even the higher
animals tend to be no cleverer than is necessary. So while many mammals
are extraordinarily efficient, they tend to be a little dull. Their
mental equipment is adequate for the everyday conditions of their life,
but it is not on sufficiently generous lines to admit of, let us say, an
interest in Nature or adventurous experiment. Mammals always tend to
"play for safety."
We hasten, however, to insert here some very interesting saving clauses.
Experimentation in Play
A glimpse of what mammals are capable of, were it necessary, may be
obtained by watching those that are playful, such as lambs and kids,
foals and calves, young foxes and others. For these young creatures let
themselves go irresponsibly, they are still unstereotyped, they test
what they and their fellows can do. The experimental character of much
of animal play is very marked.
It is now recognised by biologists that play among animals is the young
form of work, and that the playing period, often so conspicuous, is
vitally important as an apprenticeship to the serious business of life
and as an opportunity for learning the alphabet of Nature. But the
playing period is much more; it is one of the few opportunities animals
have of making experiments without too serious responsibilities. Play is
Nature's device for allowing elbow-room for new departures
(behaviour-variations) which may form part of the raw materials of
progress. Play, we repeat, gives us a glimpse of the possibilities of
the mammal mind.
Other Glimpses of Intelligence
A squirrel is just as clever as it needs to be and no more; and of some
vanishing mammals, like the beaver, not even this can be said. Humdrum
non-plastic efficiency is apt to mean stagnation. Now we have just seen
that in the play of young mammals there is an indication of unexhausted
possibilities, and we get the same impression when we think of three
other facts. (_a_) In those mammals, like dog and horse, which have
entered into active cooperative relations with man, we see that the mind
of the mammal is capable of much more than the average would lead us to
think. When man's sheltering is too complete and the domesticated
creature is passive in his grip, the intelligence deteriorates. (_b_)
When we study mammals, like the otter, which live a versatile life in a
very complex and difficult environment, we get an inspiriting picture of
the play of wits. (_c_) Thirdly, when we pass to monkeys, where the
fore-limb has become a free hand, where the brain shows a relatively
great improvement, where "words" are much used, we cannot fail to
recognise the emergence of something new--a restless inquisitiveness, a
desire to investigate the world, an unsatisfied tendency to experiment.
We are approaching the Dawn of Reason.
THE MIND OF MONKEYS
§ 8
There is a long gamut between the bushy-tailed, almost squirrel-like
marmosets and the big-brained chimpanzee. There is great variety of
attainment at different levels in the Simian tribe.
Keen Senses
To begin at the beginning, it is certain that monkeys have a first-class
sensory equipment, especially as regards sight, hearing, and touch. The
axes of the two eyes are directed forwards as in man, and a large
section of the field of vision is common to both eyes. In other words,
monkeys have a more complete stereoscopic vision than the rest of the
mammals enjoy. They look more and smell less. They can distinguish
different colours, apart from different degrees of brightness in the
coloured objects. They are quick to discriminate differences in the
shapes of things, e.g. boxes similar in size but different in shape, for
if the prize is always put in a box of the same shape they soon learn
(by association) to select the profitable one. They learn to
discriminate cards with short words or with signs printed on them,
coming down when the "Yes" card is shown, remaining on their perch when
the card says "No." Bred to a forest life where alertness is a
life-or-death quality, they are quick to respond to a sudden movement or
to pick out some new feature in their surroundings. And what is true of
vision holds also for hearing.
Power of Manipulation
Another quality which separates monkeys very markedly from ordinary
mammals is their manipulative expertness, the co-ordination of hand
and eye. This great gift follows from the fact that among monkeys the
fore-leg has been emancipated. It has ceased to be indispensable as an
organ of support; it has become a climbing, grasping, lifting, handling
organ. The fore-limb has become a free hand, and everyone who knows
monkeys at all is aware of the zest with which they use their tool. They
enjoy pulling things to pieces--a kind of dissection--or screwing the
handle off a brush and screwing it on again.
[Illustration: _Photo: W. P. Dando_
BABY ORANG
Notice the small ears and the suggestion of good temper. The mother
orang will throw prickly fruits and pieces of branches at those who
intrude on her maternal care.]
[Illustration: _Photo: Gambier Bolton._
ORANG-UTAN
A large and heavy ape, frequenting forests in Sumatra and Borneo, living
mainly in trees, where a temporary nest is made. The expression is
melancholy, the belly very protuberant, the colour yellow-brown, the
movements are cautious and slow.]
[Illustration: 1. CHIMPANZEE
2. BABY ORANG-UTAN
3. ORANG-UTAN
4. BABY CHIMPANZEES
_Photos: James's Press Agency._
In his famous book on _The Expression of the Emotions in Man and
Animals_ (1872) Charles Darwin showed that many forms of facial
expression familiar in man have their counterparts in apes and other
mammals. He also showed how important the movements of expression are as
means of communication between mother and offspring, mate and mate, kith
and kin.
The anthropoid apes show notable differences of temperament as the
photographs show. The chimpanzee is lively, cheerful, and educable. The
orang is also mild of temper, but often and naturally appears melancholy
in captivity. This is not suggested, however, by our photograph of the
adult. Both chimpanzee and orang are markedly contrasted with the fierce
and gloomy gorilla.]
Activity for Activity's Sake
Professor Thorndike hits the nail on the head when he lays stress on the
intensity of activity in monkeys--activity both of body and mind. They
are pent-up reservoirs of energy, which almost any influence will tap.
Watch a cat or a dog, Professor Thorndike says; it does comparatively
few things and is content for long periods to do nothing. It will be
splendidly active in response to some stimulus such as food or a friend
or a fight, but if nothing appeals to its special make-up, which is very
utilitarian in its interests, it will do nothing. "Watch a monkey and
you cannot enumerate the things he does, cannot discover the stimuli to
which he reacts, cannot conceive the _raison d'etre_ of his pursuits.
Everything appeals to him. He likes to be active for the sake of
activity."
This applies to mental activity as well, and the quality is one of
extraordinary interest, for it shows the experimenting mood at a higher
turn of the spiral than in any other creature, save man. It points
forward to the scientific spirit. We cannot, indeed, believe in the
sudden beginning of any quality, and we recall the experimenting of
playing mammals, such as kids and kittens, or of inquisitive adults like
Kipling's mongoose, Riki-Tiki-Tavi, which made it his business in life
to find out about things. But in monkeys the habit of restless
experimenting rises to a higher pitch. They appear to be curious about
the world. The psychologist whom we have quoted tells of a monkey which
happened to hit a projecting wire so as to make it vibrate. He went on
repeating the performance hundreds of times during the next few days. Of
course, he got nothing out of it, save fun, but it was grist to his
mental mill. "The fact of mental life is to monkeys it own reward." The
monkey's brain is "tender all over, functioning throughout, set off in
action by anything and everything."
Sheer Quickness
Correlated with the quality of restless inquisitiveness and delight in
activity for its own sake there is the quality of quickness. We mean not
merely the locomotor agility that marks most monkeys, but quickness of
perception and plan. It is the sort of quality that life among the
branches will engender, where it is so often a case of neck or nothing.
It is the quality which we describe as being on the spot, though the
phrase has slipped from its original moorings. Speaking of his Bonnet
Monkey, an Indian macaque, second cousin to the kind that lives on the
Rock of Gibraltar, Professor S. J. Holmes writes: "For keenness of
perception, rapidity of action, facility in forming good practical
judgments about ways and means of escaping pursuit and of attaining
various other ends, Lizzie had few rivals in the animal world.... Her
perceptions and decisions were so much more rapid than my own that she
would frequently transfer her attention, decide upon a line of action,
and carry it into effect before I was aware of what she was about. Until
I came to guard against her nimble and unexpected manoeuvres, she
succeeded in getting possession of many apples and peanuts which I had
not intended to give her except upon the successful performance of some
task."
Quick to Learn
Quite fundamental to any understanding of animal behaviour is the
distinction so clearly drawn by Sir Ray Lankester between the
"little-brain" type, rich in inborn or instinctive capacities, but
relatively slow to learn, and the "big-brain" type, with a relatively
poor endowment of specialised instincts, but with great educability. The
"little-brain" type finds its climax in ants and bees; the "big-brain"
type in horses and dogs, elephants and monkeys. And of all animals
monkeys are the quickest to learn, if we use the word "learn" to mean
the formation of useful associations between this and that, between a
given sense-presentation and a particular piece of behaviour.
The Case of Sally
Some of us remember Sally, the chimpanzee at the "Zoo" with which Dr.
Romanes used to experiment. She was taught to give her teacher the
number of straws he asked for, and she soon learned to do so up to five.
If she handed a number not asked for, her offer was refused; if she gave
the proper number, she got a piece of fruit. If she was asked for five
straws, she picked them up individually and placed them in her mouth,
and when she had gathered five she presented them together in her hand.
Attempts to teach her to give six to ten straws were not very
successful. For Sally "above six" meant "many," and besides, her limits
of patience were probably less than her range of computation. This was
hinted at by the highly interesting circumstance that when dealing with
numbers above five she very frequently doubled over a straw so as to
make it present two ends and thus appear as two straws. The doubling of
the straw looked like an intelligent device to save time, and it was
persistently resorted to in spite of the fact that her teacher always
refused to accept a doubled straw as equivalent to two straws. Here we
get a glimpse of something beyond the mere association of a
sound--"Five"--and that number of straws.
The Case of Lizzie
The front of the cage in which Professor Holmes kept Lizzie was made of
vertical bars which allowed her to reach out with her arm. On a board
with an upright nail as handle, there was placed an apple--out of
Lizzie's reach. She reached immediately for the nail, pulled the board
in and got the apple. "There was no employment of the method of trial
and error; there was direct appropriate action following the perception
of her relation to board, nail, and apple." Of course her ancestors may
have been adepts at drawing a fruit-laden branch within their reach, but
the simple experiment was very instructive. All the more instructive
because in many other cases the experiments indicate a gradual sifting
out of useless movements and an eventful retention of the one that pays.
When Lizzie was given a vaseline bottle containing a peanut and closed
with a cork, she at once pulled the cork out with her teeth, obeying the
instinct to bite at new objects, but she never learned to turn the
bottle upside down and let the nut drop out. She often got the nut, and
after some education she got it more quickly than she did at first, but
there was no indication that she ever perceived the fit and proper way
of getting what she wanted. "In the course of her intent efforts her
mind seemed so absorbed with the object of desire that it was never
focussed on the means of attaining that object. There was no
deliberation, and no discrimination between the important and the
unimportant elements in her behaviour. The gradually increasing facility
of her performances depended on the apparently unconscious elimination
of useless movements." This may be called learning, but it is learning
at a very low level; it is far from learning by ideas; it is hardly even
learning by experiment; it is not more than learning by experience, it
is not more than fumbling at learning!
Trial and Error
A higher note is struck in the behaviour of some more highly endowed
monkeys. In many experiments, chiefly in the way of getting into boxes
difficult to open, there is evidence (1) of attentive persistent
experiment (2) of the rapid elimination of ineffective movements, and
(3) of remembering the solution when it was discovered. Kinnaman taught
two macaques the Hampton Court Maze, a feat which probably means a
memory of movements, and we get an interesting glimpse in his
observation that they began to smack their lips audibly when they
reached the latter part of their course, and began to feel, dare one
say, "We are right this time."
In getting into "puzzle-boxes" and into "combination-boxes" (where the
barriers must be overcome in a definite order), monkeys learn by the
trial and error method much more quickly than cats and dogs do, and a
very suggestive fact emphasized by Professor Thorndike is "a process of
sudden acquisition by a rapid, often apparently instantaneous
abandonment of the unsuccessful movements and selection of the
appropriate one, which rivals in suddenness the selections made by human
beings in similar performances." A higher note still was sounded by one
of Thorndike's monkeys which opened a puzzle-box at once, eight months
after his previous experience with it. For here was some sort of
registration of a solution.
Imitation
Two chimpanzees in the Dublin Zoo were often to be seen washing the two
shelves of their cupboard and "wringing" the wet cloth in the approved
fashion. It was like a caricature of a washerwoman, and someone said,
"What mimics they are!" Now we do not know whether that was or was not
the case with the chimpanzees, but the majority of the experiments that
have been made do not lead us to attach to imitation so much importance
as is usually given to it by the popular interpreter. There are
instances where a monkey that had given up a puzzle in despair returned
to it when it had seen its neighbour succeed, but most of the
experiments suggested that the creature has to find out for itself. Even
with such a simple problem as drawing food near with a stick, it often
seems of little use to show the monkey how it is done. Placing a bit of
food outside his monkey's cage, Professor Holmes "poked it about with
the stick so as to give her a suggestion of how the stick might be
employed to move the food within reach, but although the act was
repeated many times Lizzie never showed the least inclination to use the
stick to her advantage." Perhaps the idea of a "tool" is beyond the
Bonnet Monkey, yet here again we must be cautious, for Professor L. T.
Hobhouse had a monkey of the same macaque genus which learned in the
course of time to use a crooked stick with great effect.
The Case of Peter
Perhaps the cleverest monkey as yet studied was a performing chimpanzee
called Peter, which has been generally described by Dr. Lightner Witmer.
Peter could skate and cycle, thread needles and untie knots, smoke a
cigarette and string beads, screw in nails and unlock locks. But what
Peter was thinking about all the time it was hard to guess, and there is
very little evidence to suggest that his rapid power of putting two and
two together ever rose above a sort of concrete mental experimenting,
which Dr. Romanes used to call perceptual inference. Without supposing
that there are hard-and-fast boundary lines, we cannot avoid the general
conclusion that, while monkeys are often intelligent, they seldom, if
ever, show even hints of reason, i.e. of working or playing with general
ideas. That remains Man's prerogative.
The Bustle of the Mind
In mammals like otters, foxes, stoats, hares, and elephants, what a
complex of tides and currents there must be in the brain-mind! We may
think of a stream with currents at different levels. Lowest there are
the _basal appetites_ of hunger and sex, often with eddies rising to the
surface. Then there are the _primary emotions_, such as fear of
hereditary enemies and maternal affection for offspring. Above these are
_instinctive aptitudes_, inborn powers of doing clever things without
having to learn how. But in mammals these are often expressed along
with, or as it were through, the controlled life of _intelligent
activity_, where there is more clear-cut perceptual influence.
[Illustration: _Photo: W. P. Dando._
CHIMPANZEE
An African ape, at home in the equatorial forests, a lively and playful
creature, eminently educable.]
