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Thomas Hunt Morgan
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Title: A Critique of the Theory of Evolution
Author: Thomas Hunt Morgan
Release Date: December 17, 2009 [EBook #30701]
Language: English
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* * * * *
Princeton University
THE LOUIS CLARK VANUXEM FOUNDATION
LECTURES FOR 1915-1916
* * * * *
The Louis Clark Vanuxem Foundation of Princeton University
was established in 1912 with a bequest of $25,000 under the will of Louis
Clark Vanuxem, of the Class of 1879. By direction of the executors of Mr.
Vanuxem's estate, the income of the foundation is to be used for a series
of public lectures delivered in Princeton annually, at least one half of
which shall be on subjects of current scientific interest. The lectures are
to be published and distributed among schools and libraries generally.
The following lectures have already been published or are in press:
1912-13 The Theory of Permutable Functions, by Vito Volterra
1913-14 Lectures delivered in connection with the dedication of the
Graduate College of Princeton University by Emile Boutroux, Alois
Riehl, A. D. Godley, and Arthur Shipley
1914-15 Romance, by Sir Walter Raleigh
1915-16 A Critique of the Theory of Evolution, by Thomas Hunt Morgan
* * * * *
LOUIS CLARK VANUXEM FOUNDATION
A CRITIQUE
OF THE
THEORY OF EVOLUTION
BY
THOMAS HUNT MORGAN
PROFESSOR OF EXPERIMENTAL ZOOLOGY IN
COLUMBIA UNIVERSITY
LECTURES DELIVERED AT PRINCETON UNIVERSITY
FEBRUARY 24, MARCH 1, 8, 15, 1916
PRINCETON UNIVERSITY PRESS
PRINCETON
LONDON: HUMPHREY MILFORD
OXFORD UNIVERSITY PRESS
1916
* * * * *
Copyright, 1916, by
PRINCETON UNIVERSITY PRESS
Published October, 1916
[Illustration]
* * * * *
PREFACE
Occasionally one hears today the statement that we have come to realize
that we know nothing about evolution. This point of view is a healthy
reaction to the over-confident belief that we knew everything about
evolution. There are even those rash enough to think that in the last few
years we have learned more about evolution than we might have hoped to know
a few years ago. A _critique_ therefore not only becomes a criticism of the
older evidence but an appreciation of the new evidence.
In the first lecture an attempt is made to put a new valuation on the
traditional evidence for evolution. In the second lecture the most recent
work on heredity is dealt with, for only characters that are inherited can
become a part of the evolutionary process. In the third lecture the
physical basis of heredity and the composition of the germ plasm stream are
examined in the light of new observations; while in the fourth lecture the
thesis is developed that chance variation combined with a property of
living things to manifold themselves is the key note of modern evolutionary
thought.
T. H. MORGAN
_July, 1916_
* * * * *
TABLE OF CONTENTS
CHAPTER I
A REVALUATION OF THE EVIDENCE ON
WHICH THE THEORY OF EVOLUTION WAS BASED
PAGE
PREFACE v
1. THREE KINDS OF EVOLUTION 1-7
2. THE EVIDENCE FOR ORGANIC EVOLUTION 7-27
a. The Evidence from Comparative Anatomy 7-14
b. The Evidence from Embryology 14-23
c. The Evidence from Paleontology 24-27
3. THE FOUR GREAT HISTORICAL SPECULATIONS 27-39
a. The Environment 27-31
Geoffroy St. Hilaire
b. Use and Disuse 31-34
From Lamarck to Weismann
c. The Unfolding Principle 34-36
Nägeli and Bateson
d. Natural Selection 36-39
Darwin
CHAPTER II
THE BEARING OF MENDEL'S DISCOVERY ON
THE ORIGIN OF HEREDITY CHARACTERS
1. Mendel's First Discovery--Segregation 41-52
2. Mendel's Second Discovery--Independent
Assortment 52-59
3. The Characters of Wild Animals and Plants
Follow the Same Laws of Inheritance as do
the Characters of Domesticated Animals and
Plants 59-84
a. Sexual Dimorphism 61-64
Eosin eye color of Drosophila 61-62
Color of the Clover Butterfly, Colias
philodice 62-63
Color of Papilio turnus 63
Color pattern of Papilio polytes 63-64
b. Duplication of parts 65-66
Thorax of Drosophila 65
Legs of Drosophila 65-66
c. Loss of characters 66-68
"Eyeless" of Drosophila 66-67
Vestigial wings of Drosophila 67
Bar eye of Drosophila 67-68
d. Small changes of characters 68-70
"Speck" 68
Bristles of "club" 70
e. Manifold effects of same factor 71
f. Constant but trivial effects may be the
product of factors having other vital
aspect 73
g. Sex-linked inheritance 75-80
in Drosophila ampelophila 75-76
in the wild species D. repleta 76
in man 77
in domesticated Fowls 77-78
in the wild moth, Abraxas 78-80
h. Multiple allelomorphs 81-84
in the wild Grouse Locust 81-83
in domesticated mice and rabbits 83
in Drosophila ampelophila 84
4. MUTATION AND EVOLUTION 84-88
CHAPTER III
THE FACTORIAL THEORY OF HEREDITY
AND THE COMPOSITION OF THE GERM PLASM
1. THE CELLULAR BASIS OF ORGANIC EVOLUTION
AND HEREDITY 89-98
2. THE MECHANISM OF MENDELIAN HEREDITY
DISCOVERED IN THE BEHAVIOR OF THE
CHROMOSOMES 98-102
3. THE FOUR GREAT LINKAGE GROUPS OF DROSOPHILA
AMPELOPHILA 103-118
a. Group I. 104-109
b. Group II. 109-112
c. Group III. 112-115
d. Group IV. 115-118
4. LOCALIZATION OF FACTORS IN THE CHROMOSOMES 118-142
a. The Evidence from Sex Linked Inheritance 118-137
b. The Evidence from Interference 137-138
c. The Evidence from Non-Disjunction 139-142
5. HOW MANY GENETIC FACTORS ARE THERE IN
THE GERM-PLASM OF A SINGLE INDIVIDUAL? 142-143
6. CONCLUSIONS 144
CHAPTER IV
SELECTION AND EVOLUTION
1. THE THEORY OF NATURAL SELECTION 145-161
2. HOW HAS SELECTION IN DOMESTICATED ANIMALS
AND PLANTS BROUGHT ABOUT ITS RESULTS? 161-165
3. ARE FACTORS CHANGED THROUGH SELECTION? 165-187
4. HOW DOES NATURAL SELECTION INFLUENCE
THE COURSE OF EVOLUTION? 187-193
5. CONCLUSIONS 193-194
INDEX 195-197
* * * * *
CHAPTER I
A REVALUATION OF THE EVIDENCE ON WHICH THE THEORY OF EVOLUTION WAS BASED
We use the word evolution in many ways--to include many different kinds of
changes. There is hardly any other scientific term that is used so
carelessly--to imply so much, to mean so little.
THREE KINDS OF EVOLUTION
We speak of the evolution of the stars, of the evolution of the horse, of
the evolution of the steam engine, as though they were all part of the same
process. What have they in common? Only this, that each concerns itself
with the _history_ of something. When the astronomer thinks of the
_evolution_ of the earth, the moon, the sun and the stars, he has a picture
of diffuse matter that has slowly condensed. With condensation came heat;
with heat, action and reaction within the mass until the chemical
substances that we know today were produced. This is the nebular hypothesis
of the astronomer. The astronomer explains, or tries to explain, how this
evolution took place, by an appeal to the physical processes that have been
worked out in the laboratory, processes which he thinks have existed
through all the eons during which this evolution was going on and which
were its immediate causes.
When the biologist thinks of the evolution of animals and plants, a
different picture presents itself. He thinks of series of animals that have
lived in the past, whose bones (fig. 1) and shells have been preserved in
the rocks. He thinks of these animals as having in the past given birth,
through an unbroken succession of individuals, to the living inhabitants of
the earth today. He thinks that the old, simpler types of the past have in
part changed over into the more complex forms of today.
He is thinking as the historian thinks, but he sometimes gets confused and
thinks that he is explaining evolution when he is only describing it.
[Illustration: FIG. 1. A series of skulls and feet. Eohippus, Mesohippus,
Meryhippus, Hipparion and Equus. (American Museum of Natural History. After
Matthews.)]
A third kind of evolution is one for which man himself is responsible, in
the sense that he has brought it about, often with a definite end in view.
His mind has worked slowly from stage to stage. We can often trace the
history of the stages through which his psychic processes have passed. The
evolution of the steam-boat, the steam engine, paintings, clothing,
instruments of agriculture, of manufacture, or of warfare (fig. 2)
illustrates the history of human progress. There is an obvious and striking
similarity between the evolution of man's inventions and the evolution of
the shells of molluscs and of the bones of mammals, yet in neither case
does a knowledge of the order in which these things arose explain them. If
we appeal to the psychologist he will probably tell us that human
inventions are either the result of happy accidents, that have led to an
unforeseen, but discovered use; or else the use of the invention was
foreseen. It is to the latter process more especially that the idea of
_purpose_ is applied. When we come to review the four great lines of
evolutionary thought we shall see that this human idea of purpose recurs in
many forms, suggesting that man has often tried to explain how organic
evolution has taken place by an appeal to the method which he believes he
makes use of himself in the inorganic world.
[Illustration: FIG. 2. Evolution of pole arms. (Metropolitan Museum. After
Dean.)]
What has the evolution of the stars, of the horse and of human inventions
in common? Only this, that in each case from a simple beginning through a
series of changes something more complex, or at least different, has come
into being. To lump all these kinds of changes into one and call them
evolution is no more than asserting that you believe in consecutive series
of events (which is history) causally connected (which is science); that
is, that you believe in history and that you believe in science. But let us
not forget that we may have complete faith in both without thereby offering
any explanation of either. It is the business of science to find out
_specifically_ what kinds of events were involved when the stars evolved in
the sky, when the horse evolved on the earth, and the steam engine was
evolved from the mind of man.
Is it not rather an empty generalization to say that any kind of change is
a process of evolution? At most it means little more than that you want to
intimate that miraculous intervention is not necessary to account for such
kinds of histories.
We are concerned here more particularly with the biologists' ideas of
evolution. My intention is to review the evidence on which the old theory
rested its case, in the light of some of the newer evidence of recent
years.
Four great branches of study have furnished the evidence of organic
evolution. They are:
Comparative anatomy.
Embryology.
Paleontology.
Experimental Breeding or Genetics.
_The Evidence from Comparative Anatomy_
When we study animals and plants we find that they can be arranged in
groups according to their resemblances. This is the basis of comparative
anatomy, which is only an accurate study of facts that are superficially
obvious to everyone.
The groups are based not on a single difference, but on a very large number
of resemblances. Let us take for example the group of vertebrates.
[Illustration: FIG. 3. Limb skeletons of extinct and living animals,
showing the homologous bones: 1, salamander; 2, frog; 3, turtle; 4,
Aetosaurus; 5, Pleisiosaurus; 6, Ichthyosaurus; 7, Mesosaurus; 8, duck.
(After Jordan and Kellogg.)]
The hand and the arm of man are similar to the hand and arm of the ape. We
find the same plan in the forefoot of the rat, the elephant, the horse and
the opossum. We can identify the same parts in the forefoot of the lizard,
the frog (fig. 3), and even, though less certainly, in the pectoral fins of
fishes. Comparison does not end here. We find similarities in the skull and
back bones of these same animals; in the brain; in the digestive system; in
the heart and blood vessels; in the muscles.
Each of these systems is very complex, but the same general arrangement is
found in all. Anyone familiar with the evidence will, I think, probably
reach the conclusion either that these animals have been created on some
preconceived plan, or else that they have some other bond that unites them;
for we find it difficult to believe that such complex, yet similar things
could have arisen independently. But we try to convince our students of the
truth of the theory of evolution not so much by calling their attention to
this relation as by tracing each organ from a simple to a complex
structure.
I have never known such a course to fail in its intention. In fact, I know
that the student often becomes so thoroughly convinced that he resents any
such attempt as that which I am about to make to point out that the
evidence for his conviction is not above criticism.
[Illustration: FIG. 4. Drosophila ampelophila. a, Female and b, male.]
Because we can often arrange the series of structures in a line extending
from the very simple to the more complex, we are apt to become unduly
impressed by this fact and conclude that if we found the complete series we
should find all the intermediate steps and that they have arisen in the
order of their complexity. This conclusion is not necessarily correct. Let
me give some examples that have come under my own observation. We have bred
for five years the wild fruit fly Drosophila ampelophila (fig. 4) and we
have found over a hundred and twenty-five new types that breed true. Each
has arisen independently and suddenly. Every part of the body has been
affected by one or another of these mutations. For instance many different
kinds of changes have taken place in the wings and several of these involve
the size of the wings. If we arrange the latter arbitrarily in the order of
their size there will be an almost complete series beginning with the
normal wings and ending with those of apterous flies. Several of these
types are represented in figure 5. The order in which these mutations
occurred bears no relation to their size; each originated independently
from the wild type.
[Illustration: FIG. 5. Mutants of Drosophila ampelophila arranged in order
of size of wings: (a) cut; (b) beaded; (c) stumpy; (d) another individual
of stumpy; (f) vestigial (g) apterous.]
The wings of the wild fly are straight (fig. 4). Several types have arisen
in which the wings are bent upwards and in the most extreme type the wings
are curled over the back, as seen in figure 54 (g), yet there is no
historical connection between these stages.
Mutations have occurred involving the pigmentation of the body and wings.
The head and thorax of the wild Drosophila ampelophila are grayish yellow,
the abdomen is banded with yellow and black, and the wings are gray. There
have appeared in our cultures several kinds of darker types ranging to
almost black flies (fig. 20) and to lighter types that are quite yellow. If
put in line a series may be made from the darkest flies at one end to the
light yellow flies at the other. These types, with the fluctuations that
occur within each type, furnish a complete series of gradations; yet
historically they have arisen independently of each other.
Many changes in eye color have appeared. As many as thirty or more races
differing in eye color are now maintained in our cultures. Some of them are
so similar that they can scarcely be separated from each other. It is
easily possible beginning with the darkest eye color, sepia, which is deep
brown, to pick out a perfectly graded series ending with pure white eyes.
But such a serial arrangement would give a totally false idea of the way
the different types have arisen; and any conclusion based on the existence
of such a series might very well be entirely erroneous, for the fact that
such a series exists bears no relation to the order in which its members
have appeared.
Suppose that evolution "in the open" had taken place in the same way, by
means of _discontinuous_ variation. What value then would the evidence from
comparative anatomy have in so far as it is based on a continuous series of
variants of any organ?
No one familiar with the entire evidence will doubt for a moment that these
125 races of Drosophila ampelophila belong to the same species and have had
a common origin, for while they may differ mainly in one thing they are
extremely alike in a hundred other things, and in the general relation of
the parts to each other.
It is in this sense that the evidence from comparative anatomy can be used
I think as an argument for evolution. It is the resemblances that the
animals or plants in any group have in common that is the basis for such a
conclusion; it is not because we can arrange in a continuous series any
particular variations. In other words, our inference concerning the common
descent of two or more species is based on the totality of such
resemblances that still remain in large part after each change has taken
place. In this sense the argument from comparative anatomy, while not a
demonstration, carries with it, I think, a high degree of probability.
_The Evidence from Embryology_
In passing from the egg to the adult the individual goes through a series
of changes. In the course of this development we see not only the
beginnings of the organs that gradually enlarge and change into those of
the adult animal, but also see that organs appear and later disappear
before the adult stage is reached. We find, moreover, that the young
sometimes resemble in a most striking way the adult stage of groups that we
place lower in the scale of evolution.
Many years before Darwin advanced his theory of evolution through natural
selection, the resemblance of the young of higher animals to the adults of
lower animals had attracted the attention of zoölogists and various views,
often very naïve, had been advanced to account for the resemblance. Among
these speculations there was one practically identical with that adopted by
Darwin and the post-Darwinians, namely that the higher animals repeat in
their development the _adult stages_ of lower animals. Later this view
became one of the cornerstones of the theory of organic evolution. It
reached its climax in the writings of Haeckel, and I think I may add
without exaggeration that for twenty-five years it furnished the chief
inspiration of the school of descriptive embryology. Today it is taught in
practically all textbooks of biology. Haeckel called this interpretation
the Biogenetic Law.
[Illustration: FIG. 6. Young trout (Trutta fario) six days after hatching.
(After Ziegler.)]
It was recognized, of course, that many embryonic stages could not possibly
represent ancestral animals. A young fish with a huge yolk sac attached
(fig. 6) could scarcely ever have led a happy, free life as an adult
individual. Such stages were interpreted, however, as _embryonic_ additions
to the original ancestral type. The embryo had done something on its own
account.
