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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
Naegeli 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 zoologists and various views,
often very naive, 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 zoologists 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, protege, 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
_Naegeli 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 naively
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.
Naegeli's speculation written several years after Darwin's "Origin of
Species" may be taken as a typical case. Naegeli 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.
Naegeli 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 role of the gardener who prunes the tree into this or that
shape but who has himself _produced_ nothing. As an imaginative figure of
speech Naegeli'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.
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