[Illustration: _Photo: W. S. Berridge._
YOUNG CHEETAHS, OR HUNTING LEOPARDS
Trained to hunt from time immemorial and quite easily tamed. Cheetahs
occur in India, Persia, Turkestan, and Africa.]
[Illustration: _Photo: C. Reid._
COMMON OTTER
One of the most resourceful of animals and the "most playsomest crittur
on God's earth." It neither stores nor hibernates, but survives in
virtue of its wits and because of the careful education of the young.
The otter is a roving animal, often with more than one resting-place; it
has been known to travel fifteen miles in a night.]
Higher still are the records or memories of individual experience and
the registration of individual habits, while on the surface is the
instreaming multitude of messages from the outside world, like raindrops
and hailstones on the stream, some of them penetrating deeply, being, as
we say, full of meaning. The mind of the higher animal is in some
respects like a child's mind, in having little in the way of clear-cut
ideas, in showing no reason in the strict sense, and in its
extraordinary educability, but it differs from the child's mind entirely
in the sure effectiveness of a certain repertory of responses. It is
efficient to a degree.
"Until at last arose the Man."
Man's brain is more complicated than that of the higher apes--gorilla,
orang, and chimpanzee--and it is relatively larger. But the improvements
in structure do not seem in themselves sufficient to account for man's
great advance in intelligence. The rill of inner life has become a swift
stream, sometimes a rushing torrent. Besides perceptual inference or
_Intelligence_--a sort of picture-logic, which some animals likewise
have--there is conceptual inference--or _Reason_--an internal
experimenting with general ideas. Even the cleverest animals, it would
seem, do not get much beyond playing with "particulars"; man plays an
internal game of chess with "universals." Intelligent behaviour may go a
long way with mental images; rational conduct demands general ideas. It
may be, however, that "percepts" and "concepts" differ rather in degree
than in kind, and that the passage from one to the other meant a higher
power of forming associations. A clever dog has probably a generalised
percept of man, as distinguished from a memory-image of the particular
men it has known, but man alone has the concept Man, or Mankind, or
Humanity. Experimenting with concepts or general ideas is what we call
Reason.
Here, of course, we get into deep waters, and perhaps it is wisest not
to attempt too much. So we shall content ourselves here with pointing
out that Man's advance in intelligence and from intelligence to reason
is closely wrapped up with his power of speech. What animals began--a
small vocabulary--he has carried to high perfection. But what is
distinctive is not the vocabulary so much as the habit of making
sentences, of expressing judgments in a way which admitted of
communication between mind and mind. The multiplication of words meant
much, the use of words as symbols of general ideas meant even more, for
it meant the possibility of playing the internal game of thinking; but
perhaps the most important advance of all was the means of comparing
notes with neighbours, of corroborating individual experience by social
intercourse. With words, also, it became easier to enregister outside
himself the gains of the past. It is not without significance that the
Greek Logos, which may be translated "the word," may also be translated
Mind.
§ 9
Looking Backwards
When we take a survey of animal behaviour we see a long inclined plane.
The outer world provokes simple creatures to answer back; simple
creatures act experimentally on their surroundings. From the beginning
this twofold process has been going on, receiving stimuli from the
environment and acting upon the environment, and according to the
efficiency of the reactions and actions living creatures have been
sifted for millions of years. One main line of advance has been opening
new gateways of knowledge--the senses, which are far more than five in
number. The other main line of advance has been in most general terms,
experimenting or testing, probing and proving, trying one key after
another till a door is unlocked. There is progress in multiplying the
gateways of knowledge and making them more discriminating, and there is
progress in making the modes of experimenting more wide-awake, more
controlled, and more resolute. But behind both of these is the
characteristically vital power of enregistering within the organism the
lessons of the past. In the life of the individual these enregistrations
are illustrated by memories and habituations and habits; in the life of
the race they are illustrated by reflex actions and instinctive
capacities.
Body and Mind
We must not shirk the very difficult question of the relation between
the bodily and the mental side of behaviour.
(_a_) Some great thinkers have taught that the mind is a reality by
itself which plays upon the instrument of the brain and body. As the
instrument gets worn and dusty the playing is not so good as it once
was, but the player is still himself. This theory of the essential
independence of the mind is a very beautiful one, but those who like it
when applied to themselves are not always so fond of it when it is
applied to other intelligent creatures like rooks and elephants. It may
be, however, that there is a gradual emancipation of the mind which has
gone furthest in Man and is still progressing.
(_b_) Some other thinkers have taught that the inner life of thought and
feeling is only, as it were, an echo of the really important
activity--that of the body and brain. Ideas are just foam-bells on the
hurrying streams and circling eddies of matter and energy that make up
our physiological life. To most of us this theory is impossible, because
we are quite sure that ideas and feelings and purposes, which cannot be
translated into matter and motion, are the clearest realities in our
experience, and that they count for good and ill all through our life.
They are more than the tickings of the clock; they make the wheels go
round.
(_c_) There are others who think that the most scientific position is
simply to recognise both the bodily and the mental activities as equally
important, and so closely interwoven that they cannot be separated.
Perhaps they are just the outer and the inner aspects of one
reality--the life of the creature. Perhaps they are like the concave and
convex curves of a dome, like the two sides of a shield. Perhaps the
life of the organism is always a unity, at one time appearing more
conspicuously as Mind-body, at another time as Body-mind. The most
important fact is that neither aspect can be left out. By no jugglery
with words can we get Mind out of Matter and Motion. And since we are in
ourselves quite sure of our Mind, we are probably safe in saying that in
the beginning was Mind. This is in accordance with Aristotle's saying
that there is nothing in the end which was not also in kind present in
the beginning--whatever we mean by beginning.
In conclusion
What has led to the truly wonderful result which we admire in a creature
like a dog or an otter, a horse or a hare? In general, we may say, just
two main processes--(1) testing all things, and (2) holding fast that
which is good. New departures occur and these are tested for what they
are worth. Idiosyncrasies crop up and they are sifted. New cards come
mysteriously from within into the creature's hand, and they are
played--for better or for worse. So by new variations and their sifting,
by experimenting and enregistering the results, the mind has gradually
evolved and will continue to evolve.
VIII
FOUNDATIONS OF THE UNIVERSE
THE WORLD OF ATOMS
Most people have heard of the oriental race which puzzled over the
foundations of the universe, and decided that it must be supported on
the back of a giant elephant. But the elephant? They put it on the back
of a monstrous tortoise, and there they let the matter end. If every
animal in nature had been called upon, they would have been no nearer a
foundation. Most ancient peoples, indeed, made no effort to find a
foundation. The universe was a very compact little structure, mainly
composed of the earth and the great canopy over the earth which they
called the sky. They left it, as a whole, floating in nothing. And in
this the ancients were wiser than they knew. Things do not fall down
unless they are pulled down by that mysterious force which we call
gravitation. The earth, it is true, is pulled by the sun, and would fall
into it; but the earth escapes this fiery fate by circulating at great
speed round the sun. The stars pull each other; but it has already been
explained that they meet this by travelling rapidly in gigantic orbits.
Yet we do, in a new sense of the word, need foundations of the universe.
Our mind craves for some explanation of the matter out of which the
universe is made. For this explanation we turn to modern Physics and
Chemistry. Both these sciences study, under different aspects, matter
and energy; and between them they have put together a conception of the
fundamental nature of things which marks an epoch in the history of
human thought.
§ 1
The Bricks of the Cosmos
More than two thousand years ago the first men of science, the Greeks of
the cities of Asia Minor, speculated on the nature of matter. You can
grind a piece of stone into dust. You can divide a spoonful of water
into as many drops as you like. Apparently you can go on dividing as
long as you have got apparatus fine enough for the work. But there must
be a limit, these Greeks said, and so they supposed that all matter was
ultimately composed of minute particles which were indivisible. That is
the meaning of the Greek word "atom."
Like so many other ideas of these brilliant early Greek thinkers, the
atom was a sound conception. We know to-day that matter is composed of
atoms. But science was then so young that the way in which the Greeks
applied the idea was not very profound. A liquid or a gas, they said,
consisted of round, smooth atoms, which would not cling together. Then
there were atoms with rough surfaces, "hooky" surfaces, and these stuck
together and formed solids. The atoms of iron or marble, for instance,
were so very hooky that, once they got together, a strong man could not
tear them apart. The Greeks thought that the explanation of the universe
was that an infinite number of these atoms had been moving and mixing in
an infinite space during an infinite time, and had at last hit by chance
on the particular combination which is our universe.
This was too simple and superficial. The idea of atoms was cast aside,
only to be advanced again in various ways. It was the famous Manchester
chemist, John Dalton, who restored it in the early years of the
nineteenth century. He first definitely formulated the atomic theory as
a scientific hypothesis. The whole physical and chemical science of that
century was now based upon the atom, and it is quite a mistake to
suppose that recent discoveries have discredited "atomism." An atom is
the smallest particle of a chemical element. No one has ever seen an
atom. Even the wonderful new microscope which has just been invented
cannot possibly show us particles of matter which are a million times
smaller than the breadth of a hair; for that is the size of atoms. We
can weigh them and measure them, though they are invisible, and we know
that all matter is composed of them. It is a new discovery that atoms
are not indivisible. They consist themselves of still smaller particles,
as we shall see. But the atoms exist all the same, and we may still say
that they are the bricks of which the material universe is built.
[Illustration: _Photo: Elliott & Fry._
SIR ERNEST RUTHERFORD
One of our most eminent physicists who has succeeded Sir J. J. Thomson
as Cavendish Professor of Physics at the University of Cambridge. The
modern theory of the structure of the atom is largely due to him.]
[Illustration: _Photo: Rischgitz Collection._
J. CLERK-MAXWELL
One of the greatest scientific men who have ever lived. He
revolutionised physics with his electro-magnetic theory of light, and
practically all modern researches have had their origin, direct or
indirect, in his work. Together with Faraday he constitutes one of the
main scientific glories of the nineteenth century.]
[Illustration: _Photo: Ernest H. Mills._
SIR WILLIAM CROOKES
Sir William Crookes experimented on the electric discharge in vacuum
tubes and described the phenomena as a "fourth state of matter." He was
actually observing the flight of electrons, but he did not fully
appreciate the nature of his experiments.]
[Illustration: _Photo: Photo Press_
PROFESSOR SIR W. H. BRAGG
One of the most distinguished physicists of the present day.]
But if we had some magical glass by means of which we could see into the
structure of material things, we should not see the atoms put evenly
together as bricks are in a wall. As a rule, two or more atoms first
come together to form a larger particle, which we call a "molecule."
Single atoms do not, as a rule, exist apart from other atoms; if a
molecule is broken up, the individual atoms seek to unite with other
atoms of another kind or amongst themselves. For example, three atoms of
oxygen form what we call ozone; two atoms of hydrogen uniting with one
atom of oxygen form water. It is molecules that form the mass of matter;
a molecule, as it has been expressed, is a little building of which
atoms are the bricks.
In this way we get a useful first view of the material things we handle.
In a liquid the molecules of the liquid cling together loosely. They
remain together as a body, but they roll over and away from each other.
There is "cohesion" between them, but it is less powerful than in a
solid. Put some water in a kettle over the lighted gas, and presently
the tiny molecules of water will rush through the spout in a cloud of
steam and scatter over the kitchen. The heat has broken their bond of
association and turned the water into something like a gas; though we
know that the particles will come together again, as they cool, and form
once more drops of water.
In a gas the molecules have full individual liberty. They are in a
state of violent movement, and they form no union with each other. If we
want to force them to enter into the loose sort of association which
molecules have in a liquid, we have to slow down their individual
movements by applying severe cold. That is how a modern man of science
liquefies gases. No power that we have will liquefy air at its ordinary
temperature. In _very_ severe cold, on the other hand, the air will
spontaneously become liquid. Some day, when the fires of the sun have
sunk very low, the temperature of the earth will be less than -200° C.:
that is to say, more than two hundred degrees Centigrade below
freezing-point. It will sink to the temperature of the moon. Our
atmosphere will then be an ocean of liquid air, 35 feet deep, lying upon
the solidly frozen masses of our water-oceans.
In a solid the molecules cling firmly to each other. We need a force
equal to twenty-five tons to tear asunder the molecules in a bar of iron
an inch thick. Yet the structure is not "solid" in the popular sense of
the word. If you put a piece of solid gold in a little pool of mercury,
the gold will take in the mercury _between_ its molecules, as if it were
porous like a sponge. The hardest solid is more like a lattice-work than
what we usually mean by "solid"; though the molecules are not fixed,
like the bars of a lattice-work, but are in violent motion; they vibrate
about equilibrium positions. If we could see right into the heart of a
bit of the hardest steel, we should see billions of separate molecules,
at some distance from each other, all moving rapidly to and fro.
This molecular movement can, in a measure, be made visible. It was
noticed by a microscopist named Brown that, in a solution containing
very fine suspended particles, the particles were in constant movement.
Under a powerful microscope these particles are seen to be violently
agitated; they are each independently darting hither and thither
somewhat like a lot of billiard balls on a billiard table, colliding and
bounding about in all directions. Thousands of times a second these
encounters occur, and this lively commotion is always going on, this
incessant colliding of one molecule with another is the normal
condition of affairs; not one of them is at rest. The reason for this
has been worked out, and it is now known that these particles move about
because they are being incessantly bombarded by the molecules of the
liquid. The molecules cannot, of course, be seen, but the fact of their
incessant movement is revealed to the eye by the behaviour of the
visible suspended particles. This incessant movement in the world of
molecules is called the Brownian movement, and is a striking proof of
the reality of molecular motions.
§ 2
The Wonder-World of Atoms
The exploration of this wonder-world of atoms and molecules by the
physicists and chemists of to-day is one of the most impressive triumphs
of modern science. Quite apart from radium and electrons and other
sensational discoveries of recent years, the study of ordinary matter is
hardly inferior, either in interest or audacity, to the work of the
astronomer. And there is the same foundation in both cases--marvellous
apparatus, and trains of mathematical reasoning that would have
astonished Euclid or Archimedes. Extraordinary, therefore, as are some
of the facts and figures we are now going to give in connection with the
minuteness of atoms and molecules, let us bear in mind that we owe them
to the most solid and severe processes of human thought.
Yet the principle can in most cases be made so clear that the reader
will not be asked to take much on trust. It is, for instance, a matter
of common knowledge that gold is soft enough to be beaten into gold
leaf. It is a matter of common sense, one hopes, that if you beat a
measured cube of gold into a leaf six inches square, the mathematician
can tell the thickness of that leaf without measuring it. As a matter of
fact, a single grain of gold has been beaten into a leaf seventy-five
inches square. Now the mathematician can easily find that when a single
grain of gold is beaten out to that size, the leaf must be 1/367,000 of
an inch thick, or about a thousand times thinner than the paper on
which these words are printed; yet the leaf must be several molecules
thick.