In some animals the young have structures that attach them to the mother,
as does the placenta of the mammals. In other cases the young develop
membranes about themselves--like the amnion of the chick (fig. 7) and
mammal--that would have shut off an adult animal from all intercourse with
the outside world. Hundreds of such embryonic adaptations are known to
embryologists. These were explained as adaptations and as falsifications of
the ancestral records.
[Illustration: FIG. 7. Diagram of chick showing relations of amnion,
allantois and yolk. (After Lillie.)]
At the end of the last century Weismann injected a new idea into our views
concerning the origin of variations. He urged that variations are germinal,
i.e. they first appear in the egg and the sperm as changes that later bring
about modifications in the individual. The idea has been fruitful and is
generally accepted by most biologists today. It means that the offspring of
a pair of animals are not affected by the structure or the activities of
their parents, but the germ plasm is the unmodified stream from which both
the parent and the young have arisen. Hence their resemblance. Now, it has
been found that a variation arising in the germ plasm, no matter what its
cause, may affect any stage in the development of the next individuals that
arise from it. There is no reason to suppose that such a change produces a
new character that always sticks itself, as it were, on to the end of the
old series. This idea of germinal variation therefore carried with it the
death of the older conception of evolution by superposition.
In more recent times another idea has become current, mainly due to the
work of Bateson and of de Vries--the idea that variations are
discontinuous. Such a conception does not fall easily into line with the
statement of the biogenetic "law"; for actual experience with discontinuous
variation has taught us that new characters that arise do not add
themselves to the end of the line of already existing characters but if
they affect the adult characters they change them without, as it were,
passing through and beyond them.
[Illustration: FIG. 8. Diagram of head of chick A and B, showing gill
slits, and aortic arches; and head of fish C showing aortic arches. (After
Hesse.)]
[Illustration: FIG. 9. Human embryo showing gill slits and aortic arches.
(After His; from Marshall.)]
I venture to think that these new ideas and this new evidence have played
havoc with the biogenetic "law". Nevertheless, there is an interpretation
of the facts that is entirely compatible with the theory of evolution. Let
me illustrate this by an example.
[Illustration: FIG. 10. Young fish, dorsal view, and side view, showing
gill slits. (After Kopsch.)]
The embryos of the chick (fig. 8) and of man (fig. 9) possess at an early
stage in their development gill-slits on the sides of the neck like those
of fishes. No one familiar with the relations of the parts will for a
moment doubt that the gill slits of these embryos and of the fish represent
the same structures. When we look further into the matter we find that
young fish also possess gill slits (fig. 10 and 11)--even in young stages
in their development. Is it not then more probable that the mammal and bird
possess this stage in their development simply because it has never been
lost? Is not this a more reasonable view than to suppose that the gill
slits of the embryos of the higher forms represent the adult gill slits of
the fish that in some mysterious way have been pushed back into the embryo
of the bird?
[Illustration: FIG. 11. Side views of head of embryo sharks, showing gill
slits.]
I could give many similar examples. All can be interpreted as embryonic
survivals rather than as phyletic contractions. Not one of them calls for
the latter interpretation.
The study of the cleavage pattern of the segmenting egg furnishes the most
convincing evidence that a different explanation from the one stated in the
biogenetic law is the more probable explanation.
[Illustration: FIG. 12. Cleavage stages of four types of eggs, showing the
origin of the mesenchyme cells (stippled) and mesoderm cells (darker); a,
Planarian; b, Annelid (Podarke); c, Mollusc (Crepidula), d, Mollusc
(Unio).]
It has been found that the cleavage pattern has the same general
arrangement in the early stages of flat worms, annelids and molluscs (fig.
12). Obviously these stages have never been adult ancestors, and obviously
if their resemblance has any meaning at all, it is that each group has
retained the same general plan of cleavage, possessed by their common
ancestor.
Accepting this view, let us ask, does the evidence from embryology favor
the theory of evolution? I think that it does very strongly. The embryos of
the mammal, bird, and lizard have gill slits today because gill slits were
present in the embryos of their ancestors. There is no other view that
explains so well their presence in the higher forms.
Perhaps someone will say, Well! is not this all that we have contended for!
Have you not reached the old conclusion in a roundabout way? I think not.
To my mind there is a wide difference between the old statement that the
higher animals living today have the original adult stages telescoped into
their embryos, and the statement that the resemblance between certain
characters in the embryos of higher animals and corresponding stages in the
embryos of lower animals is most plausibly explained by the assumption that
they have descended from the same ancestors, and that their common
structures are embryonic survivals.
_The Evidence from Paleontology_
The direct evidence furnished by fossil remains is by all odds the
strongest evidence that we have in favor of organic evolution. Paleontology
holds the incomparable position of being able to point directly to the
evidence showing that the animals and plants living in past times are
connected with those living at the present time, often through an unbroken
series of stages. Paleontology has triumphed over the weakness of the
evidence, which Darwin admitted was serious, by filling in many of the
missing links.
Paleontology has been criticised on the ground that she cannot pretend to
show the actual ancestors of living forms because, if in the past genera
and species were as abundant and as diverse as we find them at present, it
is very improbable that the bones of any individual that happened to be
preserved are the bones of just that species that took part in the
evolution. Paleontologists will freely admit that in many cases this is
probably true, but even then the evidence is, I think, still just as
valuable and in exactly the same sense as is the evidence from comparative
anatomy. It suffices to know that there lived in the past a particular
"group" of animals that had many points in common with those that preceded
them and with those that came later. Whether these are the actual ancestors
or not does not so much matter, for the view that from such a group of
species the later species have been derived is far more probable than any
other view that has been proposed.
With this unrivalled material and splendid series of gradations,
paleontology has constructed many stages in the past history of the globe.
But paleontologists have sometimes gone beyond this descriptive phase of
the subject and have attempted to formulate the "causes", "laws" and
"principles" that have led to the development of their series. It has even
been claimed that paleontologists are in an incomparably better position
than zoölogists to discover such principles, because they know both the
beginning and the end of the evolutionary series. The retort is obvious. In
his sweeping and poetic vision the paleontologist may fail completely to
find out the nature of the pigments that have gone into the painting of his
picture, and he may confuse a familiarity with the different views he has
enjoyed of the canvas with a knowledge of how the painting is being done.
My good friend the paleontologist is in greater danger than he realizes,
when he leaves descriptions and attempts explanation. He has no way to
check up his speculations and it is notorious that the human mind without
control has a bad habit of wandering.
When the modern student of variation and heredity--the geneticist--looks
over the different "continuous" series, from which certain "laws" and
"principles" have been deduced, he is struck by two facts: that the gaps,
in some cases, are enormous as compared with the single changes with which
he is familiar, and (what is more important) that they involve numerous
parts in many ways. The geneticist says to the paleontologist, since you do
not know, and from the nature of your case can never know, whether your
differences are due to one change or to a thousand, you can not with
certainty tell us anything about the hereditary units which have made the
process of evolution possible. And without this knowledge there can be no
understanding of the causes of evolution.
THE FOUR GREAT HISTORICAL SPECULATIONS
Looking backward over the history of the evolution theory we recognize that
during the hundred and odd years that have elapsed since Buffon, there have
been four main lines of _speculation_ concerning evolution. We might call
them the four great cosmogonies or the four modern epics of evolution.
THE ENVIRONMENT
_Geoffroy St. Hilaire_
About the beginning of the last century Geoffroy St. Hilaire, protégé, and
in some respects a disciple of Buffon, was interested as to how living
species are related to the animals and plants that had preceded them. He
was familiar with the kind of change that takes place in the embryo if it
is put into new or changed surroundings, and from this knowledge he
concluded that as the surface of the earth slowly changed--as the carbon
dioxide contents in the air altered--as land appeared--and as marine
animals left the water to inhabit it, they or their embryos responded to
the new conditions and those that responded favorably gave rise to new
creations. As the environment changed the fauna and flora changed--change
for change. Here we have a picture of progressive evolution that carries
with it an idea of mechanical necessity. If there is anything mystical or
even improbable in St. Hilaire's argument it does not appear on the
surface; for he did not assume that the response to the new environment was
always a favorable one or, as we say, an adaptation. He expressly stated
that _if_ the response was unfavorable the individual or the race died out.
He assumed that _sometimes_ the change might be favorable, i.e., that
certain species, entire groups, would respond in a direction favorable to
their existence in a new environment and these would come to inherit the
earth. In this sense he anticipated certain phases of the natural selection
theory of Darwin, but only in part; for his picture is not one of strife
within and without the species, but rather the escape of the species from
the old into a new world.
If then we recognize the intimate bond in chemical constitution of living
things and of the world in which they develop, what is there improbable in
St. Hilaire's hypothesis? Why, in a word is not more credit given to St.
Hilaire in modern evolutionary thought? The reasons are to be found, I
think, first, in that the evidence to which he appealed was meagre and
inconclusive; and, second, in that much of his special evidence does not
seem to us to be applicable. For example the monstrous forms that
development often assumes in a strange environment, and with which every
embryologist is only too familiar, rarely if ever furnish combinations, as
he supposed, that are capable of living. On the contrary, they lead rather
to the final catastrophe of the organism. And lastly, St. Hilaire's appeal
to sudden and great transformations, such as a crocodile's egg hatching
into a bird, has exposed his view to too easy ridicule.
But when all is said, St. Hilaire's conception of evolution contains
elements that form the background of our thinking to-day, for taken
broadly, the interaction between the organism and its environment was a
mechanistic conception of evolution even though the details of the theory
were inadequate to establish his contention.
In our own time the French metaphysician Bergson in his _Evolution
Creatrice_ has proposed in mystical form a thought that has at least a
superficial resemblance to St. Hilaire's conception. The response of living
things is no longer hit in one species and miss in another; it is precise,
exact; yet not mechanical in the sense at least in which we usually employ
the word mechanical. For Bergson claims that the one chief feature of
living material is that it responds favorably to the situation in which it
finds itself; at least so far as lies within the possible physical
limitations of its organization. Evolution has followed no preordained
plan; it has had no creator; it has brought about its own creation by
responding adaptively to each situation as it arose.
But note: the man of science believes that the organism responds today as
it does, because at present it has a chemical and physical constitution
that gives this response. We find a specific chemical composition and
generally a specific physical structure already existing. We have no reason
to suppose that such particular reactions would take place until a specific
chemical configuration had been acquired. Where did this constitution come
from? This is the question that the scientist asks himself. I suppose
Bergson would have to reply that it came into existence at the moment that
the first specific stimulus was applied. But if this is the answer we have
passed at once from the realm of observation to the realm of fancy--to a
realm that is foreign to our experience; for such a view assumes that
chemical and physical reactions are guided by the needs of the organism
when the reactions take place inside living beings.
USE AND DISUSE
_From Lamarck to Weismann_
The second of the four great historical explanations appeals to a change
not immediately connected with the outer world, but to one within the
organism itself.
Practice makes perfect is a familiar adage. Not only in human affairs do we
find that a part through use becomes a better tool for performing its task,
and through disuse degenerates; but in the field of animal behavior we find
that many of the most essential types of behavior have been learned through
repeated associations formed by contact with the outside.
It was not so long ago that we were taught that the instincts of animals
are the inherited experience of their ancestors--lapsed intelligence was
the current phrase.
Lamarck's name is always associated with the application of the theory of
the inheritance of acquired characters. Darwin fully endorsed this view and
made use of it as an explanation in all of his writings about animals.
Today the theory has few followers amongst trained investigators, but it
still has a popular vogue that is widespread and vociferous.
To Weismann more than to any other single individual should be ascribed the
disfavor into which this view has fallen. In a series of brilliant essays
he laid bare the inadequacy of the supposed evidence on which the
inheritance of acquired characters rested. Your neighbor's cat, for
instance, has a short tail, and it is said that it had its tail pinched off
by a closing door. In its litter of kittens one or more is found without a
tail. Your neighbor believes that here is a case of cause and effect. He
may even have known that the mother and grandmother of the cat had natural
tails. But it has been found that short tail is a dominant character;
therefore, until we know who was the father of the short-tailed kittens the
accident to its mother and the normal condition of her maternal ancestry is
not to the point.
Weismann appealed to common sense. He made few experiments to disprove
Lamarck's hypothesis. True, he cut off the tails of some mice for a few
generations but got no tailless offspring and while he gives no exact
measurements with coefficients of error he did not observe that the tails
of the descendants had shortened one whit. The combs of fighting cocks and
the tails of certain breeds of sheep have been cropped for many generations
and the practice continues today, because their tails are still long. While
in Lamarck's time there was no evidence opposed to his ingenious theory,
based as it was on an appeal to the acknowledged facts of improvement that
take place in the organs of an individual through their own functioning (a
fact that is as obvious and remarkable today as in the time of Lamarck),
yet now there is evidence as to whether the effects of use and disuse are
inherited, and this evidence is not in accord with Lamarck's doctrine.
THE UNFOLDING PRINCIPLE
_Nägeli and Bateson_
I have ventured to put down as one of the four great historical
explanations, under the heading of the unfolding principle, a conception
that has taken protean forms. At one extreme it is little more than a
mystic sentiment to the effect that evolution is the result of an inner
driving force or principle which goes under many names such as
Bildungstrieb, nisus formativus, vital force, and orthogenesis.
Evolutionary thought is replete with variants of this idea, often naïvely
expressed, sometimes unconsciously implied. Evolution once meant, in fact,
an unfolding of what pre-existed in the egg, and the term still carries
with it something of its original significance.
Nägeli's speculation written several years after Darwin's "Origin of
Species" may be taken as a typical case. Nägeli thought that there exists
in living material an innate power to grow and expand. He vehemently
protested that he meant only a mechanical principle but as he failed to
refer such a principle to any properties of matter known to physicists and
chemists his view seems still a mysterious affirmation, as difficult to
understand as the facts themselves which it purports to explain.
Nägeli compared the process of evolution to the growth of a tree, whose
ultimate twigs represent the living world of species. Natural selection
plays only the rôle of the gardener who prunes the tree into this or that
shape but who has himself _produced_ nothing. As an imaginative figure of
speech Nägeli's comparison of the tree might even today seem to hold if we
substituted "mutations" for "growth", but although we know so little about
what causes mutations there is no reason for supposing them to be due to an
inner impulse, and hence they furnish no justification for such a
hypothesis.
In his recent presidential address before the British Association Bateson
has inverted this idea. I suspect that his effort was intended as little
more than a _tour de force_. He claims for it no more than that it is a
possible line of speculation. Perhaps he thought the time had come to give
a shock to our too confident views concerning evolution. Be this as it may,
he has invented a striking paradox. Evolution has taken place through the
steady loss of inhibiting factors. Living matter was stopped down, so to
speak, at the beginning of the world. As the stops are lost, new things
emerge. Living matter has changed only in that it has become simpler.
NATURAL SELECTION
_Darwin_
Of the four great historical speculations about evolution, the doctrine of
Natural Selection of Darwin and Wallace has met with the most widespread
acceptance. In the last lecture I intend to examine this theory critically.
Here we are concerned only with its broadest aspects.
Darwin appealed to _chance variations_ as supplying evolution with the
material on which natural selection works. If we accept, for the moment,
this statement as the cardinal doctrine of natural selection it may appear
that evolution is due, (1) _not_ to an _orderly_ response of the organism
to its environment, (2) _not_ in the main to the activities of the animal
through the use or disuse of its parts, (3) _not_ to any innate principle
of living material itself, and (4) above all _not_ to purpose either from
within or from without. Darwin made quite clear what he meant by chance. By
chance he did not mean that the variations were not causal. On the contrary
he taught that in Science we mean by chance only that the particular
combination of causes that bring about a variation are not known. They are
accidents, it is true, but they are causal accidents.
In his famous book on "Animals and Plants under Domestication", Darwin
dwells at great length on the nature of the conditions that bring about
variations. If his views seem to us today at times vague, at times
problematical, and often without a secure basis, nevertheless we find in
every instance, that Darwin was searching for the _physical causes of
variation_. He brought, in consequence, conviction to many minds that there
are abundant indications, even if certain proof is lacking, that the causes
of variation are to be found in natural processes.
Today the belief that evolution takes place by means of natural processes
is generally accepted. It does not seem probable that we shall ever again
have to renew the old contest between evolution and special creation.
But this is not enough. We can never remain satisfied with a negative
conclusion of this kind. We must find out what natural causes bring about
variations in animals and plants; and we must also find out what kinds of
variations are inherited, and how they are inherited. If the circumstantial
evidence for organic evolution, furnished by comparative anatomy,
embryology and paleontology is cogent, we should be able to observe
evolution going on at the present time, i.e. we should be able to observe
the occurrence of variations and their transmission. This has actually been
done by the geneticist in the study of mutations and Mendelian heredity, as
the succeeding lectures will show.
