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CHAPTER I

A REVALUATION OF THE EVIDENCE ON WHICH THE THEORY OF EVOLUTION WAS BASED

[ 1 ]

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 [ 2 ] 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. [ 3 ]

Fig. 1. A series of skulls and feet. Eohippus, Mesohippus, Meryhippus, Hipparion and Equus. (American Museum of Natural History. After Matthews.)

[ 4 ]

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 [ 5 ] 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.

Fig. 2. Evolution of pole arms. (Metropolitan Museum. After Dean.)

[ 6 ]

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 [ 7 ] 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. [ 8 ]

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 [ 9 ] 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.

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 [ 10 ] 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 [ 11 ] 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.

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.

[ 12 ]

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 [ 13 ] 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 [ 14 ] 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 [ 15 ] 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. [ 16 ]

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 [ 17 ] world. Hundreds of such embryonic adaptations are known to embryologists. These were explained as adaptations and as falsifications of the ancestral records.

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 [ 18 ] 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 [ 19 ] they change them without, as it were, passing through and beyond them.

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.)
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 [ 20 ] compatible with the theory of evolution. Let me illustrate this by an example.

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 [ 21 ] 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?

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. [ 22 ]

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 [ 23 ] 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. [ 24 ]

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 [ 25 ] 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 [ 26 ] 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 [ 27 ] 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 [ 28 ] 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 [ 29 ] 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 [ 30 ] 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 [ 31 ] 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. [ 32 ]

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 [ 33 ] 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 [ 34 ] 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 [ 35 ] 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 [ 36 ] 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 [ 37 ] 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 [ 38 ] 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 [ 39 ] 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.