The finest gold leaf is, in fact, too thick for our purpose, and we turn
with a new interest to that toy of our boyhood the soap-bubble. If you
carefully examine one of these delicate films of soapy water, you notice
certain dark spots or patches on them. These are their thinnest parts,
and by two quite independent methods--one using electricity and the
other light--we have found that at these spots the bubble is less than
the three-millionth of an inch thick! But the molecules in the film
cling together so firmly that they must be at least twenty or thirty
deep in the thinnest part. A molecule, therefore, must be far less than
the three-millionth of an inch thick.
We found next that a film of oil on the surface of water may be even
thinner than a soap-bubble. Professor Perrin, the great French authority
on atoms, got films of oil down to the fifty-millionth of an inch in
thickness! He poured a measured drop of oil upon water. Then he found
the exact limits of the area of the oil-sheet by blowing upon the water
a fine powder which spread to the edge of the film and clearly outlined
it. The rest is safe and simple calculation, as in the case of the
beaten grain of gold. Now this film of oil must have been at least two
molecules deep, so a single molecule of oil is considerably less than a
hundred-millionth of an inch in diameter.
Innumerable methods have been tried, and the result is always the same.
A single grain of indigo, for instance, will colour a ton of water. This
obviously means that the grain contains billions of molecules which
spread through the water. A grain of musk will scent a room--pour
molecules into every part of it--for several years, yet not lose
one-millionth of its mass in a year. There are a hundred ways of showing
the minuteness of the ultimate particles of matter, and some of these
enable us to give definite figures. On a careful comparison of the best
methods we can say that the average molecule of matter is less than
the 1/125,000,000 of an inch in diameter. In a single cubic centimetre
of air--a globule about the size of a small marble--there are thirty
million trillion molecules. And since the molecule is, as we saw, a
group or cluster of atoms, the atom itself is smaller. Atoms, for
reasons which we shall see later, differ very greatly from each other in
size and weight. It is enough to say that some of them are so small that
it would take 400,000,000 of them, in a line, to cover an inch of space;
and that it takes at least a quintillion atoms of gold to weigh a single
gramme. Five million atoms of helium could be placed in a line across
the diameter of a full stop.
[Illustration: An atom is the smallest particle of a chemical element.
Two or more atoms come together to form a molecule: thus molecules form
the mass of matter. A molecule of water is made up of two atoms of
hydrogen and one atom of oxygen. Molecules of different substances,
therefore, are of different sizes according to the number and kind of
the particular atoms of which they are composed. A starch molecule
contains no less than 25,000 atoms.
Molecules, of course, are invisible. The above diagram illustrates the
_comparative_ sizes of molecules.]
[Illustration: INCONCEIVABLE NUMBERS AND INCONCEIVABLY SMALL PARTICLES
The molecules, which are inconceivably small, are, on the other hand, so
numerous that if one was able to place, end to end, all those contained
in, for example, a cubic centimetre of gas (less than a fifteenth of a
cubic inch), one would obtain a line capable of passing two hundred
times round the earth.]
[Illustration: WHAT IS A MILLION?
In dealing with the infinitely small, it is difficult to apprehend the
vast figures with which scientists confront us. A million is one
thousand thousand. We may realise what this implies if we consider that
a clock, beating seconds, takes approximately 278 hours (i.e. one week
four days fourteen hours) to tick one million times. A billion is one
million million. To tick a billion the clock would tick for over 31,735
years.
(In France and America a thousand millions is called a billion.)]
[Illustration: THE BROWNIAN MOVEMENT
A diagram, constructed from actual observations, showing the erratic
paths pursued by very fine particles suspended in a liquid, when
bombarded by the molecules of the liquid. This movement is called the
Brownian movement, and it furnishes a striking illustration of the truth
of the theory that the molecules of a body are in a state of continual
motion.]
The Energy of Atoms
And this is only the beginning of the wonders that were done with
"ordinary matter," quite apart from radium and its revelations, to which
we will come presently. Most people have heard of "atomic energy," and
the extraordinary things that might be accomplished if we could harness
this energy and turn it to human use. A deeper and more wonderful source
of this energy has been discovered in the last twenty years, but it is
well to realise that the atoms themselves have stupendous energy. The
atoms of matter are vibrating or gyrating with extraordinary vigour. The
piece of cold iron you hold in your hand, the bit of brick you pick up,
or the penny you take from your pocket is a colossal reservoir of
energy, since it consists of trillions of moving atoms. To realise the
total energy, of course, we should have to witness a transformation such
as we do in atoms of radio-active elements, about which we shall have
something to say presently.
If we put a grain of indigo in a glass of water, or a grain of musk in a
perfectly still room, we soon realise that molecules travel. Similarly,
the fact that gases spread until they fill every "empty" available space
shows definitely that they consist of small particles travelling at
great speed. The physicist brings his refined methods to bear on these
things, and he measures the energy and velocity of these infinitely
minute molecules. He tells us that molecules of oxygen, at the
temperature of melting ice, travel at the rate of about 500 yards a
second--more than a quarter of a mile a second. Molecules of hydrogen
travel at four times that speed, or three times the speed with which a
bullet leaves a rifle. Each molecule of the air, which seems so still in
the house on a summer's day, is really travelling faster than a rifle
bullet does at the beginning of its journey. It collides with another
molecule every twenty-thousandth of an inch of its journey. It is turned
from its course 5,000,000,000 times in every second by collisions. If we
could stop the molecules of hydrogen gas, and utilise their energy, as
we utilise the energy of steam or the energy of the water at Niagara, we
should find enough in every gramme of gas (about two-thousandths of a
pound) to raise a third of a ton to a height of forty inches.
I have used for comparison the speed of a rifle bullet, and in an
earlier generation people would have thought it impossible even to
estimate this. It is, of course, easy. We put two screens in the path of
the bullet, one near the rifle and the other some distance away. We
connect them electrically and use a fine time-recording machine, and the
bullet itself registers the time it takes to travel from the first to
the second screen.
Now this is very simple and superficial work in comparison with the
system of exact and minute measurements which the physicist and chemist
use. In one of his interesting works Mr. Charles R. Gibson gives a
photograph of two exactly equal pieces of paper in the opposite pans of
a fine balance. A single word has been written in pencil on one of these
papers, and that little scraping of lead has been enough to bring down
the scale! The spectroscope will detect a quantity of matter four
million times smaller even than this; and the electroscope is a million
times still more sensitive than the spectroscope. We have a
heat-measuring instrument, the bolometer, which makes the best
thermometer seem Early Victorian. It records the millionth of a degree
of temperature. It is such instruments, multiplied by the score,
which enable us to do the fine work recorded in these pages.
[Illustration: _Reproduced from "The Forces of Nature" (Messrs.
Macmillan)._
A SOAP BUBBLE
The iridescent colours sometimes seen on a soap bubble, as in the
illustration, may also be seen in very fine sections of crystals, in
glass blown into extremely fine bulbs, on the wings of dragon-flies and
the surface of oily water. The different colours correspond to different
thicknesses of the surface. Part of the light which strikes these thin
coatings is reflected from the upper surface, but another part of the
light penetrates the transparent coating and is reflected from the lower
surface. It is the mixture of these two reflected rays, their
"interference" as it is called, which produces the colours observed. The
"black spots" on a soap bubble are the places where the soapy film is
thinnest. At the black spots the thickness of the bubble is about the
three-millionth part of an inch. If the whole bubble were as thin as
this it would be completely invisible.]
§ 3
THE DISCOVERY OF X-RAYS AND RADIUM
The Discovery of Sir Wm. Crookes
But these wonders of the atom are only a prelude to the more romantic
and far-reaching discoveries of the new physics--the wonders of the
electron. Another and the most important phase of our exploration of the
material universe opened with the discovery of radium in 1898.
In the discovery of radio-active elements, a new property of matter was
discovered. What followed on the discovery of radium and of the X-rays
we shall see.
As Sir Ernest Rutherford, one of our greatest authorities, recently
said, the new physics has dissipated the last doubt about the reality of
atoms and molecules. The closer examination of matter which we have been
able to make shows positively that it is composed of atoms. But we must
not take the word now in its original Greek meaning (an "indivisible"
thing). The atoms are not indivisible. They can be broken up. They are
composed of still smaller particles.
The discovery that the atom was composed of smaller particles was the
welcome realisation of a dream that had haunted the imagination of the
nineteenth century. Chemists said that there were about eighty different
kinds of atoms--different kinds of matter--but no one was satisfied with
the multiplicity. Science is always aiming at simplicity and unity. It
may be that science has now taken a long step in the direction of
explaining the fundamental unity of all the matter. The chemist was
unable to break up these "elements" into something simpler, so he called
their atoms "indivisible" in that sense. But one man of science after
another expressed the hope that we would yet discover some fundamental
matter of which the various atoms were composed--_one primordial
substance from which all the varying forms of matter have been evolved
or built up_. Prout suggested this at the very beginning of the century,
when atoms were rediscovered by Dalton. Father Secchi, the famous Jesuit
astronomer said that all the atoms were probably evolved from ether; and
this was a very favoured speculation. Sir William Crookes talked of
"prothyl" as the fundamental substance. Others thought hydrogen was the
stuff out of which all the other atoms were composed.
The work which finally resulted in the discovery of radium began with
some beautiful experiments of Professor (later Sir William) Crookes in
the eighties.
It had been noticed in 1869 that a strange colouring was caused when an
electric charge was sent through a vacuum tube--the walls of the glass
tube began to glow with a greenish phosphorescence. A vacuum tube is one
from which nearly all the air has been pumped, although we can never
completely empty the tube. Crookes used such ingenious methods that he
reduced the gas in his tubes until it was twenty million times thinner
than the atmosphere. He then sent an electric discharge through, and got
very remarkable results. The negative pole of the electric current (the
"cathode") _gave off rays which faintly lit the molecules of the thin
gas in the tube_, and caused a pretty fluorescence on the glass walls of
the tube. What were these Rays? Crookes at first thought they
corresponded to a "new or fourth state of matter." Hitherto we had only
been familiar with matter in the three conditions of solid, liquid, and
gaseous.
Now Crookes really had the great secret under his eyes. But about twenty
years elapsed before the true nature of these rays was finally and
independently established by various experiments. The experiments proved
"that the rays consisted of a stream of negatively charged particles
travelling with enormous velocities from 10,000 to 100,000 miles a
second. In addition, it was found that the mass of each particle was
exceedingly small, about 1/1800 of the mass of a hydrogen atom, the
lightest atom known to science." _These particles or electrons, as they
are now called, were being liberated from the atom._ The atoms of matter
were breaking down in Crookes tubes. At that time, however, it was
premature to think of such a thing, and Crookes preferred to say that
the particles of the gas were electrified and hurled against the walls
of the tube. He said that it was ordinary matter in a new
state--"radiant matter." Another distinguished man of science, Lenard,
found that, when he fitted a little plate of aluminum in the glass wall
of the tube, the mysterious rays passed through this as if it were a
window. They must be waves in the ether, he said.
[Illustration: _From "Scientific Ideas of To-day_."
DETECTING A SMALL QUANTITY OF MATTER
In the left-hand photograph the two pieces of paper exactly balance. The
balance used is very sensitive, and when the single word "atoms" has
been written with a lead pencil upon one of the papers the additional
weight is sufficient to depress one of the pans as shown in the second
photograph. The spectroscope will detect less than one-millionth of the
matter contained in the word pencilled above.]
[Illustration: _Reproduced by permission of X-Rays Ltd._
THIS X-RAY PHOTOGRAPH IS THAT OF A HAND OF A SOLDIER WOUNDED IN THE
GREAT WAR
Note the pieces of shrapnel which are revealed.]
[Illustration: _Photo: National Physical Laboratory._
AN X-RAY PHOTOGRAPH OF A GOLF BALL, REVEALING AN IMPERFECT CORE]
[Illustration: _Reproduced by permission of X-Rays Ltd._
A WONDERFUL X-RAY PHOTOGRAPH
Note the fine details revealed, down to the metal tags of the bootlace
and the nails in the heel of the boot.]
§ 4
The Discovery of X-rays
So the story went on from year to year. We shall see in a moment to what
it led. Meanwhile the next great step was when, in 1895, Röntgen
discovered the X-rays, which are now known to everybody. He was
following up the work of Lenard, and he one day covered a "Crookes tube"
with some black stuff. To his astonishment a prepared chemical screen
which was near the tube began to glow. _The rays had gone through the
black stuff; and on further experiment he found that they would go
through stone, living flesh, and all sorts of "opaque" substances._ In a
short time the world was astonished to learn that we could photograph
the skeleton in a living man's body, locate a penny in the interior of a
child that had swallowed one, or take an impression of a coin through a
slab of stone.
And what are these X-rays? They are not a form of matter; they are not
material particles. X-rays were found to be a new variety of _light_
with a remarkable power of penetration. We have seen what the
spectroscope reveals about the varying nature of light wave-lengths.
Light-waves are set up by vibrations in ether,[2] and, as we shall see,
these ether disturbances are all of the same kind; they only differ as
regards wave-lengths. The X-rays which Röntgen discovered, then, are
light, but a variety of light previously unknown to us; they are ether
waves of very short length. X-rays have proved of great value in many
directions, as all the world knows, but that we need not discuss at this
point. Let us see what followed Röntgen's discovery.
[2] We refer throughout to the "ether" because, although modern
theories dispense largely with this conception, the theories of
physics are so inextricably interwoven with it that it is necessary,
in an elementary exposition, to assume its existence. The modern
view will be explained later in the article on Einstein's Theory.
While the world wondered at these marvels, the men of science were
eagerly following up the new clue to the mystery of matter which was
exercising the mind of Crookes and other investigators. In 1896
Becquerel brought us to the threshold of the great discovery.
Certain substances are phosphorescent--they become luminous after they
have been exposed to sunlight for some time, and Becquerel was trying to
find if any of these substances give rise to X-rays. One day he chose a
salt of the metal uranium. He was going to see if, after exposing it to
sunlight, he could photograph a cross with it through an opaque
substance. He wrapped it up and laid it aside, to wait for the sun, but
he found the uranium salt did not wait for the sun. Some strong
radiation from it went through the opaque covering and made an
impression of the cross upon the plate underneath. Light or darkness was
immaterial. The mysterious rays streamed night and day from the salt.