* * * * *
CHAPTER II
THE BEARING OF MENDEL'S DISCOVERY ON THE ORIGIN OF HEREDITARY CHARACTERS
Between the years 1857 and 1868 Gregor Mendel, Augustinian monk, studied
the heredity of certain characters of the common edible pea, in the garden
of the monastery at Brünn.
In his account of his work written in 1868, he said:
"It requires indeed some courage to undertake a labor of such a
far-reaching extent; it appears, however, to be the only right way by
which we can finally reach the solution of a question the importance of
which cannot be over-estimated in connection with the history of the
evolution of organic forms."
He tells us also why he selected peas for his work:
"The selection of the plant group which shall serve for experiments of
this kind must be made with all possible care if it be desired to avoid
from the outset every risk of questionable results."
"The experimental plants must necessarily
1. Possess constant differentiating characters.
2. The hybrids of such plants must, during the flowering period, be
protected from the influence of all foreign pollen, or be easily
capable of such protection."
Why do biologists throughout the world to-day agree that Mendel's discovery
is one of first rank?
A great deal might be said in this connection. What is essential may be
said in a few words. Biology had been, and is still, largely a descriptive
and speculative science. _Mendel showed by experimental proof that heredity
could be explained by a simple mechanism. His discovery has been
exceedingly fruitful._
Science begins with naïve, often mystic conceptions of its problems. It
reaches its goal whenever it can replace its early guessing by verifiable
hypotheses and predictable results. This is what Mendel's law did for
heredity.
MENDEL'S FIRST DISCOVERY--SEGREGATION
[Illustration: FIG. 13. Diagram illustrating a cross between a red (dark)
and a white variety of four o'clock (Mirabilis jalapa).]
Let us turn to the demonstration of his first law--the law of segregation.
The first case I choose is not the one given by Mendel but one worked out
later by Correns. If the common garden plant called four o'clock (Mirabilis
jalapa) with red flowers is crossed to one having white flowers, the
offspring are pink (fig. 13). The hybrid, then, is intermediate in the
color of its flowers between the two parents. If these hybrids are inbred
the offspring are white, pink and red, in the proportion of 1:2:1. All of
these had the same ancestry, yet they are of three different kinds. If we
did not know their history it would be quite impossible to state what the
ancestry of the white or of the red had been, for they might just as well
have come from pure white and pure red ancestors respectively as to have
emerged from the pink hybrids. Moreover, when we test them we find that
they are as pure as are white or red flowering plants that have had all
white or all red flowering ancestors.
Mendel's Law explains the results of this cross as shown in figure 14.
The egg cell from the white parent carries the factor for white, the pollen
cell from the red parent carries the factor for red. The hybrid formed by
their union carries both factors. The result of their combined action is to
produce flowers intermediate in color.
When the hybrids mature and their germ cells (eggs or pollen) ripen, each
carries only one of these factors, either the red or the white, but not
both. In other words, the two factors that have been brought together in
the hybrid separate in its germ cells. Half of the egg cells are white
bearing, half red bearing. Half of the pollen cells are white bearing, half
red bearing. Chance combinations at fertilization give the three classes of
individuals of the second generation.
[Illustration: FIG. 14. Diagram illustrating the history of the factors in
the germ cells of the cross shown in Fig. 13.]
The white flowering plants should forever breed true, as in fact they do.
The red flowering plants also breed true. The pink flowering plants, having
the same composition as the hybrids of the first generation, should give
the same kind of result. They do, indeed, give this result i.e. one white
to two pink to one red flowered offspring.
[Illustration: FIG. 15. Diagram illustrating a cross between special races
of white and black fowls, producing the blue (here gray) Andalusian.]
Another case of the same kind is known to breeders of poultry. One of the
most beautiful of the domesticated breeds is known as the Andalusian. It is
a slate blue bird shading into blue-black on the neck and back. Breeders
know that these blue birds do not breed true but produce white, black, and
blue offspring.
[Illustration: FIG. 16. Diagram showing history of germ cells of cross of
Fig. 15. The larger circles indicate the color of the birds; their enclosed
small circles the nature of the factors in the germ cells of such birds.]
The explanation of the failure to produce a pure race of Andalusians is
that they are like the pink flowers of the four o'clock, i.e., they are a
hybrid type formed by the meeting of the white and the black germ cells. If
the whites produced by the Andalusians are bred to the blacks (both being
pure strains), all the offspring will be blue (fig. 15); if these blues are
inbred they will give 1 white, to 2 blues, to 1 black. In other words, the
factor for white and the factor for black separate in the germ cells of the
hybrid Andalusian birds (fig. 16).
[Illustration: FIG. 17. Diagram of Mendel's cross between yellow (dominant)
and green (recessive) peas.]
The third case is Mendel's classical case of yellow and green peas (fig.
17). He crossed a plant belonging to a race having yellow peas with one
having green peas. The hybrid plants had yellow seeds. These hybrids inbred
gave three yellows to one green. The explanation (fig. 18) is the same in
principle as in the preceding cases. The only difference between them is
that the hybrid which contains both the yellow and the green factors is in
appearance not intermediate, but like the yellow parent stock. Yellow is
said therefore to be dominant and green to be recessive.
[Illustration: FIG. 18. Diagram illustrating the history of the factors in
the cross shown in Fig. 17.]
Another example where one of the contrasted characters is dominant is shown
by the cross of Drosophila with vestigial wings to the wild type with long
wings (fig. 19). The F_1 flies have long wings not differing from those of
the wild fly, so far as can be observed. When two such flies are inbred
there result three long to one vestigial.
[Illustration: FIG. 19. Diagram illustrating a cross between a fly
(Drosophila ampelophila) with long wings and a mutant fly with vestigial
wings.]
The question as to whether a given character is dominant or recessive is a
matter of no theoretical importance for the principle of segregation,
although from the notoriety given to it one might easily be misled into the
erroneous supposition that it was the discovery of this relation that is
Mendel's crowning achievement.
Let me illustrate by an example in which the hybrid standing between two
types overlaps them both. There are two mutant races in our cultures of the
fruit fly Drosophila that have dark body color, one called sooty, another
which is even blacker, called ebony (fig. 20). Sooty crossed to ebony gives
offspring that are intermediate in color. Some of them are so much like
sooty that they cannot be distinguished from sooty. At the other extreme
some of the hybrids are as dark as the lightest of the ebony flies. If
these hybrids are inbred there is a continuous series of individuals,
sooties, intermediates and ebonies. Which color here shall we call the
dominant? If the ebony, then in the second generation we count three
ebonies to one sooty, putting the hybrids with the ebonies. If the dominant
is the sooty then we count three sooties to one ebony, putting the hybrids
with the sooties. The important fact to find out is whether there actually
exist three classes in the second generation. This can be ascertained even
when, as in this case, there is a perfectly graded series from one end to
the other, by testing out individually enough of the flies to show that
one-fourth of them never produce any descendants but ebonies, one-fourth
never any but sooties, and one-half of them give rise to both ebony and
sooty.
[Illustration: FIG. 20. Cross between two allelomorphic races of
Drosophila, sooty and ebony, that give a completely graded series in F_2.]
MENDEL'S SECOND DISCOVERY--INDEPENDENT ASSORTMENT
Besides his discovery that there are pairs of characters that disjoin, as
it were, in the germ cells of the hybrid (law of segregation) Mendel made a
second discovery which also has far-reaching consequences. The following
case illustrates Mendel's second law.
If a pea that is yellow and round is crossed to one that is green and
wrinkled (fig. 21), all of the offspring are yellow and round. Inbred,
these give 9 yellow round, 3 green round, 3 yellow wrinkled, 1 green
wrinkled. All the yellows taken together are to the green as 3:1. All the
round taken together are to the wrinkled as three to one; but some of the
yellows are now wrinkled and some of the green are now round. There has
been a recombination of characters, while at the same time the results, for
each pair of characters taken separately, are in accord with Mendel's Law
of Segregation, (fig. 22). The second law of Mendel may be called the law
of independent assortment of different character pairs.
[Illustration: FIG. 21. Cross between yellow-round and green-wrinkled peas,
giving the 9: 3: 3: 1 ratio in F_2.]
We can, as it were, take the characters of one organism and recombine them
with those of a different organism. We can explain this result as due to
the assortment of factors for these characters in the germ cells according
to a definite law.
[Illustration: FIG. 22. Diagram to show the history of the factor pairs
yellow-green and round-wrinkled of the cross in Fig. 21.]
As a second illustration let me take the classic case of the combs of
fowls. If a bird with a rose comb is bred to one with a pea comb (fig. 23),
the offspring have a comb different from either. It is called a walnut
comb. If two such individuals are bred they give 9 walnut, 3 rose, 3 pea, 1
single. This proportion shows that the grandparental types differed in
respect to two pairs of characters.
[Illustration: FIG. 23. Cross between pea and rose combed fowls. (Charts of
Baur and Goldschmidt.)]
A fourth case is shown in the fruit fly, where an ebony fly with long wings
is mated to a grey fly with vestigial wings (fig. 24). The offspring are
gray with long wings. If these are inbred they give 9 gray long, 3 gray
vestigial, 3 ebony long, 1 ebony vestigial (figs. 24 and 25).
[Illustration: FIG. 24. Cross between long ebony and gray vestigial flies.]
The possibility of interchanging characters might be illustrated over and
over again. It is true not only when two pairs of characters are involved,
but when three, four, or more enter the cross.
[Illustration: FIG. 25. Diagram to show the history of the factors in the
cross shown in Fig. 24.]
It is as though we took individuals apart and put together parts of two,
three or more individuals by substituting one part for another.
Not only has this power to make whatever combinations we choose great
practical importance, it has even greater theoretical significance; for, it
follows that the individual is not in itself the unit in heredity, but that
within the germ-cells there exist smaller units concerned with the
transmission of characters.
The older mystical statement of the individual as a unit in heredity has no
longer any interest in the light of these discoveries, except as a past
phase of biological history. We see, too, more clearly that the sorting out
of factors in the germ plasm is a very different process from the influence
of these factors on the development of the organism. There is today no
excuse for confusing these two problems.
If mechanistic principles apply also to embryonic development then the
course of development is capable of being stated as a series of
chemico-physical reactions and the "_individual_" is merely a term to
express the sum total of such reactions and should not be interpreted as
something different from or more than these reactions. So long as so little
is known of the actual processes involved in development the use of the
term "individuality", while giving the appearance of profundity, in reality
often serves merely to cover ignorance and to make a mystery out of a
mechanism.
THE CHARACTERS OF WILD ANIMALS AND PLANTS FOLLOW THE SAME LAWS OF
INHERITANCE AS DO THE CHARACTERS OF DOMESTICATED ANIMALS AND PLANTS.
Darwin based many of his conclusions concerning variation and heredity on
the evidence derived from the garden and from the stock farm. Here he was
handicapped to some extent, for he had at times to rely on information much
of which was uncritical, and some of which was worthless.
Today we are at least better informed on _two_ important points; one
concerning the _kinds_ of variations that furnish to the cultivator the
materials for his selection; the other concerning the modes of inheritance
of these variations. We know now that new characters are continually
appearing in domesticated as well as in wild animals and plants, that these
characters are often sharply marked off from the original characters, and
whether the differences are great or whether they are small they are
transmitted alike according to Mendel's law.
Many of the characteristics of our domesticated animals and cultivated
plants originated long ago, and only here and there have the records of
their first appearance been preserved. In only a few instances are these
records clear and definite, while the complete history of any large group
of our domesticated products is unknown to us.
Within the last five or six years, however, from a common wild species of
fly, the fruit fly, Drosophila ampelophila, which we have brought into the
laboratory, have arisen over a hundred and twenty-five new types whose
origin is completely known. Let me call attention to a few of the more
interesting of these types and their modes of inheritance, comparing them
with wild types in order to show that the kinds of inheritance found in
domesticated races occur also in wild types. The results will show beyond
dispute that the characters of wild types are inherited in precisely the
same way as are the characters of the mutant types--a fact that is not
generally appreciated except by students of genetics, although it is of the
most far-reaching significance for the theory of evolution.
A mutant appeared in which the eye color of the female was different from
that of the male. The eye color of the mutant female is a dark eosin color,
that of the male yellowish eosin. From the beginning this difference was as
marked as it is to-day. Breeding experiments show that eosin eye color
differs from the red color of the eye of the wild fly by a single mutant
factor. Here then at a single step a type appeared that was sexually
dimorphic.
Zoölogists know that sexual dimorphism is not uncommon in wild species of
animals, and Darwin proposed the theory of sexual selection to account for
the difference between the sexes. He assumed that the male preferred
certain kinds of females differing from himself in a particular character,
and thus in time through sexual selection, the sexes came to differ from
each other.
[Illustration: FIG. 26. Clover butterfly (Colias philodice) with two types
of females, above; and one type of male, below.]
In the case of eosin eye color no such process as that postulated by Darwin
to account for the differences between the sexes was involved; for the
single mutation that brought about the change also brought in the
dimorphism with it.
In recent years zoölogists have carefully studied several cases in which
two types of female are found in the same species. In the common clover
butterfly, there is a yellow and a white type of female, while the male is
yellow (fig. 26). It has been shown that a single factor difference
determines whether the female is yellow or white. The inheritance is,
according to Gerould, strictly Mendelian.
[Illustration: FIG. 27. Papilio turnus with two types of females above and
one type of male below.]
In Papilio turnus there exist, in the southern states, two kinds of
females, one yellow like the male, one black (fig. 27). The evidence here
is not so certain, but it seems probable that a single factor difference
determines whether the female shall be yellow or black.
Finally in Papilio polytes of Ceylon and India three different types of
females appear, (fig. 28 to right) only one of which is like the male. Here
the analysis of the breeding data shows the possibility of explaining this
case as due to two pairs Mendelian factors which give in combination the
three types of female.
[Illustration: FIG. 28. Papilio polytes, with three types of female to
right and one type of male above to left.]
Taking these cases together, they furnish a much simpler explanation than
the one proposed by Darwin. They show also that characters like these shown
by wild species may follow Mendel's law.
[Illustration: FIG. 29. Mutant race of fruit fly with intercalated
duplicate mesothorax on dorsal side.]
There has appeared in our cultures a fly in which the third division of the
thorax with its appendages has changed into a segment like the second (fig.
29). It is smaller than the normal mesothorax and its wings are imperfectly
developed, but the bristles on the upper surface may have the typical
arrangement of the normal mesothorax. The mutant shows how great a change
may result from a single factor difference.
A factor that causes duplication in the legs has also been found. Here the
interesting fact was discovered (Hoge) that duplication takes place only in
the cold. At ordinary temperatures the legs are normal.
[Illustration: FIG. 30. Mutant race of fruit fly, called eyeless; a, a'
normal eye.]
In contrast to the last case, where a character is doubled, is the next one
in which the eyes are lost (fig. 30). This change also took place at a
single step. All the flies of this stock however, cannot be said to be
eyeless, since many of them show pieces of the eye--indeed the variation is
so wide that the eye may even appear like a normal eye unless carefully
examined. Formerly we were taught that eyeless animals arose in caves. This
case shows that they may also arise suddenly in glass milk bottles, by a
change in a single factor.
I may recall in this connection that wingless flies (fig. 5 f) also arose
in our cultures by a single mutation. We used to be told that wingless
insects occurred on desert islands because those insects that had the best
developed wings had been blown out to sea. Whether this is true or not, I
will not pretend to say, but at any rate wingless insects may also arise,
not through a slow process of elimination, but at a single step.
The preceding examples have all related to recessive characters. The next
one is dominant.
[Illustration: FIG. 31. Mutant race of fruit fly called bar to the right
(normal to the left). The eye is a narrow vertical bar, the outline of the
original eye is indicated.]
A single male appeared with a narrow vertical red bar (fig. 31) instead of
the broad red oval eye. Bred to wild females the new character was found to
dominate, at least to the extent that the eyes of all its offspring were
narrower than the normal eye, although not so narrow as the eye of the pure
stock. Around the bar there is a wide border that corresponds to the region
occupied by the rest of the eye of the wild fly. It lacks however the
elements of the eye. It is therefore to be looked upon as a rudimentary
organ, which is, so to speak, a by-product of the dominant mutation.
The preceding cases have all involved rather great changes in some one
organ of the body. The following three cases involve slight changes, and
yet follow the same laws of inheritance as do the larger changes.
[Illustration: FIG. 32. Mutant race of fruit fly, called speck. There is a
minute black speck at base of wing.]
