Popular Science Monthly/Volume 74/April 1909/For Darwin

From Wikisource
Jump to navigation Jump to search

FOR DARWIN[1]
By Professor T. H. MORGAN

COLOMBIA UNIVERSITY

WE have come together to-day to consider Darwin's influence on zoology. It is a hazardous task to pretend to estimate the influence of any event on the course of history so long as we can not know what the outcome had been otherwise. But to this at least we can testify, that it is the general belief of zoologists to-day that Darwin's influence in bringing about the acceptance of the theory of evolution marks a turning point in the history of their science, and I shall attempt to justify this opinion by pointing out the condition of zoology before Darwin and its subsequent course of development after 1859.

To the zoologist Darwin was above all else a zoologist. It is true he interested himself greatly in geology, but he does not stand as a leader of that science; he carried out many experiments with plants and wrote some important botanical books, and here the zoologist will yield second place to his brother, the botanist. Darwin wrote on the "Descent of Man," he studied the expression of the emotions and carried out physiological work along several lines, yet I should not rank him preeminently an anthropologist, a psychologist or a physiologist any more than a paleontologist or a botanist.

In the mind of the general public Darwinism stands to-day for evolution. The establishment of the theory of evolution is generally accepted as Darwin's chief contribution to human thought, and while Darwin did not originate this idea that forms the framework of our modern thinking, yet by general accord its acceptance is attributable, and justly so, to Darwin.

To the zoologist Darwinism means more especially evolution accounted for by the theory of natural selection, yet also many other things, to which I shall refer in the proper place.

But I shall attempt this afternoon, before all else, to convince you that the loyalty that every man of science feels towards Darwin is something greater than any special theory. I shall call it the spirit of Darwinism, the point of view, the method, the procedure, of Darwin.

In order that we may form some idea of Darwin's influence on zoology, let us examine the condition of that science prior to 1859 to see what influence zoology had on Darwin and his contemporaries. I shall not try your patience by attempting to review the history of the subject, but it would not belittle the greatness of Darwin's achievement one whit to find that brilliant discoveries had been made before his time, the theory of evolution plainly enunciated, the doctrine of spontaneous generation disproved; comparative anatomy widely studied; the important functions of the body elucidated, the foundations of the science of embryology laid, and the principles of pedigree breeding followed.

In the eighteenth century, when the study of different kinds of animals inhabiting sea and land attracted the attention of zoologists, great classifications were invented. Two main facts emerged. On the assumption of fixity of type, a classification of the different forms of animals and plants became possible. But on the other hand the more extensive the material to be classified, the more difficult it became to make such systems, for the fixity of type was often lost in apparent transitions to other types. Counter claims arose as to the superiority of one system over another, and the question of an artificial system versus a natural one was widely debated. Now, an artificial system, like the arrangement of the words in a dictionary, is obviously only a matter of convenience, but it became a question of deep philosophical importance to decide what was meant by a natural classification. To us at the present time a natural classification implies a relation due to descent; it is neither more nor less than the natural relation of a man to his ancestors. But it were a fatal mistake to read our meaning backwards to the time before Darwin.

To the great Cuvier a natural system meant an assemblage of groups having a common plan of structure, and he was enraged by Geoffroy St. Hilaire's attempt to put all animals from the bottom to the top in a straight line. A common plan of structure might only mean that idea which best expressed the outcome of a wide study of structure; but to those who tried to peer behind the scenes it meant not seldom to fathom the creation of the world; and it required no vivid imagination to add that it gives an insight into the plan by which the world was created.

A historian of the times wrote:

Yet in fact the assumption of an end or purpose in the structure of organized beings appears to be an intellectual habit, which no efforts can cast off, It has prevailed from the earliest to the latest ages of zoological research appears to be fastened upon us alike by our ignorance and our knowledge. . and the doctrine of unity of plan of all animals, and the other principles associated with this doctrine, so far as they exclude the conviction of an intelligible scheme and a discernible end, in the organization of animals, appear to be utterly erroneous.