This was something new. Here was a substance which appeared to be
producing X-rays; the rays emitted by uranium would penetrate the same
opaque substances as the X-rays discovered by Röntgen.
Discovery of Radium
Now, at the same time as many other investigators, Professor Curie and
his Polish wife took up the search. They decided to find out whether
the emission came from the uranium itself or _from something associated
with it_, and for this purpose they made a chemical analysis of great
quantities of minerals. They found a certain kind of pitchblende which
was very active, and they analysed tons of it, concentrating always on
the radiant element in it. After a time, as they successively worked out
the non-radiant matter, the stuff began to glow. In the end they
extracted from eight tons of pitchblende about half a teaspoonful of
something _that was a million times more radiant than uranium_. There
was only one name for it--Radium.
That was the starting-point of the new development of physics and
chemistry. From every laboratory in the world came a cry for radium
salts (as pure radium was too precious), and hundreds of brilliant
workers fastened on the new element. The inquiry was broadened, and, as
year followed year, one substance after another was found to possess the
power of emitting rays, that is, to be radio-active. We know to-day that
nearly every form of matter can be stimulated to radio-activity; which,
as we shall see, means that _its atoms break up into smaller and
wonderfully energetic particles which we call "electrons."_ This
discovery of electrons has brought about a complete change in our ideas
in many directions.
So, instead of atoms being indivisible, they are actually dividing
themselves, spontaneously, and giving off throughout the universe tiny
fragments of their substance. We shall explain presently what was later
discovered about the electron; meanwhile we can say that every glowing
metal is pouring out a stream of these electrons. Every arc-lamp is
discharging them. Every clap of thunder means a shower of them. Every
star is flooding space with them. We are witnessing the spontaneous
breaking up of atoms, atoms which had been thought to be indivisible.
The sun not only pours out streams of electrons from its own atoms, but
the ultra-violet light which it sends to the earth is one of the most
powerful agencies for releasing electrons from the surface-atoms of
matter on the earth. It is fortunate for us that our atmosphere absorbs
most of this ultra-violet or invisible light of the sun--a kind of light
which will be explained presently. It has been suggested that, if we
received the full flood of it from the sun, our metals would
disintegrate under its influence and this "steel civilisation" of ours
would be impossible!
But we are here anticipating, we are going beyond radium to the
wonderful discoveries which were made by the chemists and physicists of
the world who concentrated upon it. The work of Professor and Mme. Curie
was merely the final clue to guide the great search. How it was followed
up, how we penetrated into the very heart of the minute atom and
discovered new and portentous mines of energy, and how we were able to
understand, not only matter, but electricity and light, will be told in
the next chapter.
THE DISCOVERY OF THE ELECTRON AND HOW IT EFFECTED A REVOLUTION IN IDEAS
What the discovery of radium implied was only gradually realised. Radium
captivated the imagination of the world; it was a boon to medicine, but
to the man of science it was at first a most puzzling and most
attractive phenomenon. It was felt that some great secret of nature was
dimly unveiled in its wonderful manifestations, and there now
concentrated upon it as gifted a body of men--conspicuous amongst them
Sir J. J. Thomson, Sir Ernest Rutherford, Sir W. Ramsay, and Professor
Soddy--as any age could boast, with an apparatus of research as far
beyond that of any other age as the _Aquitania_ is beyond a Roman
galley. Within five years the secret was fairly mastered. Not only were
all kinds of matter reduced to a common basis, but the forces of the
universe were brought into a unity and understood as they had never been
understood before.
[Illustration: ELECTRIC DISCHARGE IN A VACUUM TUBE
The two ends, marked + and -, of a tube from which nearly all air has
been exhausted are connected to electric terminals, thus producing an
electric discharge in the vacuum tube. This discharge travels straight
along the tube, as in the upper diagram. When a magnetic field is
applied, however, the rays are deflected, as shown in the lower diagram.
The similarity of the behaviour of the electric discharge with the
radium rays (see diagram of deflection of radium rays, _post_) shows
that the two phenomena may be identified. It was by this means that the
characteristics of electrons were first discovered.]
[Illustration: THE RELATIVE SIZES OF ATOMS AND ELECTRONS
An atom is far too small to be seen. In a bubble of hydrogen gas no
larger than the letter "O" there are billions of atoms, whilst an
electron is more than a thousand times smaller than the smallest atom.
How their size is ascertained is described in the text. In this diagram
a bubble of gas is magnified to the size of the world. Adopting this
scale, _each atom_ in the bubble would then be as large as a tennis
ball.]
[Illustration: IF AN ATOM WERE MAGNIFIED TO THE SIZE OF ST. PAUL'S
CATHEDRAL, EACH ELECTRON IN THE ATOM (AS REPRESENTED BY THE CATHEDRAL)
WOULD THEN BE ABOUT THE SIZE OF A SMALL BULLET]
[Illustration: ELECTRONS STREAMING FROM THE SUN TO THE EARTH
There are strong reasons for supposing that sun-spots are huge
electronic cyclones. The sun is constantly pouring out vast streams of
electrons into space. Many of these streams encounter the earth, giving
rise to various electrical phenomena.]
§ 5
The Discovery of the Electron
Physicists did not take long to discover that the radiation from radium
was very like the radiation in a "Crookes tube." It was quickly
recognised, moreover, that both in the tube and in radium (and other
metals) the atoms of matter were somehow breaking down.
However, the first step was to recognise that there were three distinct
and different rays that were given off by such metals as radium and
uranium. Sir Ernest Rutherford christened them, after the first three
letters of the Greek alphabet, the Alpha, the Beta, and Gamma rays. We
are concerned chiefly with the second group and purpose here to deal
with that group only.[3]
[3] The "Alpha rays" were presently recognised as atoms of helium
gas, shot out at the rate of 12,000 miles a second.
The "Gamma rays" are _waves_, like the X-rays, not material particles.
They appear to be a type of X-rays. They possess the remarkable power of
penetrating opaque substances; they will pass through a foot of solid
iron, for example.
The "Beta rays," as they were at first called, have proved to be one of
the most interesting discoveries that science ever made. They proved
what Crookes had surmised about the radiations he discovered in his
vacuum tube. But it was _not_ a fourth state of matter that had been
found, but a new _property_ of matter, a property common to all atoms of
matter. The Beta rays were later christened Electrons. They are
particles of disembodied electricity, here spontaneously liberated from
the atoms of matter: only when the electron was isolated from the atom
was it recognised for the first time as a separate entity. Electrons,
therefore, are a constituent of the atoms of matter, and we have
discovered that they can be released from the atom by a variety of
agencies. Electrons are to be found everywhere, forming part of every
atom.
"An electron," Sir William Bragg says, "can only maintain a separate
existence if it is travelling at an immense rate, from one
three-hundredth of the velocity of light upwards, that is to say, at
least 600 _miles a second, or thereabouts_. Otherwise the electron
sticks to the first atom it meets." These amazing particles may travel
with the enormous velocity of from 10,000 to more than 100,000 miles a
second. It was first learned that they are of an electrical nature,
because they are bent out of their normal path if a magnet is brought
near them. And this fact led to a further discovery: to one of those
sensational estimates which the general public is apt to believe to be
founded on the most abstruse speculations. The physicist set up a little
chemical screen for the "Beta rays" to hit, and he so arranged his tube
that only a narrow sheaf of the rays poured on to the screen. He then
drew this sheaf of rays out of its course with a magnet, and he
accurately measured the shift of the luminous spot on the screen where
the rays impinged on it. But when he knows the exact intensity of his
magnetic field--which he can control as he likes--and the amount of
deviation it causes, and the mass of the moving particles, he can tell
the speed of the moving particles which he thus diverts. These particles
were being hurled out of the atoms of radium, or from the negative pole
in a vacuum tube, at a speed which, in good conditions, reached nearly
the velocity of light, i.e. nearly 186,000 miles a second.
Their speed has, of course, been confirmed by numbers of experiments;
and another series of experiments enabled physicists to determine the
size of the particles. Only one of these need be described, to give the
reader an idea how men of science arrived at their more startling
results.
Fog, as most people know, is thick in our great cities because the
water-vapour gathers on the particles of dust and smoke that are in the
atmosphere. This fact was used as the basis of some beautiful
experiments. Artificial fogs were created in little glass tubes, by
introducing dust, in various proportions, for supersaturated vapour to
gather on. In the end it was possible to cause tiny drops of rain, each
with a particle of dust at its core, to fall upon a silver mirror and be
counted. It was a method of counting the quite invisible particles of
dust in the tube; and the method was now successfully applied to the new
rays. Yet another method was to direct a slender stream of the particles
upon a chemical screen. The screen glowed under the cannonade of
particles, and a powerful lens resolved the glow into distinct sparks,
which could be counted.
In short, a series of the most remarkable and beautiful experiments,
checked in all the great laboratories of the world, settled the nature
of these so-called rays. They were streams of particles more than a
thousand times smaller than the smallest known atom. The mass of each
particle is, according to the latest and finest measurements 1/1845 of
that of an atom of hydrogen. The physicist has not been able to find any
character except electricity in them, and the name "electrons" has been
generally adopted.
The Key to many Mysteries
The Electron is an atom, of disembodied electricity; it occupies an
exceedingly small volume, and its "mass" is entirely electrical. These
electrons are the key to half the mysteries of matter. Electrons in
rapid motion, as we shall see, explain what we mean by an "electric
current," not so long ago regarded as one of the most mysterious
manifestations in nature.
"What a wonder, then, have we here!" says Professor R. K. Duncan. "An
innocent-looking little pinch of salt and yet possessed of special
properties utterly beyond even the fanciful imaginings of men of past
time; for nowhere do we find in the records of thought even the hint of
the possibility of things which we now regard as established fact. This
pinch of salt projects from its surface bodies [i.e. electrons]
possessing the inconceivable velocity of over 100,000 miles a second, a
velocity sufficient to carry them, if unimpeded, five times around the
earth in a second, and possessing with this velocity, masses a thousand
times smaller than the smallest atom known to science. Furthermore,
they are charged with negative electricity; they pass straight through
bodies considered opaque with a sublime indifference to the properties
of the body, with the exception of its mere density; they cause bodies
which they strike to shine out in the dark; they affect a photographic
plate; they render the air a conductor of electricity; they cause clouds
in moist air; they cause chemical action and have a peculiar
physiological action. Who, to-day, shall predict the ultimate service to
humanity of the beta-rays from radium!"
§ 6
THE ELECTRON THEORY, OR THE NEW VIEW OF MATTER
The Structure of the Atom
There is general agreement amongst all chemists, physicists, and
mathematicians upon the conclusions which we have so far given. We know
that the atoms of matter are constantly--either spontaneously or under
stimulation--giving off electrons, or breaking up into electrons; and
they therefore contain electrons. Thus we have now complete proof of the
independent existence of atoms and also of electrons.
When, however, the man of science tries to tell us _how_ electrons
compose atoms, he passes from facts to speculation, and very difficult
speculation. Take the letter "o" as it is printed on this page. In a
little bubble of hydrogen gas no larger than that letter there are
_trillions_ of atoms; and they are not packed together, but are
circulating as freely as dancers in a ball-room. We are asking the
physicist to take one of these minute atoms and tell us how the still
smaller electrons are arranged in it. Naturally he can only make mental
pictures, guesses or hypotheses, which he tries to fit to the facts, and
discards when they will _not_ fit.
At present, after nearly twenty years of critical discussion, there are
two chief theories of the structure of the atom. At first Sir J. J.
Thomson imagined the electrons circulating in shells (like the layers of
an onion) round the nucleus of the atom. This did not suit, and Sir E.
Rutherford and others worked out a theory that the electrons circulated
round a nucleus rather like the planets of our solar system revolving
round the central sun. Is there a nucleus, then, round which the
electrons revolve? The electron, as we saw, is a disembodied atom of
electricity; we should say, of "negative" electricity. Let us picture
these electrons all moving round in orbits with great velocity. Now it
is suggested that there is a nucleus of "positive" electricity
attracting or pulling the revolving electrons to it, and so forming an
equilibrium, otherwise the electrons would fly off in all directions.
This nucleus has been recently named the proton. We have thus two
electricities in the atom: the positive = the nucleus; the negative =
the electron. Of recent years Dr. Langmuir has put out a theory that the
electrons do not _revolve round_ the nucleus, but remain in a state of
violent agitation of some sort at fixed distances from the nucleus.
[Illustration: PROFESSOR SIR J. J. THOMSON
Experimental discoverer of the electronic constitution of matter, in the
Cavendish Physical Laboratory, Cambridge. A great investigator, noted
for the imaginative range of his hypotheses and his fertility in
experimental devices.]
[Illustration: _From the Smithsonian Report_, 1915.
ELECTRONS PRODUCED BY PASSAGE OF X-RAYS THROUGH AIR
A photograph clearly showing that electrons are definite entities. As
electrons leave atoms they may traverse matter or pass through the air
in a straight path The illustration shows the tortuous path of electrons
resulting from collision with atoms.]
[Illustration: MAGNETIC DEFLECTION OF RADIUM RAYS
The radium rays are made to strike a screen, producing visible spots of
light. When a magnetic field is applied the rays are seen to be
deflected, as in the diagram. This can only happen if the rays carry an
electric charge, and it was by experiments of this kind that we obtained
our knowledge respecting the electric charges carried by radium rays.]
[Illustration: _Reproduced by permission of "Scientific American."_
PROFESSOR R. A. MILLIKAN'S APPARATUS FOR COUNTING ELECTRONS]
But we will confine ourselves here to the facts, and leave the
contending theories to scientific men. It is now pretty generally
accepted that an atom of matter consists of a number of electrons, or
charges of negative electricity, held together by a charge of positive
electricity. It is not disputed that these electrons are in a state of
violent motion or strain, and that therefore a vast energy is locked up
in the atoms of matter. To that we will return later. Here, rather, we
will notice another remarkable discovery which helps us to understand
the nature of matter.
A brilliant young man of science who was killed in the war, Mr. Moseley,
some years ago showed that, when the atoms of different substances are
arranged in order of their weight, _they are also arranged in the order
of increasing complexity of structure_. That is to say, the heavier the
atom, the more electrons it contains. There is a gradual building up of
atoms containing more and more electrons from the lightest atom to the
heaviest. Here it is enough to say that as he took element after
element, from the lightest (hydrogen) to the heaviest (uranium) he found
a strangely regular relation between them. If hydrogen were represented
by the figure one, helium by two, lithium three, and so on up to
uranium, then uranium should have the figure ninety-two. This makes it
probable that there are in nature ninety-two elements--we have found
eighty-seven--and that the number Mr. Moseley found is the number of
electrons in the atom of each element; that is to say, the number is
arranged in order of the atomic numbers of the various elements.