At the base of the wings a minute black speck appeared (fig. 32). It was
found to be a Mendelian character. In another case the spines on the thorax
became forked or kinky (fig. 52b). This stock breeds true, and the
character is inherited in strictly Mendelian fashion.
[Illustration: FIG. 33. Mutant race of fruit fly called club. The wings
often remain unexpanded and two bristles present in wild fly (b) are absent
on side of thorax (c).]
In a certain stock a number of flies appeared in which the wing pads did
not expand (fig. 33). It was found that this peculiarity is shown in only
about twenty per cent of the individuals supposed to inherit it. Later it
was found that this stock lacked two bristles on the sides of the thorax.
By means of this knowledge the heredity of the character was easily
determined. It appears that while the expansion of the wing pads fails to
occur once in five times--probably because it is an environmental effect
peculiar to this stock,--yet the minute difference of the presence or
absence of the two lateral bristles is a constant feature of the flies that
carry this particular factor.
In the preceding cases I have spoken as though a factor influenced only one
part of the body. It would have been more accurate to have stated that the
_chief_ effect of the factor was observed in a particular part of the body.
Most students of genetics realize that a factor difference usually affects
more than a single character. For example, a mutant stock called
rudimentary wings has as its principle characteristic very short wings
(fig. 34). But the factor for rudimentary wings also produces other effects
as well. The females are almost completely sterile, while the males are
fertile. The viability of the stock is poor. When flies with rudimentary
wings are put into competition with wild flies relatively few of the
rudimentary flies come through, especially if the culture is crowded. The
hind legs are also shortened. All of these effects are the results of a
single factor-difference.
[Illustration: FIG. 34. Mutant race of fruit fly, called rudimentary.]
One may venture the guess that some of the specific and varietal
differences that are characteristic of wild types and which at the same
time appear to have no survival value, are only by-products of factors
whose most important effect is on another part of the organism where their
influence is of vital importance.
It is well known that systematists make use of characters that are constant
for groups of species, but which do not appear in themselves to have an
adaptive significance. If we may suppose that the constancy of such
characters may be only an index of the presence of a factor whose _chief_
influence is in some other direction or directions, some physiological
influence, for example, we can give at least a reasonable explanation of
the constancy of such characters.
I am inclined to think that an overstatement to the effect that each factor
may affect the entire body, is less likely to do harm than to state that
each factor affects only a particular character. The reckless use of the
phrase "unit character" has done much to mislead the uninitiated as to the
effects that a single change in the germ plasm may produce on the organism.
Fortunately, the expression "unit character" is being less used by those
students of genetics who are more careful in regard to the implications of
their terminology.
There is a class of cases of inheritance, due to the XY chromosomes, that
is called sex linked inheritance. It is shown both by mutant characters and
characters of wild species.
For instance, white eye color in Drosophila shows sex linked inheritance.
If a white eyed male is mated to a wild red eyed female (fig. 35) all the
offspring have red eyes. If these are inbred, there are three red to one
white eyed offspring, but white eyes occur only in the males. The
grandfather has transmitted his peculiarity to half of his grandsons, but
to none of his granddaughters.
[Illustration: FIG. 35. Diagram showing a cross between a white eyed male
and a red eyed female of the fruit fly. Sex linked inheritance.]
The reciprocal cross (fig. 36) is also interesting. If a white eyed female
is bred to a red eyed male, all of the daughters have red eyes and all of
the sons have white eyes. We call this criss-cross inheritance. If these
offspring are inbred, they produce equal numbers of red eyed and white eyed
females and equal numbers of red eyed and white eyed males. The ratio is 1:
1: 1: 1, or ignoring sex, 2 reds to 2 whites, and not the usual 3:1
Mendelian ratio. Yet, as will be shown later, the result is in entire
accord with Mendel's principle of segregation.
[Illustration: FIG. 36. Diagram illustrating a cross between a red eyed
male and white eyed female of the fruit fly (reciprocal cross of that shown
in Fig. 35).]
It has been shown by Sturtevant that in a wild species of Drosophila, viz.,
D. repleta, two varieties of individuals exist, in one of which the thorax
has large splotches and in the other type smaller splotches (fig. 37). The
factors that differentiate these varieties are sex linked.
[Illustration: FIG. 37. Two types of markings on thorax of Drosophila
repleta, both found "wild". They show sex linked inheritance.]
Certain types of color blindness (fig. 38) and certain other abnormal
conditions in man such as haemophilia, are transmitted as sex linked
characters.
[Illustration: FIG. 38, A. Diagram illustrating inheritance of color
blindness in man; the iris of the color-blind eye is here black.]
[Illustration: FIG. 38, B. Reciprocal of cross in Fig. 38 a.]
In domestic fowls sex linked inheritance has been found as the
characteristic method of transmission for at least as many as six
characters, but here the relation of the sexes is in a sense reversed. For
instance, if a black Langshan hen is crossed to a barred Plymouth Rock cock
(fig. 39), the offspring are all barred. If these are inbred half of the
daughters are black and half are barred; all of the sons are barred. The
grandmother has transmitted her color to half of her granddaughters but to
none of her grandsons.
[Illustration: FIG. 39. Sex-linked inheritance in domesticated birds shown
here in a cross between barred Plymouth Rock male and black Langshan
female.]
[Illustration: FIG. 40. Reciprocal of Fig. 39.]
In the reciprocal cross (fig. 40) black cock by barred hen, the daughters
are black and the sons barred--criss-cross inheritance. These inbred give
black hens and black cocks, barred hens and barred cocks.
There is a case comparable to this found in a wild species of moth, Abraxas
grossulariata. A wild variation of this type is lighter in color and is
known as A. lacticolor. When these two types are crossed they exhibit
exactly the same type of heredity as does the black-barred combination in
the domestic fowl. As shown in figure 41, lacticolor female bred to
grossulariata male gives grossulariata sons and daughters. These inbred
give grossulariata males and females and lacticolor females. Reciprocally
lacticolor male by grossulariata female, (fig. 42) gives lacticolor
daughters and grossulariata sons and these inbred give grossulariata males
and females and lacticolor males and females.
[Illustration: FIG. 41. Sex-linked inheritance in the wild moth, Abraxas
grossulariata (darker) and A. lacticolor.]
[Illustration: FIG. 42. Reciprocal of Fig. 41.]
[Illustration: FIG. 43. Four wild types of Paratettix in upper line with
three hybrids below.]
It has been found that there may be even more than two factors that show
Mendelian segregation when brought together in pairs. For example, in the
southern States there are several races of the grouse locust (Paratettix)
that differ from each other markedly in color patterns (fig. 43). When any
two individuals of these races are crossed they give, as Nabours has shown,
in F_2 a Mendelian ratio of 1: 2: 1. It is obvious, therefore, that there
are here at least nine characters, any two of which behave as a Mendelian
pair. These races have arisen in nature and differ definitely and
strikingly from each other, yet any two differ by only one factor
difference.
[Illustration: FIG. 44. Diagram illustrating four allelomorphs in mice,
viz. gray bellied gray (wild type) (above, to left); white bellied gray
(above, to right); yellow (below, to right); and black (below, to left).]
Similar relations have been found in a number of domesticated races. In
mice there is a quadruple system represented by the gray house mouse, the
white bellied, the yellow and the black mouse (fig. 44). In rabbits there
is probably a triple system, that includes the albino, the Himalayan, and
the black races. In the silkworm moth there have been described four types
of larvae, distinguished by different color markings, that form a system of
quadruple allelomorphs. In Drosophila there is a quintuple system of
factors in the sex chromosome represented by eye colors, a triple system of
body colors, and a triple system of factors for eye colors in the third
chromosome.
MUTATION AND EVOLUTION
What bearing has the appearance of these new types of Drosophila on the
theory of evolution may be asked. The objection has been raised in fact
that in the breeding work with Drosophila we are dealing with artificial
and unnatural conditions. It has been more than implied that results
obtained from the breeding pen, the seed pan, the flower pot and the milk
bottle do not apply to evolution in the "open", nature "at large" or to
"wild" types. To be consistent, this same objection should be extended to
the use of the spectroscope in the study of the evolution of the stars, to
the use of the test tube and the balance by the chemist, of the
galvanometer by the physicist. All these are unnatural instruments used to
torture Nature's secrets from her. I venture to think that the real
antithesis is not between unnatural and natural treatment of Nature, but
rather between controlled or verifiable data on the one hand, and
unrestrained generalization on the other.
If a systematist were asked whether these new races of Drosophila are
comparable to wild species, he would not hesitate for a moment. He would
call them all one species. If he were asked why, he would say, I think,
"These races differ only in one or two striking points, while in a hundred
other respects they are identical even to the minutest details." He would
add, that as large a group of wild species of flies would show on the whole
the reverse relations, _viz._, they would differ in nearly every detail and
be identical in only a few points. In all this I entirely agree with the
systematist, for I do not think such a group of types differing by one
character each, is comparable to most wild groups of species because the
difference between wild species is due to a large number of such single
differences. The characters that have been accumulated in wild species are
of significance in the maintenance of the species, or at least we are led
to infer that even though the visible character that we attend to may not
itself be important, one at least of the other effects of the factors that
represent these characters is significant. It is, of course, hardly to be
expected that _any_ random change in as complex a mechanism as an insect
would improve the mechanism, and as a matter of fact it is doubtful whether
any of the mutant types so far discovered are better adapted to those
conditions to which a fly of this structure and habits is already adjusted.
But this is beside the mark, for modern genetics shows very positively that
adaptive characters are inherited in exactly the same way as are those that
are not adaptive; and I have already pointed out that we cannot study a
single mutant factor without at the same time studying one of the factors
responsible for normal characters, for the two together constitute the
Mendelian pair.
And, finally, I want to urge on your attention a question that we are to
consider in more detail in the last lecture. Evolution of wild species
appears to have taken place by modifying and improving bit by bit the
structures and habits that the animal or plant already possessed. We have
seen that there are thirty mutant factors at least that have an influence
on eye color, and it is probable that there are at least as many normal
factors that are involved in the production of the red eye of the wild fly.
Evolution from this point of view has consisted largely in introducing new
factors that influence characters already present in the animal or plant.
Such a view gives us a somewhat different picture of the process of
evolution from the old idea of a ferocious struggle between the individuals
of a species with the survival of the fittest and the annihilation of the
less fit. Evolution assumes a more peaceful aspect. New and advantageous
characters survive by incorporating themselves into the race, improving it
and opening to it new opportunities. In other words, the emphasis may be
placed less on the competition between the individuals of a species
(because the destruction of the less fit does not _in itself_ lead to
anything that is new) than on the appearance of new characters and
modifications of old characters that become incorporated in the species,
for on these depends the evolution of the race.
* * * * *
CHAPTER III
THE FACTORIAL THEORY OF HEREDITY AND THE COMPOSITION OF THE GERM PLASM
The discovery that Mendel made with edible peas concerning heredity has
been found to apply everywhere throughout the plant and animal kingdoms--to
flowering plants, to insects, snails, crustacea, fishes, amphibians, birds,
and mammals (including man).
There must be something that these widely separated groups of plants and
animals have in common--some simple mechanism perhaps--to give such
definite and orderly series of results. There is, in fact, a mechanism,
possessed alike by animals and plants, that fulfills every requirement of
Mendel's principles.
THE CELLULAR BASIS OF ORGANIC EVOLUTION AND HEREDITY
In order to appreciate the full force of the evidence, let me first pass
rapidly in review a few familiar, historical facts, that preceded the
discovery of the mechanism in question.
[Illustration: FIG. 45. Typical cell showing the cell wall, the protoplasm
(with its contained materials); the nucleus with its contained chromatin
and nuclear sap. (After Dahlgren.)]
Throughout the greater part of the last century, while students of
evolution and of heredity were engaged in what I may call the more general,
or, shall I say, the _grosser_ aspects of the subject, there existed
another group of students who were engaged in working out the minute
structure of the material basis of the living organism. They found that
organs such as the brain, the heart, the liver, the lungs, the kidneys,
etc., are not themselves the units of structure, but that all these organs
can be reduced to a simpler unit that repeats itself a thousand-fold in
every organ. We call this unit a cell (fig. 45).
The egg is a cell, and the spermatozoon is a cell. The act of fertilization
is the union of two cells (fig. 47, upper figure). Simple as the process of
fertilization appears to us today, its discovery swept aside a vast amount
of mystical speculation concerning the rôle of the male and of the female
in the act of procreation.
Within the cell a new microcosm was revealed. Every cell was found to
contain a spherical body called the nucleus (fig. 46a). Within the nucleus
is a network of fibres, a sap fills the interstices of the network. The
network resolves itself into a definite number of threads at each division
of the cell (fig. 46 b-e). These threads we call chromosomes. Each species
of animals and plants possesses a characteristic number of these threads
which have a definite size and sometimes a specific shape and even
characteristic granules at different levels. Beyond this point our
strongest microscopes fail to penetrate. Observation has reached, for the
time being, its limit.
[Illustration: FIG. 46. A series of cells in process of cell division. The
chromosomes are the black threads and rods. (After Dahlgren.)]
The story is taken up at this point by a new set of students who have
worked in an entirely different field. Certain observations and experiments
that we have not time to consider now, led a number of biologists to
conclude that the chromosomes are the bearers of the hereditary units. If
so, there should be many such units carried by _each_ chromosome, for the
number of chromosomes is limited while the number of independently
inherited characters is large. In Drosophila it has been demonstrated not
only that there are exactly as many groups of characters that are inherited
together as there are pairs of chromosomes, but even that it is possible to
locate one of these groups in a particular chromosome and to state the
_relative position_ there of the factors for the characters. If the
validity of this evidence is accepted, the study of the cell leads us
finally in a mechanical, but not in a chemical sense, to the ultimate units
about which the whole process of the transmission of the hereditary factors
centers.
But before plunging into this somewhat technical matter (that is difficult
only because it is unfamiliar), certain facts which are familiar for the
most part should be recalled, because on these turns the whole of the
subsequent story.
[Illustration: FIG. 47. An egg, and the division of the egg--the so-called
process of cleavage. (After Selenka.)]
The thousands of cells that make up the cell-state that we call an animal
or plant come from the fertilized egg. An hour or two after fertilization
the egg divides into two cells (fig. 47). Then each half divides again.
Each quarter next divides. The process continues until a large number of
cells is formed and out of these organs mould themselves.
[Illustration: FIG. 48. Section of the egg of the beetle, Calligrapha,
showing the pigment at one end where the germ cells will later develop as
shown in the other two figures. (After Hegner.)]
At every division of the cell the chromosomes also divide. Half of these
have come from the mother, half from the father. Every cell contains,
therefore, the sum total of all the chromosomes, and if these are the
bearers of the hereditary qualities, every cell in the body, whatever its
function, has a common inheritance.
At an early stage in the development of the animal certain cells are set
apart to form the organs of reproduction. In some animals these cells can
be identified early in the cleavage (fig. 48).
The reproductive cells are at first like all the other cells in the body in
that they contain a full complement of chromosomes, half paternal and half
maternal in origin (fig. 49). They divide as do the other cells of the body
for a long time (fig. 49, upper row). At each division each chromosome
splits lengthwise and its halves migrate to opposite poles of the spindle
(fig. 49 c).
But there comes a time when a new process appears in the germ cells (fig 49
e-h). It is essentially the same in the egg and in the sperm cells. The
discovery of this process we owe to the laborious researches of many
workers in many countries. The list of their names is long, and I shall not
even attempt to repeat it. The chromosomes come together in pairs (fig. 49
a). Each maternal chromosome mates with a paternal chromosome of the same
kind.
[Illustration: FIG. 49. In the upper row of the diagram a typical process
of nuclear division, such as takes place in the early germ cells or in the
body cells. In the lower row the separation of the chromosomes that have
paired. This sort of separation takes place at one of the two reduction
divisions.]
Then follow two rapid divisions (fig. 49 f, g and 50 and 51). At one of the
divisions the double chromosomes separate so that each resulting cell comes
to contain some maternal and some paternal chromosomes, i.e. one or the
other member of each pair. At the other division each chromosome simply
splits as in ordinary cell division.
[Illustration: FIG. 50. The two maturation divisions of the sperm cell.
Four sperms result, each with half (haploid) the full number (diploid) of
chromosomes.]
The upshot of the process is that the ripe eggs (fig. 51) and the ripe
spermatozoa (fig. 50) come to contain only half the total number of
chromosomes.
[Illustration: FIG. 51. The two maturation divisions of the egg. The
divisions are unequal, so that two small polar bodies are formed one of
these subsequently divides. The three polar bodies and the egg are
comparable to the four sperms.]
When the eggs are fertilized the whole number of chromosomes is restored
again.