§ 7
The New View of Matter
Up to the point we have reached, then, we see what the new view of
Matter is. Every atom of matter, of whatever kind throughout the whole
universe, is built up of electrons in conjunction with a nucleus. From
the smallest atom of all--the atom of hydrogen--which consists of one
electron, rotating round a positively charged nucleus, to a heavy
complicated atom, such as the atom of gold, constituted of many
electrons and a complex nucleus, _we have only to do with positive and
negative units of electricity_. The electron and its nucleus are
particles of electricity. All Matter, therefore, is nothing but a
manifestation of electricity. The atoms of matter, as we saw, combine
and form molecules. Atoms and molecules are the bricks out of which
nature has built up everything; ourselves, the earth, the stars, the
whole universe.
But more than bricks are required to build a house. There are other
fundamental existences, such as the various forms of energy, which give
rise to several complex problems. And we have also to remember, that
there are more than eighty distinct elements, each with its own definite
type of atom. We shall deal with energy later. Meanwhile it remains to
be said that, although we have discovered a great deal about the
electron and the constitution of matter, and that while the physicists
of our own day seem to see a possibility of explaining positive and
negative electricity, the nature of them both is unknown. There exists
the theory that the particles of positive and negative electricity,
which make up the atoms of matter, are points or centres of disturbances
of some kind in a universal ether, and that all the various forms of
energy are, in some fundamental way, aspects of the same primary entity
which constitutes matter itself.
But the discovery of the property of radio-activity has raised many
other interesting questions, besides that which we have just dealt with.
In radio-active elements, such as uranium for example, the element is
breaking down; in what we call radio-activity we have a manifestation of
the spontaneous change of elements. What is really taking place is a
transmutation of one element into another, from a heavier to a lighter.
The element uranium spontaneously becomes radium, and radium passes
through a number of other stages until it, in turn, becomes lead. Each
descending element is of lighter atomic weight than its predecessor. The
changing process, of course, is a very slow one. It may be that all
matter is radio-active, or can be made so. This raises the question
whether all the matter in the universe may not undergo disintegration.
There is, however, another side of the question, which the discovery of
radio-activity has brought to light, and which has effected a revolution
in our views. We have seen that in radio-active substances the elements
are breaking down. Is there a process of building up at work? If the
more complicated atoms are breaking down into simpler forms, may there
not be a converse process--a building up from simpler elements to more
complicated elements? It is probably the case that both processes are at
work.
There are some eighty-odd chemical elements on the earth to-day: are
they all the outcome of an inorganic evolution, element giving rise to
element, going back and back to some primeval stuff from which they
were all originally derived infinitely long ago? Is there an evolution
in the inorganic world which may be going on, parallel to that of the
evolution of living things; or is organic evolution a continuation of
inorganic evolution? We have seen what evidence there is of this
inorganic evolution in the case of the stars. We cannot go deeply into
the matter here, nor has the time come for any direct statement that can
be based on the findings of modern investigation. Taking it altogether
the evidence is steadily accumulating, and there are authorities who
maintain that already the evidence of inorganic evolution is convincing
enough. The heavier atoms would appear to behave as though they were
evolved from the lighter. The more complex forms, it is supposed, have
_evolved_ from the simpler forms. Moseley's discovery, to which
reference has been made, points to the conclusion that the elements are
built up one from another.
§ 8
Other New Views
We may here refer to another new conception to which the discovery of
radio-activity has given rise. Lord Kelvin, who estimated the age of the
earth at twenty million years, reached this estimate by considering the
earth as a body which is gradually cooling down, "losing its primitive
heat, like a loaf taken from the oven, at a rate which could be
calculated, and that the heat radiated by the sun was due to
contraction." Uranium and radio-activity were not known to Kelvin, and
their discovery has upset both his arguments. Radio-active substances,
which are perpetually giving out heat, introduce an entirely new factor.
We cannot now assume that the earth is necessarily cooling down; it may
even, for all we know, be getting hotter. At the 1921 meeting of the
British Association, Professor Rayleigh stated that further knowledge
had extended the probable period during which there had been life on
this globe to about one thousand million years, and the total age of
the earth to some small multiple of that. The earth, he considers, is
not cooling, but "contains an internal source of heat from the
disintegration of uranium in the outer crust." On the whole the estimate
obtained would seem to be in agreement with the geological estimates.
The question, of course, cannot, in the present state of our knowledge,
be settled within fixed limits that meet with general agreement.
[Illustration: MAKING THE INVISIBLE VISIBLE
Radium, as explained in the text, emits rays--the "Alpha," the "Beta"
(electrons), and "Gamma" rays. The above illustration indicates the
method by which these invisible rays are made visible, and enables the
nature of the rays to be investigated. To the right of the diagram is
the instrument used, the Spinthariscope, making the impact of radium
rays visible on a screen.
The radium rays shoot out in all directions; those that fall on the
screen make it glow with points of light. These points of light are
observed by the magnifying lens.
A. Magnifying lens. B. A zinc sulphite screen. C. A needle on whose
point is placed a speck of radium.
The lower picture shows the screen and needle magnified.]
[Illustration: THE THEORY OF ELECTRONS
An atom of matter is composed of electrons. We picture an atom as a sort
of miniature solar system, the electrons (particles of negative
electricity) rotating round a central nucleus of positive electricity,
as described in the text. In the above pictorial representation of an
atom the whirling electrons are indicated in the outer ring. Electrons
move with incredible speed as they pass from one atom to another.]
[Illustration: ARRANGEMENTS OF ATOMS IN A DIAMOND
The above is a model (seen from two points of view) of the arrangement
of the atoms in a diamond. The arrangement is found by studying the
X-ray spectra of the diamond.]
As we have said, there are other fundamental existences which give rise
to more complex problems. The three great fundamental entities in the
physical universe are matter, ether, and energy; so far as we know,
outside these there is nothing. We have dealt with matter, there remain
ether and energy. We shall see that just as no particle of matter,
however small, may be created or destroyed, and just as there is no such
thing as empty space--ether pervades everything--so there is no such
thing as _rest_. Every particle that goes to make up our solid earth is
in a state of perpetual unremitting vibration; energy "is the universal
commodity on which all life depends." Separate and distinct as these
three fundamental entities--matter, ether, and energy--may appear, it
may be that, after all, they are only different and mysterious phases of
an essential "oneness" of the universe.
§ 9
The Future
Let us, in concluding this chapter, give just one illustration of the
way in which all this new knowledge may prove to be as valuable
practically as it is wonderful intellectually. We saw that electrons are
shot out of atoms at a speed that may approach 160,000 miles a second.
Sir Oliver Lodge has written recently that a seventieth of a grain of
radium discharges, at a speed a thousand times that of a rifle bullet,
thirty million electrons a second. Professor Le Bon has calculated that
it would take 1,340,000 barrels of powder to give a bullet the speed of
one of these electrons. He shows that the smallest French copper
coin--smaller than a farthing--contains an energy equal to eighty
million horsepower. A few pounds of matter contain more energy than we
could extract from millions of tons of coal. Even in the atoms of
hydrogen at a temperature which we could produce in an electric furnace
the electrons spin round at a rate of nearly a hundred trillion
revolutions a second!
Every man asks at once: "Will science ever tap this energy?" If it does,
no more smoke, no mining, no transit, no bulky fuel. The energy of an
atom is of course only liberated when an atom passes from one state to
another. The stored up energy is fortunately fast bound by the electrons
being held together as has been described. If it were not so "the earth
would explode and become a gaseous nebula"! It is believed that some day
we shall be able to release, harness, and utilise atomic energy. "I am
of opinion," says Sir William Bragg, "that atom energy will supply our
future need. A thousand years may pass before we can harness the atom,
or to-morrow might see us with the reins in our hands. That is the
peculiarity of Physics--research and 'accidental' discovery go hand in
hand." Half a brick contains as much energy as a small coal-field. The
difficulties are tremendous, but, as Sir Oliver Lodge reminds us, there
was just as much scepticism at one time about the utilisation of steam
or electricity. "Is it to be supposed," he asks, "that there can be no
fresh invention, that all the discoveries have been made?" More than one
man of science encourages us to hope. Here are some remarkable words
written by Professor Soddy, one of the highest authorities on
radio-active matter, in our chief scientific weekly (_Nature_, November
6, 1919):
The prospects of the successful accomplishment of artificial
transmutation brighten almost daily. The ancients seem to have had
something more than an inkling that the accomplishment of
transmutation would confer upon men powers hitherto the prerogative
of the gods. But now we know definitely that the material aspect of
transmutation would be of small importance in comparison with the
control over the inexhaustible stores of internal atomic energy to
which its successful accomplishment would inevitably lead. It has
become a problem, no longer redolent of the evil associations of the
age of alchemy, but one big with the promise of a veritable physical
renaissance of the whole world.
If that "promise" is ever realised, the economic and social face of the
world will be transformed.
Before passing on to the consideration of ether, light, and energy, let
us see what new light the discovery of the electron has thrown on the
nature and manipulation of electricity.
WHAT IS ELECTRICITY?
The Nature of Electricity
There is at least one manifestation in nature, and so late as twenty
years ago it seemed to be one of the most mysterious manifestations of
all, which has been in great measure explained by the new discoveries.
Already, at the beginning of this century, we spoke of our "age of
electricity," yet there were few things in nature about which we knew
less. The "electric current" rang our bells, drove our trains, lit our
rooms, but none knew what the current was. There was a vague idea that
it was a sort of fluid that flowed along copper wires as water flows in
a pipe. We now suppose that it is _a rapid movement of electrons from
atom to atom_ in the wire or wherever the current is.
Let us try to grasp the principle of the new view of electricity and see
how it applies to all the varied electrical phenomena in the world about
us. As we saw, the nucleus of an atom of matter consists of positive
electricity which holds together a number of electrons, or charges of
negative electricity.[4] This certainly tells us to some extent what
electricity is, and how it is related to matter, but it leaves us with
the usual difficulty about fundamental realities. But we now know that
electricity, like matter, is atomic in structure; a charge of
electricity is made up of a number of small units or charges of a
definite, constant amount. It has been suggested that the two kinds of
electricity, i.e. positive and negative, are right-handed and
left-handed vortices or whirlpools in ether, or rings in ether, but
there are very serious difficulties, and we leave this to the future.
[4] The words "positive" and "negative" electricity belong to the
days when it was regarded as a fluid. A body overcharged with the
fluid was called positive; an undercharged body was called negative.
A positively-electrified body is now one whose atoms have lost some
of their outlying electrons, so that the positive charge of
electricity predominates. The negatively-electrified body is one
with more than the normal number of electrons.
§ 10
What an Electric Current is
The discovery of these two kinds of electricity has, however, enabled us
to understand very fairly what goes on in electrical phenomena. The
outlying electrons, as we saw, may pass from atom to atom, and this, on
a large scale, is the meaning of the electric current. In other words,
we believe an electric current to be a flow of electrons. Let us take,
to begin with, a simple electrical "cell," in which a feeble current is
generated: such a cell as there is in every house to serve its electric
bells.
In the original form this simple sort of "battery" consisted of a plate
of zinc and a plate of copper immersed in a chemical. Long before
anything was known about electrons it was known that, if you put zinc
and copper together, you produce a mild current of electricity. We know
now what this means. Zinc is a metal the atoms of which are particularly
disposed to part with some of their outlying electrons. Why, we do not
know; but the fact is the basis of these small batteries. Electrons from
the atoms of zinc pass to the atoms of copper, and their passage is a
"current." Each atom gives up an electron to its neighbour. It was
further found long ago that if the zinc and copper were immersed in
certain chemicals, which slowly dissolve the zinc, and the two metals
were connected by a copper wire, the current was stronger. In modern
language, there is a brisker flow of electrons. The reason is that
the atoms of zinc which are stolen by the chemical leave their
detachable electrons behind them, and the zinc has therefore more
electrons to pass on to the copper.
[Illustration: DISINTEGRATION OF ATOMS
An atom of Uranium, by ejecting an Alpha particle, becomes Uranium X.
This substance, by ejecting Beta and Gamma rays, becomes Radium. Radium
passes through a number of further changes, as shown in the diagram, and
finally becomes lead. Some radio-active substances disintegrate much
faster than others. Thus Uranium changes very slowly, taking
5,000,000,000 years to reach the same stage of disintegration that
Radium A reaches in 3 minutes. As the disintegration proceeds, the
substances become of lighter and lighter atomic weights. Thus Uranium
has an atomic weight of 238, whereas lead has an atomic weight of only
206. The breaking down of atoms is fully explained in the text.]
[Illustration: _Reproduced by permission from "The Interpretation of
Radium" (John Murray)._
SILK TASSEL ELECTRIFIED
The separate threads of the tassel, being each electrified with the same
kind of electricity, repel one another, and thus the tassel branches out
as in the photograph.]
[Illustration: SILK TASSEL DISCHARGED BY THE RAYS FROM RADIUM
When the radium rays, carrying an opposite electric charge to that on
the tassel, strikes the threads, the threads are neutralised, and hence
fall together again.]
[Illustration: A HUGE ELECTRIC SPARK
This is an actual photograph of an electric spark. It is leaping a
distance of about 10 feet, and is the discharge of a million volts. It
is a graphic illustration of the tremendous energy of electrons.]
[Illustration: _From "Scientific Ideas of To-day_."
ELECTRICAL ATTRACTION BETWEEN COMMON OBJECTS
Take an ordinary flower-vase well dried and energetically rub it with a
silk handkerchief. The vase which thus becomes electrified will attract
any light body, such as a feather, as shown in the above illustration.]
Such cells are now made of zinc and carbon, immersed in sal-ammoniac,
but the principle is the same. The flow of electricity is a flow of
electrons; though we ought to repeat that they do not flow in a body, as
molecules of water do. You may have seen boys place a row of bricks,
each standing on one end, in such order that the first, if it is pushed,
will knock over the second, the second the third, and so on to the last.
There is a flow of _movement_ all along the line, but each brick moves
only a short distance. So an electron merely passes to the next atom,
which sends on an electron to a third atom, and so on. In this case,
however, the movement from atom to atom is so rapid that the ripple of
movement, if we may call it so, may pass along at an enormous speed. We
have seen how swiftly electrons travel.
But how is this turned into power enough even to ring a bell? The actual
mechanical apparatus by which the energy of the electron current is
turned into sound, or heat, or light will be described in a technical
section later in this work. We are concerned here only with the
principle, which is clear. While zinc is very apt to part with
electrons, copper is just as obliging in facilitating their passage
onward. Electrons will travel in this way in most metals, but copper is
one of the best "conductors." So we lengthen the copper wire between the
zinc and the carbon until it goes as far as the front door and the bell,
which are included in the circuit. When you press the button at the
door, two wires are brought together, and the current of electrons
rushes round the circuit; and at the bell its energy is diverted into
the mechanical apparatus which rings the bell.
Copper is a good conductor--six times as good as iron--and is therefore
so common in electrical industries. Some other substances are just as
stubborn as copper is yielding, and we call them "insulators," because
they resist the current instead of letting it flow. Their atoms do not
easily part with electrons. Glass, vulcanite, and porcelain are very
good insulators for this reason.
What the Dynamo does
But even several cells together do not produce the currents needed in
modern industry, and the flow is produced in a different manner. As the
invisible electrons pass along a wire they produce what we call a
magnetic field around the wire, they produce a disturbance in the
surrounding ether. To be exact, it is through the ether surrounding the
wire that the energy originated by the electrons is transmitted. To set
electrons moving on a large scale we use a "dynamo." By means of the
dynamo it is possible to transform mechanical energy into electrical
energy. The modern dynamo, as Professor Soddy puts it, may be looked
upon as an electron pump. We cannot go into the subject deeply here, we
would only say that a large coil of copper wire is caused to turn round
rapidly between the poles of a powerful magnet. That is the essential
construction of the "dynamo," which is used for generating strong
currents. We shall see in a moment how magnetism differs from
electricity, and will say here only that round the poles of a large
magnet there is a field of intense disturbance which will start a flow
of electrons in any copper that is introduced into it. On account of the
speed given to the coil of wire its atoms enter suddenly this magnetic
field, and they give off crowds of electrons in a flash.
It is found that a similar disturbance is caused, though the flow is in
the _opposite_ direction, when the coil of wire leaves the magnetic
field. And as the coil is revolving very rapidly we get a powerful
current of electricity that runs in alternate directions--an
"alternating" current. Electricians have apparatus for converting it
into a continuous current where this is necessary.
A current, therefore, means a steady flow of the electrons from atom to
atom. Sometimes, however, a number of electrons rush violently and
explosively from one body to another, as in the electric spark or the
occasional flash from an electric tram or train. The grandest and most
spectacular display of this phenomenon is the thunderstorm. As we saw
earlier, a portentous furnace like the sun is constantly pouring floods
of electrons from its atoms into space. The earth intercepts great
numbers of these electrons. In the upper regions of the air the stream
of solar electrons has the effect of separating positively-electrified
atoms from negatively-electrified ones, and the water-vapour, which is
constantly rising from the surface of the sea, gathers more freely round
the positively-electrified atoms, and brings them down, as rain, to the
earth. Thus the upper air loses a proportion of positive electricity, or
becomes "negatively electrified." In the thunderstorm we get both kinds
of clouds--some with large excesses of electrons, and some deficient in
electrons--and the tension grows until at last it is relieved by a
sudden and violent discharge of electrons from one cloud to another or
to the earth--an electric spark on a prodigious scale.
§ 11
Magnetism
We have seen that an electric current is really a flow of electrons. Now
an electric current exhibits a magnetic effect. The surrounding space is
endowed with energy which we call electro-magnetic energy. A piece of
magnetised iron attracting other pieces of iron to it is the popular
idea of a magnet. If we arrange a wire to pass vertically through a
piece of cardboard and then sprinkle iron filings on the cardboard we
shall find that, on passing an electric current through the wire, the
iron filings arrange themselves in circles round it. The magnetic force,
due to the electric current, seems to exist in circles round the wire,
an ether disturbance being set up. Even a single electron, when in
movement, creates a magnetic "field," as it is called, round its path.
There is no movement of electrons without this attendant field of
energy, and their motion is not stopped until that field of energy
disappears from the ether. The modern theory of magnetism supposes that
all magnetism is produced in this way. All magnetism is supposed to
arise from the small whirling motions of the electrons contained in the
ultimate atoms of matter. We cannot here go into the details of the
theory nor explain why, for instance, iron behaves so differently from
other substances, but it is sufficient to say that here, also, the
electron theory provides the key. This theory is not yet definitely
_proved_, but it furnishes a sufficient theoretical basis for future
research. The earth itself is a gigantic magnet, a fact which makes the
compass possible, and it is well known that the earth's magnetism is
affected by those great outbreaks on the sun called sun-spots. Now it
has been recently shown that a sun-spot is a vast whirlpool of electrons
and that it exerts a strong magnetic action. There is doubtless a
connection between these outbreaks of electronic activity and the
consequent changes in the earth's magnetism. The precise mechanism of
the connection, however, is still a matter that is being investigated.
ETHER AND WAVES
Ether and Waves
The whole material universe is supposed to be embedded in a vast medium
called the ether. It is true that the notion of the ether has been
abandoned by some modern physicists, but, whether or not it is
ultimately dispensed with, the conception of the ether has entered so
deeply into the scientific mind that the science of physics cannot be
understood unless we know something about the properties attributed to
the ether. The ether was invented to explain the phenomena of light, and
to account for the flow of energy across empty space. Light takes time
to travel. We see the sun at any moment by the light that left it 8
minutes before. It has taken that 8 minutes for the light from the
sun to travel that 93,000,000 miles odd which separates it from our
earth. Besides the fact that light takes time to travel, it can be shown
that light travels in the form of waves. We know that sound travels in
waves; sound consists of waves in the air, or water or wood or whatever
medium we hear it through. If an electric bell be put in a glass jar and
the air be pumped out of the jar, the sound of the bell becomes feebler
and feebler until, when enough air has been taken out, we do not hear
the bell at all. Sound cannot travel in a vacuum. We continue to _see_
the bell, however, so that evidently light can travel in a vacuum. The
invisible medium through which the waves of light travel is the ether,
and this ether permeates all space _and all matter_. Between us and the
stars stretch vast regions empty of all matter. But we see the stars;
their light reaches us, even though it may take centuries to do so. We
conceive, then, that it is the universal ether which conveys that light.
All the energy which has reached the earth from the sun and which,
stored for ages in our coal-fields, is now used to propel our trains and
steamships, to heat and light our cities, to perform all the
multifarious tasks of modern life, was conveyed by the ether. Without
that universal carrier of energy we should have nothing but a stagnant,
lifeless world.
[Illustration: _Photo: Leadbeater._
AN ELECTRIC SPARK
An electric spark consists of a rush of electrons across the space
between the two terminals. A state of tension is established in the
ether by the electric charges, and when this tension passes a certain
limit the discharge takes place.]
[Illustration: _From "Scientific Ideas of To-day."_
AN ETHER DISTURBANCE AROUND AN ELECTRON CURRENT
In the left-hand photograph an electric current is passing through the
coil, thus producing a magnetic field and transforming the poker into a
magnet. The poker is then able to support a pair of scissors. As soon as
the electric current is broken off, as in the second photograph, the
ether disturbance ceases. The poker loses its magnetism, and the
scissors fall.]
We have said that light consists of waves. The ether may be considered
as resembling, in some respects, a jelly. It can transmit vibrations.
The waves of light are really excessively small ripples, measuring from
crest to crest. The distance from crest to crest of the ripples in a
pond is sometimes no more than an inch or two. This distance is
enormously great compared to the longest of the wave-lengths that
constitute light. We say the longest, for the waves of light differ in
length; the colour depends upon the length of the light. Red light has
the longest waves and violet the shortest. The longest waves, the waves
of deep-red light, are seven two hundred and fifty thousandths of an
inch in length (7/250,000 inch). This is nearly twice the length of
deep-violet light-waves, which are 1/67,000 inch. But light-waves, the
waves that affect the eye, are not the only waves carried by the ether.
Waves too short to affect the eye can affect the photographic plate, and
we can discover in this way the existence of waves only half the length
of the deep-violet waves. Still shorter waves can be discovered, until
we come to those excessively minute rays, the X-rays.
Below the Limits of Visibility
But we can extend our investigations in the other direction; we find
that the ether carries many waves longer than light-waves. Special
photographic emulsions can reveal the existence of waves five times
longer than violet-light waves. Extending below the limits of visibility
are waves we detect as heat-waves. Radiant heat, like the heat from a
fire, is also a form of wave-motion in the ether, but the waves our
senses recognise as heat are longer than light-waves. There are longer
waves still, but our senses do not recognise them. But we can detect
them by our instruments. These are the waves used in wireless
telegraphy, and their length may be, in some cases, measured in miles.
These waves are the so-called electro-magnetic waves. Light, radiant
heat, and electro-magnetic waves are all of the same nature; they differ
only as regards their wave-lengths.
LIGHT--VISIBLE AND INVISIBLE
If Light, then, consists of waves transmitted through the ether, what
gives rise to the waves? Whatever sets up such wonderfully rapid series
of waves must be something with an enormous vibration. We come back to
the electron: all atoms of matter, as we have seen, are made up of
electrons revolving in a regular orbit round a nucleus. These electrons
may be affected by out-side influences, they may be agitated and their
speed or vibration increased.
Electrons and Light
The particles even of a piece of cold iron are in a state of vibration.
No nerves of ours are able to feel and register the waves they emit, but
your cold poker is really radiating, or sending out a series of
wave-movements, on every side. After what we saw about the nature of
matter, this will surprise none. Put your poker in the fire for a time.
The particles of the glowing coal, which are violently agitated,
communicate some of their energy to the particles of iron in the poker.
They move to and fro more rapidly, and the waves which they create are
now able to affect your nerves and cause a sensation of heat. Put the
poker again in the fire, until its temperature rises to 500° C. It
begins to glow with a dull red. Its particles are now moving very
violently, and the waves they send out are so short and rapid that they
can be picked up by the eye--we have _visible_ light. They would still
not affect a photographic plate. Heat the iron further, and the crowds
of electrons now send out waves of various lengths which blend into
white light. What is happening is the agitated electrons flying round in
their orbits at a speed of trillions of times a second. Make the iron
"blue hot," and it pours out, in addition to light, the _invisible_
waves which alter the film on the photographic plate. And beyond these
there is a long range of still shorter waves, culminating in the X-rays,
which will pass between the atoms of flesh or stone.
Nearly two hundred and fifty years ago it was proved that light
travelled at least 600,000 times faster than sound. Jupiter, as we saw,
has moons, which circle round it. They pass behind the body of the
planet, and reappear at the other side. But it was noticed that, when
Jupiter is at its greatest distance from us, the reappearance of the
moon from behind it is 16 minutes and 36 seconds later than when the
planet is nearest to us. Plainly this was because light took so long to
cover the additional distance. The distance was then imperfectly known,
and the speed of light was underrated. We now know the distance, and we
easily get the velocity of light.
No doubt it seems far more wonderful to discover this within the walls
of a laboratory, but it was done as long ago as 1850. A cogged wheel is
so mounted that a ray of light passes between two of the teeth and is
reflected back from a mirror. Now, slight as is the fraction of a second
which light takes to travel that distance, it is possible to give such
speed to the wheel that the next tooth catches the ray of light on its
return and cuts it off. The speed is increased still further until the
ray of light returns to the eye of the observer through the notch _next_
to the one by which it had passed to the mirror! The speed of the wheel
was known, and it was thus possible again to gather the velocity of
light. If the shortest waves are 1/67,000 of an inch in length, and
light travels at 186,000 miles a second, any person can work out that
about 800 trillion waves enter the eye in a second when we see "violet."
Sorting out Light-waves
The waves sent out on every side by the energetic electrons become
faintly visible to us when they reach about 1/35,000 of an inch. As they
become shorter and more rapid, as the electrons increase their speed, we
get, in succession, the colours red, orange, yellow, green, blue,
indigo, and violet. Each distinct sensation of colour means a wave of
different length. When they are all mingled together, as in the light of
the sun, we get white light. When this white light passes through glass,
the speed of the waves is lessened; and, if the ray of light falls
obliquely on a triangular piece of glass, the waves of different lengths
part company as they travel through it, and the light is spread out in a
band of rainbow-colour. The waves are sorted out according to their
lengths in the "obstacle race" through the glass. Anyone may see this
for himself by holding up a wedge-shaped piece of crystal between the
sunlight and the eye; the prism separates the sunlight into its
constituent colours, and these various colours will be seen quite
readily. Or the thing may be realised in another way. If the seven
colours are painted on a wheel as shown opposite page 280 (in the
proportion shown), and the wheel rapidly revolved on a pivot, the wheel
will appear a dull white, the several colours will not be seen. But
_omit_ one of the colours, then the wheel, when revolved, will not
appear white, but will give the impression of one colour, corresponding
to what the union of six colours gives. Another experiment will show
that some bodies held up between the eye and a white light will not
permit all the rays to pass through, but will intercept some; a body
that intercepts all the seven rays except red will give the impression
of red, or if all the rays except violet, then violet will be the colour
seen.
[Illustration: _Photo: H. J. Shepstone._
LIGHTNING
In a thunderstorm we have the most spectacular display in lightning of a
violent and explosive rush of electrons (electricity) from one body to
another, from cloud to cloud, or to the earth. In this wonderful
photograph of an electrical storm note the long branched and undulating
flashes of lightning. Each flash lasts no longer than the one
hundred-thousandth part of a second of time.]
[Illustration: LIGHT WAVES
Light consists of waves transmitted through the ether. Waves of light
differ in length. The colour of the light depends on the wave-length.
Deep-red waves (the longest) are 7/250000 inch and deep-violet waves
1/67000 inch. The diagram shows two wave-motions of different
wave-lengths. From crest to crest, or from trough to trough, is the
length of the wave.]
[Illustration: THE MAGNETIC CIRCUIT OF AN ELECTRIC CURRENT
The electric current passing in the direction of the arrow round the
electric circuit generates in the surrounding space circular magnetic
circuits as shown in the diagram. It is this property which lies at the
base of the electro-magnet and of the electric dynamo.]
[Illustration: THE MAGNET
The illustration shows the lines of force between two magnets. The lines
of force proceed from the north pole of one magnet to the south pole of
the other. They also proceed from the north to the south poles of the
same magnet. These facts are shown clearly in the diagram. The north
pole of a magnet is that end of it which turns to the north when the
magnet is freely suspended.]