THE MECHANISM OF MENDELIAN HEREDITY DISCOVERED IN THE BEHAVIOR OF THE
CHROMOSOMES
If the factors in heredity are carried in the chromosomes and if the
chromosomes are definite structures, we should anticipate that there should
be as many _groups_ of characters as there are kinds of chromosomes. In
only one case has a sufficient number of characters been studied to show
whether there is any correspondence between the number of hereditary groups
of characters and the number of chromosomes. In the fruit fly, Drosophila
ampelophila, we have found about 125 characters that are inherited in a
perfectly definite way. On the opposite page is a list of some of them.
It will be observed in this list that the characters are arranged in four
groups, Groups I, II, III and IV. Three of these groups are equally large
or nearly so; Group IV contains only two characters. The characters are put
into these groups because in heredity the members of each group tend to be
inherited together, i.e., if two or more enter the cross together they tend
to remain together through subsequent generations. On the other hand, any
member of one group is inherited entirely independently of any member of
the other groups; in the same way as Mendel's yellow-green pair of
characters is inherited independently of the round-wrinkled pair.
_Group I_ _Group II_ _Group III_ _Group IV_
Abnormal Antlered Band Bent
Bar Apterous Beaded Eyeless
Bifid Arc Cream III
Bow Balloon Deformed
Cherry Black Dwarf
Chrome Blistered Ebony
Cleft Comma Giant
Club Confluent Kidney
Depressed Cream II Low crossing over
Dot Curved Maroon
Eosin Dachs Peach
Facet Extra vein Pink
Forked Fringed Rough
Furrowed Jaunty Safranin
Fused Limited Sepia
Green Little crossover Sooty
Jaunty Morula Spineless
Lemon Olive Spread
Lethals, 13 Plexus Trident
Miniature Purple Truncate intensifier
Notch Speck Whitehead
Reduplicated Strap White ocelli
Ruby Streak
Rudimentary Trefoil
Sable Truncate
Shifted Vestigial
Short
Skee
Spoon
Spot
Tan
Truncate intensifier
Vermilion
White
Yellow
If the factors for these characters are carried by the chromosomes, then we
should expect that those factors that are carried by the same chromosome
would be inherited together, provided the chromosomes are definite
structures in the cell.
[Illustration: FIG. 52. Chromosomes (diploid) of D. ampelophila. The sex
chromosomes are XX in the female and XY in the male. There are three other
pairs of chromosomes.]
In the chromosome group of Drosophila, (fig. 52) there are _four_ pairs of
chromosomes, three of nearly the same size and one much smaller. Not only
is there agreement between the number of hereditary groups and the number
of the chromosomes, but even the size relations are the same, for there are
three great groups of characters and three pairs of large chromosomes, and
one small group of characters and one pair of small chromosomes.
THE FOUR GREAT LINKAGE GROUPS OF DROSOPHILA AMPELOPHILA
The following description of the characters of the wild fly may be useful
in connection with the account of the modifications of these characters
that appear in the mutants.
The head and thorax of the wild fly are grayish-yellow, the abdomen is
banded with alternate stripes of yellow and black. In the male, (fig. 4 to
right), there are three narrow bands and a black tip. In the female there
are five black bands (fig. 4 to left). The wings are gray with a surface
texture of such a kind that at certain angles they are iridescent. The eyes
are a deep, solid, brick-red. The minute hairs that cover the body have a
very definite arrangement that is most obvious on the head and thorax.
There is a definite number of larger hairs called bristles or chaetae which
have a characteristic position and are used for diagnostic purposes in
classifying the species. On the foreleg of the male there is a comb-like
organ formed by a row of bristles; it is absent in the female. The comb is
a secondary sexual character, and it is, so far as known, functionless.
Some of the characters of the mutant types are shown in figures 53, 54, 55,
56. The drawing of a single fly is often used here to illustrate more than
one character. This is done to economize space, but of course there would
be no difficulty in actually bringing together in the same individual any
two or more characters belonging to the same group (or to different
groups). Without colored figures it is not possible to show many of the
most striking differences of these mutant races; at most dark and light
coloring can be indicated by the shading of the body, wings, or eyes.
_Group I_
In the six flies drawn in figure 53 there are shown five different wing
characters. The first of these types (a) is called cut, because the ends of
the wings look as though they had been cut to a point. The antennae are
displaced downward and appressed and their bristle-like aristae are
crumpled.
[Illustration: FIG. 53. Group I. (See text)]
The second figure (b) represents a fly with a notch in the ends of the
wings. This character is dominant, but the same factor that produces the
notch in the wings is also a recessive lethal factor; because of this
latter effect of the character no males of this race exist, and the females
of the race are never pure but hybrid. Every female with notch wings bred
to a wild male, will produce in equal numbers notch winged daughters and
daughters with normal wings. There will be half as many sons as daughters.
The explanation of this peculiar result is quite simple. Every notch winged
female has one X chromosome that carries the factor for notch and one X
chromosome that is "normal". Daughters receiving the former chromosomes are
notched because the factor for notch is dominant, but they are not killed
since the lethal effect of the notch factor is recessive to the normal
allelomorph carried by the other chromosome that the daughters get from
their father. This normal factor is recessive for notch but dominant for
life. This same figure (b) is used here to show three other sex linked
characters. The spines on the thorax are twisted or kinky, which is due to
a factor called "forked". The effect is best seen on the thorax, but all
spines on the body are similarly modified; even the minute hairs are also
affected. Ruby eye color might be here represented--if the eyes in the
figure were colored. The lighter color of the body and antennae is intended
to indicate that the character tan is also present. The light color of the
antennae is the most certain way of identifying tan. The tan flies are
interesting because they have lost the positive heliotropism that is so
marked a feature in the behavior of D. ampelophila. As this peculiarity of
the tan flies is inherited like all the other sex linked characters, it
follows that when a tan female is bred to a wild male all the sons inherit
the recessive tan color and indifference to light, while the daughters show
the dominant sex linked character of their father, i.e., they are "gray",
and go to the light. Hence when such a brood is disturbed the females fly
to the light, but the males remain behind.
One of the first mutants that appeared in D. ampelophila was called
rudimentary on account of the condition of the wings (c). The same mutation
has appeared independently several times. In the drawing (c) the dark body
color is intended to indicate "sable" and the lighter color of the eyes is
intended to indicate eosin. This eye color, which is an allelomorph of
white, is also interesting because in the female the color is deeper than
in the male. In other cases of sex linked factors the character is the same
in the two sexes.
In the fourth figure (d) the third and fourth longitudinal veins of the
wing are _fused_ into one vein from the base of the wing to the level of
the first cross-vein and in addition converge and meet near their outer
ends. The shape of the eye is represented in the figure as different from
the normal, due to another factor called "bar". This is a dominant
character, the hybrid condition being also narrow, but not so narrow as the
pure type. Vermilion eye color might also be here represented--due to a
factor that has appeared independently on several occasions.
In the fifth figure (e) the wings are shorter and more pointed than in the
wild fly. This character is called miniature. The light color of the
drawing may be taken to represent yellow body color, and the light color of
the eye white eye color.
In the last figure (f) the wings are represented as pads, essentially in
the same condition that they are in when the fly emerges from the pupa
case. Not all the flies of this stock have the wings in this condition;
some have fully expanded wings that appear normal in all respects.
Nevertheless, about the same percentage of offspring show the pads
irrespective of whether the parents had pads or expanded wings.
The flies of this stock show, however, another character, which is a
product of the same factor, and which is constant, i.e., repeated in all
individuals. The two bristles on the sides of the thorax are constantly
absent in this race. The lighter color of the eye in the figure may be
taken to indicate buff--a faint yellowish color. The factor for this eye
color is another allelomorph of white.
There are many other interesting characters that belong to the first group,
such as abnormal abdomen, short legs, duplication of the legs, etc. In
fact, any part of the body may be affected by a sex-linked factor.
_Group II_
In the first figure (a) of figure 54 that contains members of Group II the
wings are almost entirely absent or "vestigial". This condition arose at a
single step and breeds true, although it appears to be influenced to some
extent by temperature, also by modifiers that sometimes appear in the
stock. Purple eye color belongs in Group II; it resembles the color of the
eye of the wild fly but is darker and more translucent.
[Illustration: FIG. 54. Group II. (See text.)]
In the second figure (b) the wing is again long and narrow and sometimes
bent back on itself, as shown here. In several respects the wing resembles
strap (d) but seems to be due to another factor, called antler,
insufficiently studied as yet.
In the third figure (c) the wings turn up at the end. This is brought about
by the presence of the factor called jaunty.
In the fourth figure the wings are long and narrow and several of the veins
are unrepresented. This character, "strap", is very variable and has not
yet been thoroughly studied. On the thorax there is a deep black mark
called trefoil. Even in the wild fly there is a three pronged mark on the
thorax present in many individuals. Trefoil is a further development and
modification of this mark and is due to a special factor.
In the fifth figure (e) the wings are arched. The factor is called arc. The
dark color of the body, and especially of the wings, indicates the factor
for black.
The sixth figure (f) shows the wings "curved" downwards. In addition there
is present a minute black speck at the base of each wing, due to another
factor called speck.
In the seventh figure (g) the wing is truncate. Its end is obliquely
squared instead of rounded; it may be longer than the body, or shorter when
other modifying factors are present. The mutation that produces this type
of wing is of not infrequent occurrence. It has been shown by Muller and
Altenburg that there are at least two factors that modify this
character--the chief factor is present in the second chromosome; alone it
produces the truncate wing in only a certain percentage of cases, but when
the modifiers are also present about ninety percent of the individuals may
show the truncate condition of the wing. But the presence of these factors
makes the stock very infertile, so that it is difficult to maintain.
In the eighth figure (h) the legs are shortened owing to the absence of a
segment of the tarsus. The stock is called dachs--a nickname given to it
because the short legs suggested the dachshund.
_Group III_
In figure 55, (a), a mutant type called bithorax is shown. The old
metathorax is replaced by another mesothorax thrust in between the normal
mesothorax and the abdomen. It carries a pair of wings that do not
completely unfold. On this new mesothorax the characteristic arrangement of
the bristles is shown. Thus at a single step a typical region of the body
has doubled. The character is recessive.
[Illustration: FIG. 55. Group III. (See text.)]
The size of the adult fly of D. ampelophila varies greatly according to the
amount of nourishment obtained by the larva. After the fly emerges its size
remains nearly constant, as in many insects. Two races have, however, been
separated by Bridges that are different in size as a result of a genetic
factor. The first of these, called dwarf, is represented by figure 55, (b).
The race is minute, although of course its size is variable, depending on
food and other conditions. The same figure shows the presence of another
factor, "sooty", that makes the fly very dark. Maroon eye color might be
here represented, due to still another factor.
In the third figure (c) the other mutation in size is shown. It is called
"giant". The flies are twice the size of wild flies. An eye color, called
peach, might here be represented. It is an allelomorph of pink.
In the fourth figure (d) the mutant called dichaete is shown. It is
characterized by the absence of two of the bristles on the thorax. Other
bristles may also be absent, but not so constantly as the two just
mentioned. Another effect of the same factor is the spread-out condition of
the wings. The very dark eye color in this figure may be taken to indicate
the presence of another factor, "sepia", which causes the eyes to assume a
brown color that becomes black with age. Most of the other mutations in eye
color that have occurred tend to give a lighter color: this one, which is
also recessive, makes the eye darker.
In the fifth figure (e) the color of the darkest fly is due to a factor
called ebony, which is an allelomorph of sooty.
In the sixth figure (f) the wings are beaded, i.e., the margin is defective
at intervals, giving a beaded-like outline to the wings. This condition is
very variable and much affected by other factors that influence the shape
of the wings. The lighter eye color of the drawing may be taken to
represent pink.
In the seventh figure (g) the wings are curled up over the back. This is a
recessive character.
_Group IV_
Only two mutants have been obtained that do not belong to any of the
preceding groups; these are put together in Group IV. It has been shown
that they are linked to each other and the linkage is so close that it has
thus far been impossible to obtain the dominant recessive. One of these
mutants, called "eyeless" (fig. 56, a, a^1), is variable--the eyes are
often entirely absent or represented by one or more groups of ommatidia.
The outline of the original eye, so to speak, is strongly marked out and
its area might be called a rudimentary organ, if such a statement has any
meaning here.
[Illustration: FIG. 56. Group IV. (See text.)]
The other figure (b) represents "bent", so called from the shape of the
wings. This mutant is likewise very variable, often indistinguishable from
the wild type, yet when well developed strikingly different from any other
mutant.
This brief account of a few of the mutant races that can be most easily
represented by uncolored figures will serve to show how all parts of the
body may change, some of the changes being so slight that they would be
overlooked except by an expert, others so great that in the character
affected the flies depart far from the original species.
_It is important to note that mutations in the first chromosome are not
limited to any part of the body nor do they affect more frequently a
particular part. The same statement holds equally for all of the other
chromosomes. In fact, since each factor may affect visibly several parts of
the body at the same time there are no grounds for expecting any special
relation between a given chromosome and special regions of the body. It can
not too insistently be urged that when we say a character is the product of
a particular factor we mean no more than that it is the most conspicuous
effect of the factor._
If, then, as these and other results to be described point to the
chromosomes as the bearers of the Mendelian factors, and if, as will be
shown presently, these factors have a definite location in the chromosomes
it is clear that the location of the factors in the chromosomes bears no
spatial relation to the location of the parts of the body to each other.
LOCALIZATION OF FACTORS IN THE CHROMOSOMES
_The Evidence from Sex Linked Inheritance_
When we follow the history of pairs of chromosomes we find that their
distribution in successive generations is paralleled by the inheritance of
Mendelian characters. This is best shown in the sex chromosomes (fig. 57).
In the female there are two of these chromosomes that we call the X
chromosomes; in the male there are also two but one differs from those of
the female in its shape, and in the fact that it carries none of the normal
allelomorphs of the mutant factors. It is called the Y chromosome.
The course followed by the sex chromosomes and that by the characters in
the case of sex linked inheritance are shown in the next diagram of
Drosophila illustrating a cross between a white eyed male and a red eyed
female.
[Illustration: FIG. 57. Scheme of sex determination in Drosophila type.
Each _mature_ egg contains one X, each mature sperm contains one X, or a Y
chromosome. Chance union of any egg with any sperm will give either XX
(female) or XY (male).]
[Illustration: FIG. 58. Cross between white eyed male of D. ampelophila and
red eyed female. The sex chromosomes are indicated by the rods. A black rod
indicates that the chromosome carries the factor for red; the open
chromosome the factor for white eye color.]
The first of these represents a cross between a white eyed male and a red
eyed female (fig. 58, top row). The X chromosome in the male is represented
by an open bar, the Y chromosome is bent. In the female the two X
chromosomes are black. Each egg of such a female will contain one "black" X
after the polar bodies have been thrown off. In the male there will be two
classes of sperm--the female-producing, carrying the (open) X, and the
male-producing, carrying the Y chromosome. Any egg fertilized by an X
bearing sperm will produce a female that will have red eyes because the X
(black) chromosome it gets from the mother carries the dominant factor for
red. Any egg fertilized by a Y-bearing sperm will produce a male that will
also have red eyes because he gets his (black) X chromosome from his
mother.
When, then, these two F_1 flies (second row) are inbred the following
combinations are expected. Each egg will contain a black X (red eye
producing) or a white X (white eye producing) after the polar bodies have
been extruded. The male will produce two kinds of sperms, of which the
female producing will contain a black X (red eye producing). Since any egg
may by chance be fertilized by any sperm there will result the four classes
of individuals shown on the bottom row of the diagram. All the females will
have red eyes, because irrespective of the two kinds of eggs involved all
the female-producing sperm carry a black X. Half of the males have red eyes
because half of the eggs have had each a red-producing X chromosome. The
other half of the males have white eyes, because the other half of the eggs
had each a white-producing X chromosome. Other evidence has shown that the
Y chromosome of the male is indifferent, so far as these Mendelian factors
are concerned.
[Illustration: FIG. 59. Cross between red eyed male and white eyed female;
reciprocal cross of Fig. 58.]
The reciprocal experiment is illustrated in figure 59. A white eyed female
is mated to a red eyed male (top row). All the mature eggs of such a female
contain one white-producing X chromosome represented by the open bar in the
diagram. The red eyed male contains female-producing X-bearing sperm that
carry the factor for red eye color, and male-producing Y chromosomes. Any
egg fertilized by an X-bearing sperm will become a red eyed female because
the X chromosome that comes from the father carries the dominant factor for
red eye color. Any egg fertilized by a Y-bearing sperm will become a male
with white eyes because the only X chromosome that the male contains comes
from his mother and is white producing.