The Fate of the World
Professor Soddy has given an interesting picture of what might happen
when the sun's light and heat is no longer what it is. The human eye
"has adapted itself through the ages to the peculiarities of the sun's
light, so as to make the most of that wave-length of which there is
most.... Let us indulge for a moment in these gloomy prognostications,
as to the consequences to this earth of the cooling of the sun with the
lapse of ages, which used to be in vogue, but which radio-activity has
so rudely shaken. Picture the fate of the world when the sun has become
a dull red-hot ball, or even when it has cooled so far that it would no
longer emit light to us. That does not all mean that the world would be
in inky darkness, and that the sun would not emit light to the people
then inhabiting this world, if any had survived and could keep
themselves from freezing. To such, if the eye continued to adapt itself
to the changing conditions, our blues and violets would be ultra-violet
and invisible, but our dark heat would be light and hot bodies would be
luminous to them which would be dark to us."
§ 12
What the Blue "Sky" means
We saw in a previous chapter how the spectroscope splits up light-waves
into their colours. But nature is constantly splitting the light into
its different-lengthed waves, its colours. The rainbow, where dense
moisture in the air acts as a spectroscope, is the most familiar
example. A piece of mother-of-pearl, or even a film of oil on the street
or on water, has the same effect, owing to the fine inequalities in its
surface. The atmosphere all day long is sorting out the waves. The blue
"sky" overhead means that the fine particles in the upper atmosphere
catch the shorter waves, the blue waves, and scatter them. We can make a
tubeful of blue sky in the laboratory at any time. The beautiful
pink-flush on the Alps at sunrise, the red glory that lingers in the
west at sunset, mean that, as the sun's rays must struggle through
denser masses of air when it is low on the horizon, the long red waves
are sifted out from the other shafts.
Then there is the varied face of nature which, by absorbing some waves
and reflecting others, weaves its own beautiful robe of colour. Here and
there is a black patch, which _absorbs_ all the light. White surfaces
_reflect_ the whole of it. What is reflected depends on the period of
vibration of the electrons in the particular kind of matter. Generally,
as the electrons receive the flood of trillions of waves, they absorb
either the long or the medium or the short, and they give us the
wonderful colour-scheme of nature. In some cases the electrons continue
to radiate long after the sunlight has ceased to fall upon them. We get
from them "black" or invisible light, and we can take photographs by it.
Other bodies, like glass, vibrate in unison with the period of the
light-waves and let them stream through.
Light without Heat
There are substances--"phosphorescent" things we call them--which give
out a mysterious cold light of their own. It is one of the problems
of science, and one of profound practical interest. If we could produce
light without heat our "gas bill" would shrink amazingly. So much energy
is wasted in the production of heat-waves and ultra-violet waves which
we do not want, that 90 per cent. or more of the power used in
illumination is wasted. Would that the glow-worm, or even the dead
herring, would yield us its secret! Phosphorus is the one thing we know
as yet that suits the purpose, and--it smells! Indeed, our artificial
light is not only extravagant in cost, but often poor in colour. The
unwary person often buys a garment by artificial light, and is disgusted
next morning to find in it a colour which is not wanted. The colour
disclosed by the sun was not in the waves of the artificial light.
[Illustration: ROTATING DISC OF SIR ISAAC NEWTON FOR MIXING COLOURS
The Spectroscope sorts out the above seven colours from sunlight (which
is compounded of these seven colours). If painted in proper proportions
on a wheel, as shown in the coloured illustration, and the wheel be
turned rapidly on a pivot through its centre, only a dull white will be
perceived. If one colour be omitted, the result will be one colour--the
result of the union of the remaining six.]
Beyond the waves of violet light are the still shorter and more rapid
waves--the "ultra-violet" waves--which are precious to the photographer.
As every amateur knows, his plate may safely be exposed to light that
comes through a red or an orange screen. Such a screen means "no
thoroughfare" for the blue and "beyond-blue" waves, and it is these
which arrange the little grains of silver on the plate. It is the same
waves which supply the energy to the little green grains of matter
(chlorophyll) in the plant, preparing our food and timber for us, as
will be seen later. The tree struggles upward and spreads out its leaves
fanwise to the blue sky to receive them. In our coal-measures, the
mighty dead forests of long ago, are vast stores of sunlight which we
are prodigally using up.
The X-rays are the extreme end, the highest octave, of the series of
waves. Their power of penetration implies that they are excessively
minute, but even these have not held their secret from the modern
physicist. From a series of beautiful experiments, in which they were
made to pass amongst the atoms of a crystal, we learned their length. It
is about the ten-millionth of a millimetre, and a millimetre is about
the 1/25 of an inch!
One of the most recent discoveries, made during a recent eclipse of the
sun, is that light is subject to gravitation. A ray of light from a star
is bent out of its straight path when it passes near the mass of the
sun. Professor Eddington tells us that we have as much right to speak of
a pound of light as of a pound of sugar. Professor Eddington even
calculates that the earth receives 160 tons of light from the sun every
year!
ENERGY: HOW ALL LIFE DEPENDS ON IT
As we have seen in an earlier chapter, one of the fundamental entities
of the universe is matter. A second, not less important, is called
energy. Energy is indispensable if the world is to continue to exist,
since all phenomena, including life, depend on it. Just as it is humanly
impossible to create or to destroy a particle of matter, so is it
impossible to create or to destroy energy. This statement will be more
readily understood when we have considered what energy is.
Energy, like matter, is indestructible, and just as matter exists in
various forms so does energy. And we may add, just as we are ignorant of
what the negative and positive particles of electricity which constitute
matter really are, so we are ignorant of the true nature of energy. At
the same time, energy is not so completely mysterious as it once was. It
is another of nature's mysteries which the advance of modern science has
in some measure unveiled. It was only during the nineteenth century that
energy came to be known as something as distinct and permanent as matter
itself.
Forms of Energy
The existence of various forms of energy had been known, of course, for
ages; there was the energy of a falling stone, the energy produced by
burning wood or coal or any other substance, but the essential
_identity_ of all these forms of energy had not been suspected. The
conception of energy as something which, like matter, was constant in
amount, which could not be created nor destroyed, was one of the great
scientific acquisitions of the past century.
[Illustration: WAVE SHAPES
Wave-motions are often complex. The above illustration shows some fairly
complicated wave shapes. All such wave-motions can be produced by
superposing a number of simple wave forms.]
[Illustration: THE POWER OF A MAGNET
The illustration is that of a "Phoenix" electric magnet lifting scrap
from railway trucks. The magnet is 52 inches in diameter and lifts a
weight of 26 tons. The same type of magnet, 62 inches in diameter, lifts
a weight of 40 tons.]
[Illustration: _Photo: The Locomotive Publishing Co., Ltd._
THE SPEED OF LIGHT
A train travelling at the rate of sixty miles per hour would take rather
more than seventeen and a quarter days to go round the earth at the
equator, i.e. a distance of 25,000 miles. Light, which travels at the
rate of 186,000 miles per second, would take between one-seventh and
one-eighth of a second to go the same distance.]
[Illustration: ROTATING DISC OF SIR ISAAC NEWTON FOR MIXING COLOURS
The Spectroscope sorts out the above seven colours from sunlight (which
is compounded of these seven colours). If painted in proper proportions
on a wheel, as shown in the coloured illustration, and the wheel turned
rapidly on a pivot through its centre, only a dull white will be
perceived. If one colour be omitted, the result will be one colour--the
result of the union of the remaining six.]
It is not possible to enter deeply into this subject here. It is
sufficient if we briefly outline its salient aspects. Energy is
recognised in two forms, kinetic and potential. The form of energy which
is most apparent to us is the _energy of motion_; for example, a rolling
stone, running water, a falling body, and so on. We call the energy of
motion _kinetic energy_. Potential energy is the energy a body has in
virtue of its position--it is its capacity, in other words, to acquire
kinetic energy, as in the case of a stone resting on the edge of a
cliff.
Energy may assume different forms; one kind of energy may be converted
directly or indirectly into some other form. The energy of burning coal,
for example, is converted into heat, and from heat energy we have
mechanical energy, such as that manifested by the steam-engine. In this
way we can transfer energy from one body to another. There is the energy
of the great waterfalls of Niagara, for instance, which are used to
supply the energy of huge electric power stations.
What Heat is
An important fact about energy is, that all energy _tends to take the
form of heat energy_. The impact of a falling stone generates heat; a
waterfall is hotter at the bottom than at the top--the falling particles
of water, on striking the ground, generate heat; and most chemical
changes are attended by heat changes. Energy may remain latent
indefinitely in a lump of wood, but in combustion it is liberated, and
we have heat as a result. The atom of radium or of any other
radio-active substance, as it disintegrates, generates heat. "Every hour
radium generates sufficient heat to raise the temperature of its own
weight of water, from the freezing point to the boiling point." And what
is heat? _Heat is molecular motion._ The molecules of every substance,
as we have seen on a previous page, are in a state of continual motion,
and the more vigorous the motion the hotter the body. As wood or coal
burns, the invisible molecules of these substances are violently
agitated, and give rise to ether waves which our senses interpret as
light and heat. In this constant movement of the molecules, then, we
have a manifestation of the energy of motion and of heat.
That energy which disappears in one form reappears in another has been
found to be universally true. It was Joule who, by churning water, first
showed that a measurable quantity of mechanical energy could be
transformed into a measurable quantity of heat energy. By causing an
apparatus to stir water vigorously, that apparatus being driven by
falling weights or a rotating flywheel or by any other mechanical means,
the water became heated. A certain amount of mechanical energy had been
used up and a certain amount of heat had appeared. The relation between
these two things was found to be invariable. Every physical change in
nature involves a transformation of energy, but the total quantity of
energy in the universe remains unaltered. This is the great doctrine of
the Conservation of Energy.
§ 13
Substitutes for Coal
Consider the source of nearly all the energy which is used in modern
civilisation--coal. The great forests of the Carboniferous epoch now
exists as beds of coal. By the burning of coal--a chemical
transformation--the heat energy is produced on which at present our
whole civilisation depends. Whence is the energy locked up in the coal
derived? From the sun. For millions of years the energy of the sun's
rays had gone to form the vast vegetation of the Carboniferous era and
had been transformed, by various subtle processes, into the potential
energy that slumbers in those immense fossilized forests.
The exhaustion of our coal deposits would mean, so far as our knowledge
extends at present, the end of the world's civilisation. There are other
known sources of energy, it is true. There is the energy of falling
water; the great falls of Niagara are used to supply the energy of huge
electric power stations. Perhaps, also, something could be done to
utilise the energy of the tides--another instance of the energy of
moving water. And attempts have been made to utilise directly the energy
of the sun's rays. But all these sources of energy are small compared
with the energy of coal. A suggestion was made at a recent British
Association meeting that deep borings might be sunk in order to utilise
the internal heat of the earth, but this is not, perhaps, a very
practical proposal. By far the most effective substitutes for coal would
be found in the interior energy of the atom, a source of energy which,
as we have seen, is practically illimitable. If the immense electrical
energy in the interior of the atom can ever be liberated and controlled,
then our steadily decreasing coal supply will no longer be the bugbear
it now is to all thoughtful men.
The stored-up energy of the great coal-fields can be used up, but we
cannot replace it or create fresh supplies. As we have seen, energy
cannot be destroyed, but it can become _unavailable_. Let us consider
what this important fact means.
§ 14
Dissipation of Energy
Energy may become dissipated. Where does it go? since if it is
indestructible it must still exist. It is easier to ask the question
than to give a final answer, and it is not possible in this OUTLINE,
where an advanced knowledge of physics is not assumed on the part of the
reader, to go fully into the somewhat difficult theories put forward by
physicists and chemists. We may raise the temperature, say, of iron,
until it is white-hot. If we stop the process the temperature of the
iron will gradually settle down to the temperature of surrounding
bodies. As it does so, where does its previous energy go? In some
measure it may pass to other bodies in contact with the piece of iron,
but ultimately the heat becomes radiated away in space where we cannot
follow it. It has been added to the vast reservoir of _unavailable_ heat
energy of uniform temperature. It is sufficient here to say that if all
bodies had a uniform temperature we should experience no such thing as
heat, because heat only travels from one body to another, having the
effect of cooling the one and warming the other. In time the two bodies
acquire the same temperature. The sum-total of the heat in any body is
measured in terms of the kinetic energy of its moving molecules.
There must come a time, so far as we can see at present, when, even if
all the heat energy of the universe is not radiated away into empty
infinite space, yet a uniform temperature will prevail. If one body is
hotter than another it radiates heat to that body until both are at the
same temperature. Each body may still possess a considerable quantity of
heat energy, which it has absorbed, but that energy, so far as reactions
between those two bodies are concerned, _is now unavailable_. The same
principle applies whatever number of bodies we consider. Before heat
energy can be utilised we must have bodies with different temperature.
If the whole universe were at some uniform temperature, then, although
it might possess an enormous amount of heat energy, this energy would be
unavailable.
What a Uniform Temperature would mean
And what does this imply? It implies a great deal: for if all the energy
in the world became unavailable, the universe, as it now is, would cease
to be. It is possible that, by the constant interchange of heat
radiations, the whole universe is tending to some uniform temperature,
in which case, although all molecular motion would not have ceased, it
would have become unavailable. In this sense it may be said that the
universe is running down.
[Illustration: NIAGARA FALLS
The energy of this falling water is prodigious. It is used to generate
thousands of horse-power in great electrical installations. The power is
used to drive electric trams in cities 150 to 250 miles away.]
[Illustration: _Photo: Stephen Cribb._
TRANSFORMATION OF ENERGY
An illustration of Energy. The chemical energy brought into existence by
firing the explosive manifesting itself as mechanical energy, sufficient
to impart violent motion to tons of water.]
[Illustration: _Photo: Underwood & Underwood._
"BOILING" A KETTLE ON ICE
When a kettle containing liquid air is placed on ice it "boils" because
the ice is intensely hot _when compared with the very low temperature of
the liquid air_.]
If all the molecules of a substance were brought to a standstill, that
substance would be at the absolute zero of temperature. There could be
nothing colder. The temperature at which all molecular motions would
cease is known: it is -273° C. No body could possibly attain a lower
temperature than this: a lower temperature could not exist. Unless there
exists in nature some process, of which we know nothing at present,
whereby energy is renewed, our solar system must one day sink to this
absolute zero of temperature. The sun, the earth, and every other body
in the universe is steadily radiating heat, and this radiation cannot go
on for ever, because heat continually tends to diffuse and to equalise
temperatures.
But we can see, theoretically, that there is a way of evading this law.
If the chaotic molecular motions which constitute heat could be
_regulated_, then the heat energy of a body could be utilised directly.
Some authorities think that some of the processes which go on in the
living body do not involve any waste energy, that the chemical energy of
food is transformed directly into work without any of it being
dissipated as useless heat energy. It may be, therefore, that man will
finally discover some way of escape from the natural law that, while
energy cannot be destroyed, it has a tendency to become unavailable.