When these two F_1 flies are inbred (middle row) the following combinations
are expected. Half the eggs will contain each a white producing X
chromosome and half red producing. The female-producing sperms will each
contain a white X and the male-producing sperms will each contain an
indifferent Y chromosome. Chance meetings of egg and sperm will give the
four F_2 classes (bottom row). These consist of white eyed and red eyed
females and white eyed and red eyed males. The ratio here is 1:1 and not
three to one (3:1) as in other Mendelian cases. But Mendel's law of
segregation is not transgressed, as the preceding analysis has shown; for,
the chromosomes have followed strictly the course laid down on Mendel's
principle for the distribution of factors. The peculiar result in this case
is due to the fact that the F_1 male gets his single factor for eye color
from his mother only and it is linked to or contained in a body (the X
chromosome) that is involved in producing the females, while the mate of
this body--the Y chromosome--is indifferent with regard to these factors,
yet active as a mate to X in synapsis.
[Illustration: FIG. 60. Diagram of sex determination in type with XX female
and XO male (after Wilson).]
In man there are several characters that show exactly this same kind of
inheritance. Color blindness, or at least certain kinds of color blindness,
appear to follow the same scheme. A color blind father transmits through
his daughters his peculiarity to half of his grandsons, but to none of his
grand-daughters (fig. 38A). The result is the same as in the case of the
white eyed male of Drosophila. Color blind women are rather unusual, which
is expected from the method of inheritance of this character, but in the
few known cases where such color blind women have married normal husbands
the sons have inherited the peculiarity from the mother (fig. 38B). Here
again the result is the same as for the similar combination in Drosophila.
[Illustration: FIG. 61. Spermatogenesis in man. There are 47 chromosomes
(diploid) in the male. After reduction half of the sperm carry 24
chromosomes (one of which is X) and half carry 23 chromosomes (no X).]
In man the sex formula appears to be XX for the female and XO for the male
(fig. 60), and since the relation is essentially the same as that in
Drosophila the chromosome explanation is the same. According to von
Winiwarter there are 48 chromosomes in the female and 47 in the male (fig.
61). After the extrusion of the polar bodies there are 24 chromosomes in
the egg. In the male at one of the two maturation divisions the X
chromosome passes to one pole undivided (fig. 61, C). In consequence there
are two classes of sperms in man; female producing containing 24
chromosomes, and male producing containing 23 chromosomes. If the factor
for color blindness is carried by the X chromosome its inheritance in man
works out on the same chromosome scheme and in the same way as does white
eye color (or any other sex linked character) in the fly, for the O sperm
in man is equivalent to the Y sperm in the fly.
In these cases we have been dealing with a single pair of characters. Let
us now take a case where two pairs of sex linked characters enter the cross
at the same time, and preferably a case where the two recessives enter the
cross from the same parent.
If a female with white eyes and yellow wings is crossed to a wild male with
red eyes and gray wings (fig. 62), the sons are yellow and have white eyes
and the daughters are gray and have red eyes. If two F_1 flies are mated
they will produce the following classes.
[Illustration: FIG. 62. Cross between a white eyed, yellow winged female of
D. ampelophila and a red eyed, gray winged male. Two pairs of sex linked
characters, viz., white-red and yellow-gray are involved. (See text.)]
Yellow Gray Yellow Gray
White Red Red White
------------ -------------
| |
99.% 1.%
Not only have the two grandparental combinations reappeared, but in
addition two new combinations, viz., grey white and yellow red. The two
original combinations far exceed in numbers the new or exchange
combinations. If we follow the history of the X chromosomes we discover
that the _larger classes_ of grandchildren appear in accord with the way in
which the X chromosomes are transmitted from one generation to the next.
The _smaller classes_ of grandchildren, the exchange combinations or
cross-overs, as we call them, can be explained by the assumption that at
some stage in their history an interchange of parts has taken place between
the chromosomes. This is indicated in the diagrams.
The most important fact brought out by the experiment is that the factors
that went in together tend to stick together. It makes no difference in
what combination the members of the two pairs of characters enter, they
tend to remain in that combination.
If one admits that the sex chromosomes carry these factors for the
sex-linked characters--and the evidence is certainly very strong in favor
of this view--it follows necessarily from these facts that at some time in
their history there has been an interchange between the two sex chromosomes
in the female.
There are several stages in the conjugation of the chromosomes at which
such an interchange between the members of a pair might occur. There is
further a small amount of direct evidence, unfortunately very meagre at
present, showing that an interchange does actually occur.
At the ripening period of the germ cell the members of each pair of
chromosomes come together (fig. 49, e). In several forms they have been
described as meeting at one end and then progressively coming to lie side
by side as shown in fig. 63, e, f, g, h, i. At the end of the process they
appear to have completely united along their length (fig. 63, j, k, l). It
is always a maternal and a paternal chromosome that meet in this way and
always two of the same kind. It has been observed that as the members of a
pair come together they occasionally twist around each other (fig. 63, g,
l, and 64, and 65). In consequence a part of one chromosome comes to be now
on one side and now on the other side of its mate.
[Illustration: FIG. 63. Conjugation of chromosomes (side to side union) in
the spermatogenesis of Batracoseps. (After Janssens.)]
When the chromosomes separate at the next division of the germ cell the
part on one side passes to one pole, the part on the other to the opposite
pole, (figs. 64 and 65). Whenever the chromosomes do not untwist at this
time there must result an interchange of pieces where they were crossed
over each other.
[Illustration: FIG. 64. Scheme to illustrate a method of crossing over of
the chromosomes.]
Janssens has found at the time of separation evidence in favor of the view
that some such interchange probably takes place.
We find this same process of interchange of characters taking place in each
of the other three groups of Drosophila. An example will show this for the
Group II.
[Illustration: FIG. 65. Scheme to illustrate double crossing over.]
If a black vestigial male is crossed to a gray long-winged female (fig. 66)
the offspring are gray long. If an F_1 female is back-crossed to a black
vestigial male the following kinds of flies are produced:
Black Gray Black Gray
vestigial long long vestigial
----------------- -----------------
| |
83% 17%
The combinations that entered are more common in the F_2 generations than
the cross-over classes, showing that there is linkage of the factors that
entered together.
Another curious fact is brought out if instead of back-crossing the F_1
female we back-cross the F_1 male to a black vestigial female. Their
offspring are now of only two kinds, black vestigial and gray long. This
means that in the male there is no crossing-over or interchange of pieces.
This relation holds not only for the Group II but for all the other groups
as well.
Why interchange takes place in the female of Drosophila and not in the male
we do not know at present. We might surmise that when in the male the
members of a pair come together they do not twist around each other, hence
no crossing-over results.
[Illustration: FIG. 66. Cross between black vestigial and gray long flies.
Two pairs of factors involved in the second group. The F_1 female is back
crossed (to right) to black vestigial male; and the F_1 male is back
crossed to black vestigial female (to left). Crossing over takes place in
the F_1 female but not in the F_1 male.]
Crossing-over took place between white and yellow only once in a hundred
times. Other characters show different values, but the same value under the
same conditions is obtained from the same pair of characters.
[Illustration: FIG. 67. Map of four chromosomes of D. ampelophila locating
those factors in each group that have been most fully studied.]
If we assume that the nearer together the factors lie in the chromosome the
less likely is a twist to occur between them, and conversely the farther
apart they lie the more likely is a twist to occur between them, we can
understand how the linkage is different for different pairs of factors.
On this basis we have made out chromosomal maps for each chromosome (fig.
67). The diagram indicates those loci that have been most accurately
placed.
_The Evidence from Interference_
There is a considerable body of information that we have obtained that
corroborates the location of the factors in the chromosome. This evidence
is too technical to take up in any detail, but there is one result that is
so important that I must attempt to explain it. If, as I assume, crossing
over is brought about by twisting of the chromosomes, and if owing to the
material of the chromosomes there is a most frequent distance of internode,
then, when crossing over between nodes takes place at same level at a-b in
figure 68, the region on each side of that point, a to A and b to B, should
be protected, so to speak, from further crossing over. This in fact we have
found to be the case. No other explanation so far proposed will account for
this extraordinary relation.
[Illustration: FIG. 68. Scheme to indicate that when the members of a pair
of chromosomes cross (at a-b) the region on each side is protected
inversely to the distance from a-b.]
What advantage, may be asked, is there in obtaining numerical data of this
kind? It is this:--whenever a new character appears we need only determine
in which of the four groups it lies and its distance from two members
within that group. With this information we can predict with a high degree
of probability what results it will give with any other member of any
group. Thus we can do on paper what would require many months of labor by
making the actual experiment. In a word we can predict what will happen in
a situation where prediction is impossible without this numerical
information.
_The Evidence from Non-Disjunction_
In the course of the work on Drosophila exceptions appeared in one strain
where certain individuals did not conform to the scheme of sex linked
inheritance. For a moment the hypothesis seemed to fail, but a careful
examination led to the suspicion that in this strain something had happened
to the sex chromosomes. It was seen that if in some way the X chromosomes
failed to disjoin in certain eggs, the exceptions could be explained. The
analysis led to the suggestion that if the Y chromosome had got into the
female line the results would be accounted for, since its presence there
would be expected to cause this peculiar non-disjunction of the X
chromosomes.
That this was the explanation was shown when the material was examined. The
females that gave these results were found by Bridges to have two X's and a
Y chromosome.
The normal chromosome group of the female is shown in figure 52 and the
chromosome group of one of the exceptional females is shown in figure 69.
In a female of this kind there are three sex chromosomes X X Y which are
homologous in the sense that in normal individuals the two present are
mates and separate at the reduction division. If in the X X Y individual X
and X conjugate and separate at reduction and the unmated Y is free to move
to either pole of the spindle, two kinds of mature eggs will result, viz.,
X and XY. If, on the other hand, X and Y conjugate and separate at
reduction and the remaining X is free to go to either pole, four kinds of
eggs will result--XY--X--XX--Y. As a total result four kinds of eggs are
expected: viz. many XY and X eggs and a few XX and Y eggs.
[Illustration: FIG. 69. Figure of the chromosome group of an XXY female,
that gives non-disjunction.]
These four kinds of eggs may be fertilized either by female-producing
sperms or male-producing sperms, as indicated in the diagram (fig. 70).
[Illustration: FIG. 70. Scheme showing the results of fertilizing white
bearing eggs (4 kinds) resulting from non-disjunction. The upper half of
the diagram gives the results when these eggs are fertilized by normal red
bearing, female producing sperm, the lower half by normal, male producing
sperm.]
If such an XXY female carried white bearing Xs (open X in the figures), and
the male carried a red bearing X (black X in the figures) it will be seen
that there should result an exceptional class of sons that are red, and an
exceptional class of daughters that are white. Tests of these exceptions
show that they behave subsequently in heredity as their composition
requires. Other tests may also be made of the other classes of offspring.
Bridges has shown that they fulfill all the requirements predicted. Thus a
result that seemed in contradiction with the chromosome hypothesis has
turned out to give a brilliant confirmation of that theory both genetically
and cytologically.
HOW MANY GENETIC FACTORS ARE THERE IN THE GERM-PLASM OF A SINGLE INDIVIDUAL
In passing I invite your attention to a speculation based on our maps of
the chromosomes--a speculation which I must insist does not pretend to be
more than a guess but has at least the interest of being the first guess
that we have ever been in position to make as to how many factors go
towards the makeup of the germ plasm.
We have found practically no factors less than .04 of a unit apart. If our
map includes the entire length of the chromosomes and if we assume factors
are uniformly distributed along the chromosome at distances equal to the
shortest distance yet observed, viz. .04, then we can calculate roughly how
many hereditary factors there are in Drosophila. The calculation gives
about 7500 factors. The reader should be cautioned against accepting the
above assumptions as strictly true, for crossing-over values are known to
differ according to different environmental conditions (as shown by Bridges
for age), and to differ even in different parts of the chromosome as a
result of the presence of specific genetic factors (as shown by
Sturtevant). Since all the chromosomes except the X chromosomes are double
we must double our estimate to give the _total_ number of factors, but the
half number is the number of the different kinds of factors of Drosophila.
CONCLUSIONS
I have passed in review a long series of researches as to the nature of the
hereditary material. We have in consequence of this work arrived within
sight of a result that seemed a few years ago far beyond our reach. The
mechanism of heredity has, I think, been discovered--discovered not by a
flash of intuition but as the result of patient and careful study of the
evidence itself.
With the discovery of this mechanism I venture the opinion that the problem
of heredity has been solved. We know how the factors carried by the parents
are sorted out to the germ cells. The explanation does not pretend to state
how factors arise or how they influence the development of the embryo. But
these have never been an integral part of the doctrine of heredity. The
problems which they present must be worked out in their own field. So, I
repeat, the mechanism of the chromosomes offers a satisfactory solution of
the traditional problem of heredity.
* * * * *
CHAPTER IV
SELECTION AND EVOLUTION
Darwin's Theory of Natural Selection still holds today first place in every
discussion of evolution, and for this very reason the theory calls for
careful scrutiny; for it is not difficult to show that the expression
"natural selection" is to many men a metaphor that carries many meanings,
and sometimes different meanings to different men. While I heartily agree
with my fellow biologists in ascribing to Darwin himself, and to his work,
the first place in biological philosophy, yet recognition of this claim
should not deter us from a careful analysis of the situation in the light
of work that has been done since Darwin's time.
THE THEORY OF NATURAL SELECTION
In his great book on the _Origin of Species_, Darwin tried to do two
things: first, to show that the evidence bearing on evolution makes that
explanation probable. No such great body of evidence had ever been brought
together before, and it wrought, as we all know, a revolution in our modes
of thinking.
Darwin also set himself the task of showing _how_ evolution might have
taken place. He pointed to the influence of the environment, to the effects
of use and disuse, and to natural selection. It is to the last theory that
his name is especially attached. He appealed to a fact familiar to
everyone, that no two individuals are identical and that some of the
differences that they show are inherited. He argued that those individuals
that are best suited to their environment are the most probable ones to
survive and to leave most offspring. In consequence their descendants
should in time replace through competition the less well-adapted
individuals of the species. This is the process Darwin called natural
selection, and Spencer the survival of the fittest.
Stated in these general terms there is nothing in the theory to which
anyone is likely to take exception. But let us examine the argument more
critically.
[Illustration: FIG. 71. Series of leaves of a tree arranged according to
size. (After de Vries.)]
If we measure, or weigh, or classify any character shown by the individuals
of a population, we find differences. We recognize that some of the
differences are due to the varied experiences that the individuals have
encountered in the course of their lives, i.e. to their environment, but we
also recognize that some of the differences may be due to individuals
having different inheritances--different germ plasms. Some familiar
examples will help to bring home this relation.
If the leaves of a tree are arranged according to size (fig. 71), we find a
continuous series, but there are more leaves of medium size than extremes.
If a lot of beans be sorted out according to their weights, and those
between certain weights put into cylinders, the cylinders, when arranged
according to the size of the beans, will appear as shown in figure 72. An
imaginary line running over the tops of the piles will give a curve (fig.
73) that corresponds to the curve of probability (fig. 74).
[Illustration: FIG. 72. Beans put into cylindrical jars according to the
sizes of the beans. The jars arranged according to size of contained beans.
(After de Vries.)]
[Illustration: FIG. 73. A curve resulting from arrangement of beans
according to size. (After de Vries.)]
If we stand men in lines according to their height (fig. 75) we get a
similar arrangement.
[Illustration: FIG. 74. Curve of probability.]
[Illustration: FIG. 75. Students arranged according to size. (After
Blakeslee.)]
The differences in size shown by the individual beans or by the individual
men are due in part to heredity, in part to the environment in which they
have developed. This is a familiar fact of almost every-day observation. It
is well shown in the following example. In figure 76 the two boys and the
two varieties of corn, which they are holding, differ in height. The
pedigrees of the boys (fig. 77) make it probable that their height is
largely inherited and the two races of corn are known to belong to a tall
and a short race respectively. Here, then, the chief effect or difference
is due to heredity. On the other hand, if individuals of the same race
develop in a favorable environment the result is different from the
development in an unfavorable environment, as shown in figure 78. Here to
the right the corn is crowded and in consequence dwarfed, while to the left
the same kind of corn has had more room to develop and is taller.
[Illustration: FIG. 76. A short and a tall boy each holding a stalk of
corn--one stalk of a race of short corn, the other of tall corn. (After
Blakeslee.)]
[Illustration: FIG. 77. Pedigree of boys shown in Fig. 76. (After
Blakeslee.)]
Darwin knew that if selection of particular kinds of individuals of a
population takes place the next generation is affected. If the taller men
of a community are selected _the average_ of their offspring will be taller
than the average of the former population. If selection for tallness again
takes place, still taller men will _on the average_ arise. If, amongst
these, selection again makes a choice the process would, he thought,
continue (fig. 79).