The primary reservoir of energy is the atom; it is the energy of the
atom, the atom of elements in the sun, the stars, the earth, from which
nature draws for all her supply of energy. Shall we ever discover how we
can replenish the dwindling resources of energy, or find out how we can
call into being the at present unavailable energy which is stored up in
uniform temperature?
It looks as if our successors would witness an interesting race,
between the progress of science on the one hand and the depletion of
natural resources upon the other. The natural rate of flow of energy
from its primary atomic reservoirs to the sea of waste heat energy
of uniform temperature, allows life to proceed at a complete pace
sternly regulated by the inexorable laws of supply and demand,
which the biologists have recognised in their field as the struggle
for existence.[5]
[5] _Matter and Energy_, by Professor Soddy.
It is certain that energy is an actual entity just as much as matter,
and that it cannot be created or destroyed. Matter and ether are
receptacles or vehicles of energy. As we have said, what these entities
really are in themselves we do not know. It may be that all forms of
energy are in some fundamental way aspects of the same primary entity
which constitutes matter: how all matter is constituted of particles of
electricity we have already seen. The question to which we await an
answer is: What is electricity?
§ 15
MATTER, ETHER, AND EINSTEIN
The supreme synthesis, the crown of all this progressive conquest of
nature, would be to discover that the particles of positive and negative
electricity, which make up the atoms of matter, are points or centres of
disturbances of some kind in a universal ether, and that all our
"energies" (light, magnetism, gravitation, etc.) are waves or strains of
some kind set up in the ether by these clusters of electrons.
It is a fascinating, tantalising dream. Larmor suggested in 1900 that
the electron is a tiny whirlpool, or "vortex," in ether; and, as such a
vortex may turn in either of two opposite ways, we seem to see a
possibility of explaining positive and negative electricity. But the
difficulties have proved very serious, and the nature of the electron is
unknown. A recent view is that it is "a ring of negative electricity
rotating about its axis at a high speed," though that does not carry us
very far. The unit of positive electricity is even less known. We must
be content to know the general lines on which thought is moving toward
the final unification.
We say "unification," but it would be a grave error to think that ether
is the only possible basis for such unity, or to make it an essential
part of one's philosophy of the universe. Ether was never more than an
imagined entity to which we ascribed the most extraordinary properties,
and which seemed then to promise considerable aid. It was conceived as
an elastic solid of very great density, stretching from end to end of
the universe, transmitting waves from star to star at the rate of
186,000 miles a second; yet it was believed that the most solid matter
passed through it as if it did not exist.
Some years ago a delicate experiment was tried for the purpose of
detecting the ether. Since the earth, in travelling round the sun, must
move through the ether if the ether exists, there ought to be a stream
of ether flowing through every laboratory; just as the motion of a ship
through a still atmosphere will make "a wind." In 1887 Michelson and
Morley tried to detect this. Theoretically, a ray of light in the
direction of the stream ought to travel at a different rate from a ray
of light against the stream or across it. They found no difference, and
scores of other experiments have failed. This does not prove that there
is no ether, as there is reason to suppose that our instruments would
appear to shrink in precisely the same proportion as the alteration of
the light; but the fact remains that we have no proof of the existence
of ether. J. H. Jeans says that "nature acts as if no such thing
existed." Even the phenomena of light and magnetism, he says, do not
imply ether; and he thinks that the hypothesis may be abandoned. The
primary reason, of course, for giving up the notion of the ether is
that, as Einstein has shown, there is no way of detecting its existence.
If there is an ether, then, since the earth is moving through it, there
should be some way of detecting this motion. The experiment has been
tried, as we have said, but, although the method used was very
sensitive, no motion was discovered. It is Einstein who, by
revolutionising our conceptions of space and time, showed that no such
motion ever could be discovered, whatever means were employed, and that
the usual notion of the ether must be abandoned. We shall explain this
theory more fully in a later section.
INFLUENCE OF THE TIDES: ORIGIN OF THE MOON: THE EARTH SLOWING DOWN
§ 16
Until comparatively recent times, until, in fact, the full dawn of
modern science, the tides ranked amongst the greatest of nature's
mysteries. And, indeed, what agency could be invoked to explain this
mysteriously regular flux and reflux of the waters of the ocean? It is
not surprising that that steady, rhythmical rise and fall suggested to
some imaginative minds the breathing of a mighty animal. And even when
man first became aware of the fact that this regular movement was
somehow associated with the moon, was he much nearer an explanation?
What bond could exist between the movements of that distant world and
the diurnal variation of the waters of the earth? It is reported that an
ancient astronomer, despairing of ever resolving the mystery, drowned
himself in the sea.
The Earth Pulled by the Moon
But it was part of the merit of Newton's mighty theory of gravitation
that it furnished an explanation even of this age-old mystery. We can
see, in broad outlines at any rate, that the theory of universal
attraction can be applied to this case. For the moon, Newton taught us,
pulls every particle of matter throughout the earth. If we imagine that
part of the earth's surface which comprises the Pacific Ocean, for
instance, to be turned towards the moon, we see that the moon's pull,
_acting on the loose and mobile water_, would tend to heap it up into a
sort of mound. The whole earth is pulled by the moon, but the water
is more free to obey this pull than is the solid earth, although small
tides are also caused in the earth's solid crust. It can be shown also
that a corresponding hump would tend to be produced on the other side of
the earth, owing, in this case, to the tendency of the water, being more
loosely connected, to lag behind the solid earth. If the earth's surface
were entirely fluid the rotation of the earth would give the impression
that these two humps were continually travelling round the world, once
every day. At any given part of the earth's surface, therefore, there
would be two humps daily, i.e. two periods of high water. Such is the
simplest possible outline of the gravitational theory of the tides.
[Illustration: THE CAUSE OF TIDES
The tides of the sea are due to the pull of the moon, and, in lesser
degree, of the sun. The whole earth is pulled by the moon, but the loose
and mobile water is more free to obey this pull than is the solid earth,
although small tides are also caused in the earth's solid crust. The
effect which the tides have on slowing down the rotation of the earth is
explained in the text.]
[Illustration: _Photo: G. Brocklehurst._
THE AEGIR ON THE TRENT
An exceptionally smooth formation due to perfect weather conditions. The
wall-like formation of these tidal waves (see next page also) will be
noticed. The reason for this is that the downward current in the river
heads the sea-water back, and thus helps to exaggerate the advancing
slope of the wave. The exceptional spring tides are caused by the
combined operation of the moon and the sun, as is explained in the
text.]
[Illustration: _Photo: G. Brocklehurst._
A BIG SPRING TIDE, THE AEGIR ON THE TRENT]
The actually observed phenomena are vastly more complicated, and the
complete theory bears very little resemblance to the simple form we have
just outlined. Everyone who lives in the neighbourhood of a port knows,
for instance, that high water seldom coincides with the time when the
moon crosses the meridian. It may be several hours early or late. High
water at London Bridge, for instance, occurs about one and a half hours
after the moon has passed the meridian, while at Dublin high water
occurs about one and a half hours before the moon crosses the meridian.
The actually observed phenomena, then, are far from simple; they have,
nevertheless, been very completely worked out, and the times of high
water for every port in the world can now be prophesied for a
considerable time ahead.
The Action of Sun and Moon
It would be beyond our scope to attempt to explain the complete theory,
but we may mention one obvious factor which must be taken into account.
Since the moon, by its gravitational attraction, produces tides, we
should expect that the sun, whose gravitational attraction is so much
stronger, should also produce tides and, we would suppose at first
sight, more powerful tides than the moon. But while it is true that the
sun produces tides, it is not true that they are more powerful than
those produced by the moon. The sun's tide-producing power is, as a
matter of fact, less than half that of the moon. The reason of this is
that _distance_ plays an enormous rôle in the production of tides. The
mass of the sun is 26,000,000 times that of the moon; on the other hand
it is 386 times as far off as the moon. This greater distance more than
counterbalances its greater mass, and the result, as we have said, is
that the moon is more than twice as powerful. Sometimes the sun and moon
act together, and we have what are called spring tides; sometimes they
act against one another, and we have neap tides. These effects are
further complicated by a number of other factors, and the tides, at
various places, vary enormously. Thus at St. Helena the sea rises and
falls about three feet, whereas in the Bay of Fundy it rises and falls
more than fifty feet. But here, again, the reasons are complicated.
§ 17
Origin of the Moon
But there is another aspect of the tides which is of vastly greater
interest and importance than the theory we have just been discussing. In
the hands of Sir George H. Darwin, the son of Charles Darwin, the tides
had been made to throw light on the evolution of our solar system. In
particular, they have illustrated the origin and development of the
system formed by our earth and moon. It is quite certain that, long ages
ago, the earth was rotating immensely faster than it is now, and that
the moon was so near as to be actually in contact with the earth. In
that remote age the moon was just on the point of separating from the
earth, of being thrown off by the earth. Earth and moon were once one
body, but the high rate of rotation caused this body to split up into
two pieces; one piece became the earth we now know, and the other became
the moon. Such is the conclusion to which we are led by an examination
of the tides. In the first place let us consider the energy produced by
the tides. We see evidences of this energy all round the word's
coastlines. Estuaries are scooped out, great rocks are gradually reduced
to rubble, innumerable tons of matter are continually being set in
movement. Whence is this energy derived? Energy, like matter, cannot be
created from nothing; what, then, is the source which makes this
colossal expenditure possible.
The Earth Slowing down
The answer is simple, but startling. _The source of tidal energy is the
rotation of the earth._ The massive bulk of the earth, turning every
twenty-four hours on its axis, is like a gigantic flywheel. In virtue of
its rotation it possesses an enormous store of energy. But even the
heaviest and swiftest flywheel, if it is doing work, or even if it is
only working against the friction of its bearings, cannot dispense
energy for ever. It must, gradually, slow down. There is no escape from
this reasoning. It is the rotation of the earth which supplies the
energy of the tides, and, as a consequence, the tides must be slowing
down the earth. The tides act as a kind of brake on the earth's
rotation. These masses of water, _held back by the moon_, exert a kind
of dragging effect on the rotating earth. Doubtless this effect,
measured by our ordinary standards, is very small; it is, however,
continuous, and in the course of the millions of years dealt with in
astronomy, this small but constant effect may produce very considerable
results.
But there is another effect which can be shown to be a necessary
mathematical consequence of tidal action. It is the moon's action on the
earth which produces the tides, but they also react on the moon. The
tides are slowing down the earth, and they are also driving the moon
farther and farther away. This result, strange as it may seem, does not
permit of doubt, for it is the result of an indubitable dynamical
principle, which cannot be made clear without a mathematical discussion.
Some interesting consequences follow.
Since the earth is slowing down, it follows that it was once rotating
faster. There was a period, a long time ago, when the day comprised only
twenty hours. Going farther back still we come to a day of ten hours,
until, inconceivable ages ago, the earth must have been rotating on its
axis in a period of from three to four hours.
At this point let us stop and inquire what was happening to the moon. We
have seen that at present the moon is getting farther and farther away.
It follows, therefore, that when the day was shorter the moon was
nearer. As we go farther back in time we find the moon nearer and nearer
to an earth rotating faster and faster. When we reach the period we have
already mentioned, the period when the earth completed a revolution in
three or four hours, we find that the moon was so near as to be almost
grazing the earth. This fact is very remarkable. Everybody knows that
there is a _critical velocity_ for a rotating flywheel, a velocity
beyond which the flywheel would fly into pieces because the centrifugal
force developed is so great as to overcome the cohesion of the molecules
of the flywheel. We have already likened our earth to a flywheel, and we
have traced its history back to the point where it was rotating with
immense velocity. We have also seen that, at that moment, the moon was
barely separated from the earth. The conclusion is irresistible. In an
age more remote the earth _did_ fly in pieces, and one of those pieces
is the moon. Such, in brief outline, is the tidal theory of the origin
of the earth-moon system.
The Day Becoming Longer
At the beginning, when the moon split off from the earth, it obviously
must have shared the earth's rotation. It flew round the earth in the
same time that the earth rotated, that is to say, the month and the day
were of equal length. As the moon began to get farther from the earth,
the month, because the moon took longer to rotate round the earth, began
to get correspondingly longer. The day also became longer, because the
earth was slowing down, taking longer to rotate on its axis, but the
month increased at a greater rate than the day. Presently the month
became equal to two days, then to three, and so on. It has been
calculated that this process went on until there were twenty-nine days
in the month. After that the number of days in the month began to
decrease until it reached its present value or magnitude, and will
continue to decrease until once more the month and the day are equal. In
that age the earth will be rotating very slowly. The braking action of
the tides will cause the earth always to keep the same face to the moon;
it will rotate on its axis in the same time that the moon turns round
the earth. If nothing but the earth and moon were involved this state of
affairs would be final. But there is also the effect of the solar tides
to be considered. The moon makes the day equal to the month, but the sun
has a tendency, by still further slowing down the earth's rotation on
its axis, to make the day equal to the year. It would do this, of
course, by making the earth take as long to turn on its axis as to go
round the sun. It cannot succeed in this, owing to the action of the
moon, but it can succeed in making the day rather longer than the month.
Surprising as it may seem, we already have an illustration of this
possibility in the satellites of Mars. The Martian day is about one
half-hour longer than ours, but when the two minute satellites of Mars
were discovered it was noticed that the inner one of the two revolved
round Mars in about seven hours forty minutes. In one Martian day,
therefore, one of the moons of Mars makes more than three complete
revolutions round that planet, so that, to an inhabitant of Mars, there
would be more than three months in a day.
BIBLIOGRAPHY
ARRHENIUS, SVANTE, _Worlds in the Making_.
CLERK-MAXWELL, JAMES, _Matter and Motion_.
DANIELL, ALFRED, _A Text-Book of the Principles of Physics_.
DARWIN, SIR G. H., _The Tides_.
HOLMAN, _Matter, Energy, Force and Work_.
KAPP, GISBERT, _Electricity_.
KELVIN, LORD, _Popular Lectures and Addresses_. Vol. i. _Constitution
of Matter._
LOCKYER, SIR NORMAN, _Inorganic Evolution_.
LODGE, SIR OLIVER, _Electrons_ and _The Ether of Space_.
PERRIN, JEAN, _Brownian Movement and Molecular Reality_.
SODDY, FREDERICK, _Matter and Energy_ and _The Interpretation of Radium_.
THOMPSON, SILVANUS P., _Light, Visible and Invisible_.
THOMSON, SIR J. J., _The Corpuscular Theory of Matter_.
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