[Illustration: FIG. 78. A race of corn reared under different conditions.]
We now recognize that this statement contains an important truth, but we
have found that it contains only a part of the truth. Any one who repeats
for himself this kind of selection experiment will find that while his
average class will often change in the direction of his selection, the
process slows down as a rule rather suddenly (fig. 80). He finds, moreover,
that the limits of variability are not necessarily transcended as the
process continues even although the average may for a while be increased.
More tall men may be produced by selection of this kind, but the tallest
men are not necessarily any taller than the tallest in the original
population.
[Illustration: FIG. 79. Curves showing how (hypothetically) selection might
be supposed to bring about progress in direction of selection. (After
Goldschmidt.)]
Selection, then, has not produced anything new, but only more of certain
kinds of individuals. Evolution, however, means producing more new things,
not more of what already exists.
Darwin seems to have thought that the range of variation shown by the
offspring of a given individual about that type of individual would be as
wide as the range shown by the original population (fig. 79), but Galton's
work has made it clear that this is not the case in a general or mixed
population. If the offspring of individuals continued to show, as Darwin
seems to have thought, as wide a range on each side of their parents' size,
so to speak, as did the original population, then it would follow that
selection could slide successive generations along in the direction of
selection.
[Illustration: FIG. 80. Diagram illustrating the results of selection for
extra bristles in D. ampelophila. Selection at first produces decided
effects which soon slow down and then cease. (MacDowell.)]
Darwin himself was extraordinarily careful, however, in the statements he
made in this connection and it is rather by implication than by actual
reference that one can ascribe this meaning to his views. His
contemporaries and many of his followers, however, appear to have accepted
this _sliding scale_ interpretation as the cardinal doctrine of evolution.
If this is doubted or my statement is challenged then one must explain why
de Vries' mutation theory met with so little enthusiasm amongst the older
group of zoölogists and botanists; and one must explain why Johannsen's
splendid work met with such bitter opposition from the English school--the
biometricians--who amongst the post-Darwinian school are assumed to be the
lineal descendants of Darwin.
And in this connection we should not forget that just this sort of process
was supposed to take place in the inheritance of use and disuse. What is
gained in one generation forms the basis for further gains in the next
generation. Now, Darwin not only believed that acquired characters are
inherited but turned more and more to this explanation in his later
writings. Let us, however, not make too much of the matter; for it is much
less important to find out whether Darwin's ideas were vague, than it is to
make sure that our own ideas are clear.
If I have made several statements here that appear dogmatic let me now
attempt to justify them, or at least give the evidence which seems to me to
make them probable.
The work of the Danish botanist, Johannsen, has given us the most carefully
analyzed case of selection that has ever been obtained. There are,
moreover, special reasons why the material that he used is better suited to
give definite information than any other so far studied. Johannsen worked
with the common bean, weighing the seeds or else measuring them. These
beans if taken from many plants at random give the typical curve of
probability (fig. 74). The plant multiplies by self-fertilization. Taking
advantage of this fact Johannsen kept the seeds of each plant separate from
the others, and raised from them a new generation. When curves were made
from these new groups it was found that some of them had different modes
from that of the original general population (fig. 81 A-E, bottom group).
They are shown in the upper groups (A, B, C, D, E). But do not understand
me to say that the offspring of each bean gave a different mode.
[Illustration: FIG. 81. Pure lines of beans. The lower figure gives the
general population, the other figures give the pure lines within the
population. (After Johannsen.)]
On the contrary, some of the lines would be the same.
The result means that the general population is made up of definite kinds
of individuals that may have been sorted out.
That his conclusion is correct is shown by rearing a new generation from
any plant or indeed from several plants of any one of these lines. Each
line repeats the same modal class. There is no further breaking up into
groups. Within the line it does not matter at all whether one chooses a big
bean or a little one--they will give the same result. In a word, the germ
plasm in each of these lines is pure, or homozygous, as we say. The
differences that we find between the weights (or sizes) of the individual
beans are due to external conditions to which they have been subjected.
In a word, Johannsen's work shows that the frequency distribution of a pure
line is due to factors that are extrinsic to the germ plasm. It does not
matter then which individuals in a pure line are used to breed from, for
they all carry the same germ plasm.
We can now understand more clearly how selection acting on a general
population brings about results in the direction of selection.
An individual is picked out from the population in order to get a
particular kind of germ plasm. Although the different classes of
individuals may overlap, so that one can not always judge an individual
from its appearance, nevertheless on the whole chance favors the picking
out of the kind of germ plasm sought.
In species with separate sexes there is the further difficulty that two
individuals must be chosen for each mating, and superficial examination of
them does not insure that they belong to the same group--their germ plasm
cannot be inspected. Hence selection of biparental forms is a precarious
process, now going forward, now backwards, now standing still. In time,
however, the process forward is almost certain to take place if the
selection is from a heterogeneous population. Johannsen's work was
simplified because he started with pure lines. In fact, had he not done so
his work would not have been essentially different from that of any
selection experiment of a pure race of animals or plants. Whether Johannsen
realized the importance of the condition or not is uncertain--curiously he
laid no emphasis on it in the first edition of his "Elemente der exakten
Erblichkeitslehre".
It has since been pointed out by Jennings and by Pearl that a race that
reproduces by self-fertilization as does this bean, automatically becomes
pure in all of the factors that make up its germ plasm. Since
self-fertilization is the normal process in this bean the purity of the
germ plasm already existed when Johannsen began to experiment.
HOW HAS SELECTION IN DOMESTICATED ANIMALS AND PLANTS BROUGHT ABOUT ITS
RESULTS?
If then selection does not bring about transgressive variation in a general
population, how can selection produce anything new? If it can not produce
anything new, is there any other way in which selection becomes an agent in
evolution?
We can get some light on this question if we turn to what man has done with
his domesticated animals and plants. Through selection, i.e., artificial
selection, man has undoubtedly brought about changes as remarkable as any
shown by wild animals and plants. We know, moreover, a good deal about how
these changes have been wrought.
(1) By crossing different wild species or by crossing wild with races
already domesticated new combinations have been made. Parts of one
individual have been combined with parts of others, creating new
combinations. It is possible even that characters that are entirely new may
be produced by the interaction of factors brought into recombination.
(2) New characters appear from time to time in domesticated and in wild
species. These, like the mutants in Drosophila, are fully equipped at the
start. Since they breed true and follow Mendel's laws it is possible to
combine them with characters of the wild type or with those of other mutant
races.
Amongst the new mutant factors there may be some whose chief effect is on
the character that the breeder is already selecting. Such a modification
will be likely to attract attention. Superficially it may appear that the
factor for the original character has varied, while the truth may be that
another factor has appeared that has modified a character already present.
In fact, many or all Mendelian factors that affect the same organ may be
said to be modifiers of each other's effects. Thus the factor for vermilion
causes the eye to be one color, and the factor for eosin another color,
while eosin vermilion is different from both. Eosin may be said to be a
modifier of vermilion or vermilion of eosin. In general, however, it is
convenient to use the term "modifier" for cases in which the factor causes
a detectable change in a character already present or conspicuous.
[Illustration: FIG. 82. Scheme to indicate influence of the modifying
factors, cream and whiting. Neither produces any effect alone but they
modify other eye colors such as eosin.]
One of the most interesting, and at the same time most treacherous, kinds
of modifying factors is that which produces an effect _only_ when some
other factor is present. Thus Bridges has shown that there is a factor
called "cream" that does not affect the red color of the eye of the wild
fly, yet makes "eosin" much paler (fig. 82). Another factor "whiting" which
produces no effect on red makes eosin entirely white. Since cream or
whiting may be carried by red eyed flies without their presence being seen
until eosin is used, the experimenter must be continually on the lookout
for such factors which may lead to erroneous conclusions unless detected.
As yet breeders have not realized the important rôle that modifiers have
played in their results, but there are indications at least that the
heaping up of modifying factors has been one of the ways in which highly
specialized domesticated animals have been produced. Selection has
accomplished this result not by changing factors, but by picking up
modifying factors. The demonstration of the presence of these factors has
already been made in some cases. Their study promises to be one of the most
instructive fields for further work bearing on the selection hypothesis.
In addition to these well recognized methods by which artificial selection
has produced new things we come now to a question that is the very crux of
the selection theory today. Our whole conception of selection turns on the
answer that we give to this matter and if I appear insistent and go into
some detail it is because I think that the matter is worth very careful
consideration.
ARE FACTORS CHANGED THROUGH SELECTION?
As we have seen, the variation that we find from individual to individual
is due in part to the environment; this can generally be demonstrated.
Other differences in an ordinary population are recognized as due to
different genetic (hereditary) combinations. No one will dispute this
statement. But is all the variability accounted for in these two ways? May
not a factor itself fluctuate? Is it not _a priori_ probable that factors
do fluctuate? Why, in a word, should we regard factors as inviolate when we
see that everything else in organisms is more or less in amount? I do not
know of any _a priori_ reason why a factor may not fluctuate, unless it is,
as I like to think, a chemical molecule. We are, however, dealing here not
with generalities but with evidence, and there are three known methods by
means of which it has been shown that variability, other than environmental
or recombinational, is not due to variability in a factor, nor to various
"potencies" possessed by the same factors.
(1) By making the stock uniform for all of its factors--chief factors and
modifiers alike. Any change in such a stock produced by selection would
then be due to a change in one or more of the factors themselves.
Johannsen's experiment is an example of this sort.
[Illustration: FIG. 83 a. Drosophila ampelophila with truncate wings.]
(2) The second method is one that is capable of _demonstrating_ that the
effects of selection are actually due to modifiers. It has been worked out
in our laboratory, chiefly by Muller, and used in a particular case to
demonstrate that selection produced its effect by isolating modifying
factors. For example, a mutant type called truncate appeared, characterized
by shorter wings, usually square at the end, (fig. 83a). The wings varied
from those of normal length to wings much shorter (fig. 83b). For three
years the mutant stock was bred from individuals having the shorter wings
until at last a stock was obtained in which some of the individuals had
wings much shorter than the body. By means of linkage experiments it was
shown that at least three factors were present that modified the wings.
These were isolated by means of their linkage relations, and their mutual
influence on the production of truncate wings was shown.
[Illustration: FIG. 83 b. Series of wings of different length shown by
truncate stock of D. ampelophila.]
An experiment of this kind can only be carried out in a case where the
groups of linked gens are known. At present Drosophila is the only animal
(or plant) sufficiently well known to make this test possible, but this
does not prove that the method is of no value. On the contrary it shows
that any claim that factors can themselves be changed can have no finality
until the claim can be tested out by means of the linkage test. For
instance, bar eye (fig. 31) arose as a mutation. All our stock has
descended from a single original mutant. But Zeleny has shown that
selection within our stock will make the bar eye narrower or broader
according to the direction of selection. It remains to be shown in this
case how selection has produced its effects, and this can be done by
utilizing the same process that was used in the case of truncate.
Another mutant stock called beaded (fig. 84), has been bred for five years
and selected for wings showing more beading. In extreme cases the wings
have been reduced to mere stumps (see stumpy, fig. 5), but the stock shows
great variability. It is probable here as Dexter has shown, that a number
of mutant factors that act as modifiers have been picked up in the course
of the selection, and when it is recalled that during those five years over
125 new characters have appeared elsewhere it does not seem improbable that
factors also have appeared that modify the wings of this stock.
[Illustration: FIG. 84. Two flies showing beaded wings.]
(3) The third method is one that has been developed principally by East for
plants; also by MacDowell for rabbits and flies. The method does not claim
to prove that modifiers are present, but it shows why certain results are
in harmony with that expectation and can not be accounted for on the basis
that a factor has changed. Let me give an example. When a Belgian hare with
large body was crossed to a common rabbit with a small body the hybrid was
intermediate in size. When the hybrid was crossed back to the smaller type
it produced rabbits of various sizes in apparently a continuous series.
MacDowell made measurements of the range of variability in the first and in
the second generations.
_Classification in relation to parents based on skull lengths and ulna
lengths, to show the relative variability of two measurements and of
the first generation (F_1) and the back cross (B. C.)_
CHARACTER|GENERATION|-13|-12|-11|-10| -9| -8| -7| -6| -5| -4| -3| -2| -1|
---------+----------+---+---+---+---+---+---+---+---+---+---+---+---+---+
Length of{ F_1 | | | | | | | | | | | | | |
skull { B.C. | | | | | | | | | | | | | 3|
Length of{ F_1 | | | | | | | | | | | | | |
ulna { B.C. | 1| | | | | 1| | 2| 3| 1| 2| 4| 4|
_same table continued_
CHARACTER|GENERATION| 0| 1| 2| 3| 4| 5| 6| 7| 8| 9| 10| 11| 12|
---------+----------+---+---+---+---+---+---+---+---+---+---+---+---+---+
Length of{ F_1 | | | | | | | | 2| 2| 8| 5| 10| 7|
skull { B.C. | 6| 4| 13| 18| 42| 32| 38| 34| 16| 16| 8| 4| 3|
Length of{ F_1 | | 1| | | 2| | 1| 1| 1| 2| 2| 5| 3|
ulna { B.C. | 12| 11| 20| 26| 17| 19| 18| 15| 12| 13| 15| 11| 5|
_same table continued_
CHARACTER|GENERATION| 13| 14| 15| 16| 17| 18| 19| 20| 21| 22| 23| 24| 25|
---------+----------+---+---+---+---+---+---+---+---+---+---+---+---+---+
Length of{ F_1 | 3| 2| 2| | | | | | | | | | |
skull { B.C. | 1| | | | | | | | | | | | |
Length of{ F_1 | 1| 7| 3| 2| 1| | | | 2| | 1| | 1|
ulna { B.C. | 2| 4| 2| 2| | | 1| 1| | | | | |
He found that the variability was smaller in the first generation than in
the second generation (back cross). This is what is expected if several
factor-differences were involved, because the hybrids of the first
generation are expected to be more uniform in factorial composition than
are those in the second generation which are produced by recombination of
the factors introduced through their grandparents. Excellent illustrations
of the same kinds of results have been found in Indian corn. As shown in
figure 85 the length of the cob in F_1 is intermediate between the parent
types while in F_2 the range is wider and both of the original types are
recovered. East states that similar relations have been found for 18
characters in corn. Emerson has recently furnished further illustrations of
the same relations in the length of stalks in beans.
[Illustration: FIG. 85. Cross between two races of Indian corn, one with
short cobs and one with long cobs. The range of variability in F_1 is less
than that in F_2. (After East.)]
A similar case is shown by a cross between fantail and common pigeons (fig.
86). The latter have twelve feathers in the tail, while the selected race
from which the fantails came had between 28 and 38 feathers in the tail.
The F_1 offspring (forty-one individuals) showed (fig. 87) between 12 and
20 tail feathers, while in F_2 the numbers varied between 12 and 25. Here
one of the grand-parental types reappears in large numbers, while the
extreme of the other grand-parental type did not reappear (in the counts
obtained), although the F_2 number would probably overlap the lower limits
of the race of fantail grandparents had not a selected (surviving) lot been
taken for the figures given in the table.
[Illustration: FIG. 86. Cross of pigeon with normal tail P_1 and fantail
P_1; F_1, bird below.]
[Illustration: FIG. 87. Cross of normal and fantail pigeons. (See Fig. 86.)
The F_2 range is wider than that of F_1. The normal grand-parental type of
12 feathers was recovered in F_2 but the higher numbers characteristic of
fantails were not recovered.]
The preceding account attempts to point out how I should prefer to
interpret the problem of selection in the light of the most recent work on
breeding. But I would give a very incomplete account of the whole situation
if I neglected to include some important work which has led some of my
fellow-workers to a very different conclusion.
[Illustration: FIG. 88. Scheme to show classes of hooded rats used by
Castle. (After Castle.)]
Castle in particular is the champion of a view based on his results with
hooded rats. Starting with individuals which have a narrow black stripe
down the back he selected for a narrower stripe in one direction and for a
broader stripe in the other. As the diagram shows (fig. 88) Castle has
succeeded in producing in one direction a race in which the dorsal stripe
has disappeared and in the other direction a race in which the black has
extended over the back and sides, leaving only a white mark on the belly.
Neither of these extremes occurs, he believes, in the ordinary hooded race
of domesticated rats. In other words no matter how many of them came under
observation the extreme types of his experiment would not be found.
Castle claims that the factor for hoodedness must be a single Mendelian
unit, because if hooded rats are crossed to wild gray rats with uniform
coat and their offspring are inbred there are produced in F_2 three uniform
rats to one hooded rat. Castle advances the hypothesis that factors--by
which he means Mendelian factors--may themselves vary in much the same way
as do the characters that they stand for. He argues, in so many words, that
since we judge a factor by the kind of character it produces, when the
character varies the factor that stands for it may have changed.
As early as 1903 Cuénot had carried out experiments with spotted mice
similar to those of Castle with rats. Cuénot found that spotted crossed to
uniform coat color gave in F_2 a ratio of three uniform to one spotted, yet
selection of those spotted mice with more white in their coat produced mice
in successive generations that had more and more white. Conversely Cuénot
showed that selection of those spotted mice that had more color in their
coat produced mice with more and more color and less white. Cuénot does not
however bring up in this connection the question as to how selection in
these spotted mice brings about its results.
Without attempting to discuss these results at the length that they deserve
let me briefly state why I think Castle's evidence fails to establish his
conclusion.
In the first place one of the premises may be wrong. The three to one ratio
in F_2 by no means proves that all conditions of hoodedness are due to one
factor. The result shows at most that one factor that gives the hooded
types is a simple Mendelian factor. The changes in this type may be caused
by modifying factors that can show an effect only when hoodedness is itself
present. That this is not an imaginary objection but a real one is shown by
an experiment that Castle himself made which furnishes the ground for the
second objection.
Second. If the factor has really changed its potency, then if a very dark
individual from one end of the series is crossed to a wild rat and the
second generation raised we should expect that the hooded F_2 rats would
all be dark like their dark grandparent. When Castle made this test he
found that there were many grades of hooded rats in the F_2 progeny. They
were darker, it is true, as a group than were the original hooded group at
the beginning of the selection experiment, but they gave many intermediate
grades. Castle attempts to explain this by the assumption that the factor
made pure by selection became contaminated by its normal allelomorph in the
F_1 parent, but not only does this assumption appear to beg the whole
question, but it is in flat contradiction with what we have observed in
hundreds of Mendelian cases where no evidence for such a contamination
exists.
Later Castle crossed some of the extracted rats of average grade (3.01)
from the plus series to the same wild race and got F_2 hooded rats from
this cross. These F_2 hooded rats did not further approach the ordinary
range but were nearer the extreme selected plus hooded rats (3.33) than
were the F_2's extracted from the first cross (2.59). Castle concludes from
this that multiple factors can not account for the result. As a matter of
fact, Castle's evidence _as published_ does not establish his conclusion
because the wild rats used in the second experiment may have carried plus
modifiers. This could only be determined by suitable tests which Castle
does not furnish. This is the crucial point, without which the evidence
carries no conviction.
Furthermore, from Castle's point of view, these latest results would seem
to increase the difficulty of interpretation of his first F_2 extracted
cross, and it is now the first result that calls for explanation if one
accepts his later conclusion.
These and other objections that might be taken up show, I think, that
Castle's experiment with hooded rats fails entirely to establish his
contention of change in potency of the germ or of contamination of factors,
while on the contrary they are in entire accord with the view that he is
dealing with a case of modifying factors.
[Illustration: FIG. 89. Races of Paramecium. (After Jennings.)]
Equally important are the results that Jennings has obtained with certain
protozoa. Paramecium multiplies by dividing across in the middle, each half
replacing its lacking part. Both the small nucleus (micronucleus) and the
large nucleus (macronucleus) divide at each division of the body. Jennings
found that while individuals descended from a single paramecium vary in
size (fig. 89), yet the population from a large individual is the same as
the population derived from a small individual. In other words, selection
produces no result and the probable explanation is, of course, that the
different sizes of individuals are due to the environment, while the
constancy of the type is genetic. Jennings found a number of races of
paramecium of different sizes living under natural conditions. The largest
individual of a small race might overlap the smallest individual of other
larger races (fig. 89); nevertheless each kind reproduced its particular
race. The results are like those of Johannsen in a general way, but differ
in that reproduction takes place in paramecium by direct division instead
of through self-fertilization as in beans, and also in that the paramecia
were probably not homozygous. Since, however, so far as known no
"reduction" takes place in paramecium at each division, the genetic
composition of parent and offspring should be the same. Whether
pseudo-parthenogenesis that Woodruff and Erdmann have found occurring in
paramecium at intervals involves a redistribution of the hereditary factors
is not clear. Jennings's evidence seems incompatible with such a view.
[Illustration: FIG. 90. Stylonychia showing division into two. (After
Stein.)]
More recently one of Jennings's students, Middleton, has made a careful
series of selection experiments with Stylonychia (fig. 90) in which he
selected for lines showing more rapid or slower rates of division. His
observations seem to show that his selection separated two such lines that
came from the same original stock. The rapidity of the effects of selection
seems to preclude the explanation that pseudo-parthenogenesis has
complicated the results. Nevertheless, the results are of such a kind as to
suggest that they were due to selection of vegetative (somatic) differences
and that no genetic change of factors was involved, for his conclusion that
the rapidity with which the effects gained by long selection might be
suddenly reversed when selection was reversed is hardly consistent with an
interpretation of the results based on changes in the "potencies" of the
factors present.
Equally striking are the interesting experiments that Jennings has recently
carried out with Difflugia (fig. 91). This protozoon secretes a shell about
itself which has a characteristic shape, and often carries spines. The
opening at one end of the shell through which the protoplasm protrudes to
make the pseudopodia is surrounded by a rim having a characteristic
pattern. The protoplasm contains several nuclei and in addition there is
scattered material or particles called chromidia that are supposed to be
chromatic in nature and related to the material of the nuclei, possibly by
direct interchange.
[Illustration: FIG. 91. Difflugia Corona. (After Cash.)]
When Difflugia divides, part of the protoplasm protrudes from the opening
and a new shell is secreted about this mass which becomes a daughter
individual. The behavior of the nucleus and of the chromidia at this time
is obscure, but there is some evidence that their materials may be
irregularly distributed between parent and offspring. If this is correct,
and if in the protozoa the chromatin has the same influence that it seems
to have in higher animals, the mode of reproduction in Difflugia would be
expected to give little more than random sampling of the germ plasm.
[Illustration: FIG. 92. Races of Difflugia. (After Leidy.)]
Jennings was able by means of selection to get from the descendants of one
original individual a number of different types that themselves bred true,
except in so far as selection could affect another change in them. In this
connection it is interesting to note that Leidy has published figures of
Difflugia (fig. 92) that show that a great many "types" exist. If through
sexual union (a process that occurs in Difflugia) the germ plasm
(chromatin) of these wild types has in times past been recombined, then
selection would be expected to separate certain types again, if, at
division, irregular sampling of the germ plasm takes place. Until these
points are settled the bearing of these important experiments of Jennings
on the general problem of selection is uncertain.
HOW DOES NATURAL SELECTION INFLUENCE THE COURSE OF EVOLUTION?
The question still remains: Does selection play any rôle in evolution, and,
if so, in what sense? Does the elimination of the unfit influence the
course of evolution, except in the negative sense of leaving more room for
the fit? There is something further to be said in this connection, although
opinions may differ as to whether the following interpretation of the term
"natural selection" is the only possible one.
[Illustration: FIG. 93. Evolution of elephant's skulls. (After Dendy.)]
If through a mutation a character appears that is neither advantageous nor
disadvantageous, but indifferent, the chance that it may become established
in the race is extremely small, although by good luck such a thing may
occur rarely. It makes no difference whether the character in question is a
dominant or a recessive one, the chance of its becoming established is
exactly the same. If through a mutation a character appears that has an
_injurious_ effect, however slight this may be, it has practically no
chance of becoming established.
[Illustration: FIG. 94. Evolution of elephant's trunk. (After Lull.)]
If through a mutation a character appears that has a _beneficial_ influence
on the individual, the chance that the individual will survive is
increased, not only for itself, but for all of its descendants that come to
inherit this character. It is this increase in the number of individuals
possessing a particular character, that might have an influence on the
course of evolution. This gives a better chance for improvement by several
successive steps; but not because the species is more likely to mutate
again in the same direction. An imaginary example will illustrate how this
happens: When elephants had trunks less than a foot long, the chance of
getting trunks more than one foot long was in proportion to the length of
trunks already present and to the number of individuals; but increment in
trunk length is no more likely to occur from an animal having a trunk more
than one foot long than from an animal with a shorter trunk.
The case is analogous to tossing pennies. At any stage in the game the
chance of accumulating a hundred heads is in proportion to the number of
heads already obtained, and to the number of throws still to be made. But
the number of heads obtained has no influence on the number of heads that
will appear in the next throw.
[Illustration: FIG. 95. Evolution of elephant's trunk: above Maeritherium,
in the middle Tetrabelodon (After Lancaster); below African elephants
(After Gambier Bolton).]
Owing then to this property of the germ plasm to duplicate itself in a
large number of samples not only is an opportunity furnished to an
advantageous variation to become extensively multiplied, but the presence
of a large number of individuals of a given sort prejudices the probable
future result.
The question may be raised as to whether it is desirable to call selection
a _creative_ process. There are so many supernatural and mystical
implications that hang around the term creative that one can not be too
careful in stating in what sense the term is to be used. If by creative is
meant that something is made out of nothing, then of course there is no
need for the scientist to try to answer such a question. But if by a
creative process is meant that something is made out of something else,
then there are two alternatives to be reckoned with.
First, if it were true that selection of an individual of a certain kind
determines that new variations in the same direction occur as a consequence
of the selection, then selection would certainly be creative. How this
could occur might be quite unintelligible, but of course it might be
claimed that the point is not whether we can explain how creation takes
place, but whether we can get verifiable evidence that such a kind of thing
happens. This possibility is disposed of by the fact that there is no
evidence that selection determines the direction in which variation occurs.
Second, if you mean by a creative process that by picking out a certain
kind of individual and multiplying its numbers a better chance is furnished
that a certain end result will be obtained, such a process may be said to
be creative. This is, I think, the proper use of the term creative in a
mechanistic sense.
CONCLUSIONS
In reviewing the evidence relating to selection I have tried to handle the
problem as objectively as I could.
The evidence shows clearly that the characters of wild animals and plants,
as well as those of domesticated races, are inherited both in the wild and
in the domesticated forms according to Mendel's Law.
The causes of the mutations that give rise to new characters we do not
know, although we have no reason for supposing that they are due to other
than natural processes.
Evolution has taken place by the incorporation into the race of those
mutations that are beneficial to the life and reproduction of the organism.
Natural selection as here defined means both the increase in the number of
individuals that results after a beneficial mutation has occurred (owing to
the ability of living matter to propagate) and also that this preponderance
of certain kinds of individuals in a population makes some further results
more probable than others. More than this, natural selection can not mean,
if factors are fixed and are not changed by selection.
* * * * *
INDEX
Abnormal abdomen 109
Abraxas 78-81
Allantois 17
Allelomorphs 83-84
Altenburg 112
Amnion 16-17
Andalusian fowl 45, 46
Annelids 22
Antlered wing 111
Apterous wing 11
Arc wing 111
Aristae 104
Bar eye 67, 108, 169
Bateson 18, 34, 36
Beaded wing 11, 115
Beans 147-149, 157
Belgian hare 171
Bent wing 116
Bergson 30, 31
Bildungstrieb 34
Biogenetic law 15, 18, 19, 21
Biometricians 156
Bird 21, 23
Bithorax 65, 112, 113
Black body color 111, 133
Blakeslee 152
Bridges 114, 143, 163
British Association 36
Brünn 40
Buff eye color 109
Bufon 27
Castle 176-180
Cat 33
Cell 90, 91
Chance variations 37
Chick 16, 17, 20
Chromatin 184
Chromosome group of Drosophila 102
Chromosomes 91, 95, 96, 98, 130, 131, 132
Cleavage 21, 22, 94
Clover butterfly 62
Club wing 69, 70, 108
Colias philodice 62
Color blindness 77, 125
Comb of Drosophila 103
Combs of fowls 33, 54
Comparative anatomy 7, 8, 9, 14
Corn 150, 153, 172
Correns 41
Cosmogonies 27
Cream eye color 163, 164
Crepidula 22
Criss-cross inheritance 78
Crossing over 131-133
Cuénot 178
Curled wing 115
Curved wing 111
Curve of probability 149
Cut wing 11, 104
Dachs legs 112
Dahlgren 62
Darwin 15, 24, 28, 32, 35-37, 64, 145, 146, 152, 154-156
Dendy 188
De Vries 18, 147, 156
Dexter 170
Dichaete 114
Difflugia 184-187
Discontinuous variation 13
Disuse 31
Drosophila ampelophila 10, 12, 13, 48-50, 60, 75, 84, 85, 93, 100, 103,
119, 155, 162, 169
Drosophila repleta 76
Duplication of legs 109
Dwarf 114
East 170, 172
Ebony 50, 55, 56, 115
Egg 91, 94
Elephant 191
Elephants' skulls 188
Elephants' trunks 190
Embryology 13-23
Emerson 172
Environment 27
Eosin eye color 61, 107, 163
Erdmann 183
Evolution Creatrice 30
Evolution--three kinds of 1, 2, 4
Eye color 13
Eyeless 66, 115
Factorial theory 89
Factors of Drosophila 143
Fantails 172, 175
Fertilization 91
Fish 16, 20, 21
Flatworms 22
Fluctuations 12
Forked bristles 106
Fowl 77
Fused veins 107, 108
Galton 154
Geneticist 26
Germ-plasm 142
Geoffroy St. Hilaire 27
Giant 114
Gill-slits 20, 21, 23
Groups I, II, III, IV 100-118
Haeckel 15
Haemophilia 77
Heliotropism 106, 107
Himalyan rabbits 83
History 1, 6
Hoge 66
Horse, evolution of 6
Indian corn 172, 173
Interference 137, 138
Janssens 132
Jaunty wing 111
Jennings 161, 181-184, 186
Johannsen 156, 157, 159-161, 166, 182
Lamarck 31-34
Langshan 77
Leaves 147
Leidy 186
Lethal 105
Linkage groups 103
Lizard 23
Localization of factors 118
MacDowell 155, 170, 171
Macritherium 191
Mammal 16, 21, 23
Man 20, 77, 125, 126
Map of Chromosomes 136
Maroon eye color 114
Mendel 40, 41, 52, 89
Mendelian heredity 39
Mendel's law 41-59, 64, 124
Mendel's second law 52
Mesenchyme cells 22
Mesoderm cells 22
Metaphysician 30
Mice 33, 178
Middleton 183
Miniature wing 108
Mirabilis 42
Modifiers 163, 164, 170, 171
Molluscs 22
Mouse 83
Muller 112, 167
Mutations 35, 39, 84
Nägeli 34, 35
Natural Selection 36, 145, 146, 187-194
Nisus formativus 34
Non-disjunction 139-142
Notch wing 104-106
Nucleus 91
Origin of Species 35, 145
Orthogenesis 34
Paleontology 24-27
Papilio polytes 63
Papilio turnus 63
Paramecium 181, 182
Paratettix 81
Peach eye color 114
Pea comb 54
Pearl 161
Peas 47
Pigeons 172, 174, 175
Pink eye color 114, 115
Planarian 22
Plymouth Rock 77
Podarke 22
Polar bodies 126
Pole arms 5
Protozoa 181
Pseudo-parthenogenesis 183
Purple eye color 109
Purpose 4
Rabbits 83, 170
Rats 176-180
Reduction division 182
Reproductive cells 96
Ruby eye color 106
Rudimentary organ 116
Rudimentary wing 70, 71, 107
Sable body color 107
Science definition of 6
Segregation 41
Selenka 94
Sepia eye color 13, 114
Sex chromosomes 118
Sex linked inheritance 75, 118-130
Sexual dimorphism 62
Sheep 33
Single comb 54
Sooty body color 50, 114, 115
Speck 68, 69, 111
Spencer 145
Spermatozoön 91, 98
Stars, evolution of 6
St. Hilaire 27-30
Strap wing 110, 111
Stumpy wing 11
Sturtevant 76, 143
Stylonychia 183
Survival of the fittest 146
Systematist 85
Tails 33
Tan flies 106, 107
Tetrabelodon 191
Trefoil 111
Truncate wing 111, 112, 167, 168
Unfolding principle 34
Unio 22
Unit character 74, 75
Use 31
Variation discontinuous 13
Vermilion eye color 108, 163
Vestigial wing 11, 55, 56, 109, 133
Vital force 34
Wallace 36
Walnut comb 54
Weismann 17, 31-33
Wilson, E. B. 125
Wingless 67
Winiwarter 126
White eye color 13, 75, 119-130
Whiting eye color 163, 164
Woodruff 183
Yellow body color 108, 133
Yolk sac 16, 17
Zeleny 169
* * * * *
Corrections made to printed original.
page 104, "shown in figures 53, 54, 55, 56": '52, 53, 54, 55' in original.
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