Contrast, in passing, this pious conviction with Geoffroy's modest lines:

I ascribe no intention to God, for I mistrust the feeble powers of my reason. I observe facts merely and go on. I can not make nature an intelligent being who does nothing in vain, who acts by the shortest mode, who does all for the best.

Thus arose in the eighteenth and nineteenth centuries the dogma of the fixity of species—a dogma based, it is true, on a direct appeal to fact as well as to conscience. But this dogma contained the germ of its own undoing, in so far as it appealed for its support to observations that every man might make for himself. Yet so influential were its advocates, so convinced of its truth, that more than one assault was made before it crumbled away.

It is no small pleasure to repeat to-day the names of those bold and original thinkers, who braved the displeasure of their compatriots and the contempt of their times, who brought forward evidence and argument to disprove the teaching of the schools. Their work, it is true, failed in the sense that it received no sufficient meed of praise or word of commendation, but who will deny that a seed was sown that in time bore fruit? Foremost, I think, ranks the great Lamarck, the centenary of whose "Philosophic Zoologique" is celebrated this year in France—a bold spirit, whose ideas, based on a wide familiarity with facts, live and bear fruit to-day. Geoffroy St. Hilaire, advanced thinker and philosopher of nature, opponent of the great anatomist Cuvier, brought the problem of evolution to the bar of judgment, losing the decision, it is true, but his ideas a later generation hold in high esteem. Erasmus Darwin, grandfather of our Darwin, author of "The Zoonomia," celebrated in verse "The Botanic Garden" and the "Loves of the Plants," and even before Lamarck, advocated the principle of evolution and the thory of inheritance of acquired characters. Herbert Spencer, adopting the idea of evolution, laid thereon the elaborate superstructure of his philosophy. Robert Chambers, too, kept alive the central idea of change in the organic world in his "Vestiges of Creation." Others there were, besides, in different lands, but these especially were nearer to Darwin and his times.

We come now to the years between 1837 and 1844, when Darwin was making his memorable notes on the relation between varieties and species. Reading through his letters of this period one is surprised to find how little he was impressed by the history of zoology and the influences of his own time, and how much he based his conclusions on the results of his own close observations, his accumulation of data, and careful consideration of facts. In regard to Lamarck, Darwin states in his autobiography, that in 1835 when he was at Edinburgh University, Dr. Grant "burst forth in high admiration of Lamarck and his views on evolution. I listened in silent astonishment, and as far as I can judge, without any effect on my mind."

In later years, after reading Lamarck, Darwin wrote Lyell, in 1859:

You often allude to Lamarck's work; I do not know what you think about it, but it appeared to me extremely poor; I got not one fact or idea from it.

Writing to Lyell in 1863, he says:

You refer repeatedly to my view as a modification of Lamarck's doctrine of development and progress. . . . Plato, Buffon, my grandfather before Lamarck, and others, propounded the obvious views that if species were not created separately they must have come from other species, and I can see nothing else in common between the "Origin" [of Species] and Lamarck.

Darwin wrote to Hooker in 1844:

Heaven forfend me from Lamarck's nonsense of a "tendency to progression," "adaptations from the slow willing of animals," etc. But the conclusions I am led to are not widely different from his; though the means of change are wholly so.

Darwin had read "The Zoonomia" of his grandfather prior to 1825 in which "similar views [to those of Lamarck] are mentioned but without producing any effect" on him. He continues, with his usual candor:

Nevertheless it is probable that the hearing rather early in life such views maintained and praised may have favored my upholding them under a different form in my "Origin of Species."

It is a regrettable fact that Darwin did not appreciate Lamarck's work. The failure of Lamarck's writings to produce any apparent influence on Darwin may be attributed, I think, to the form in which Lamarck's views are presented. He uses facts as illustrations of his ideas, while with Darwin the facts are all important as furnishing the evidence on which a theory is to be established. He misunderstood Lamarck's view in regard to the inheritance of acquired characters, yet held himself the same opinion in the main as had Lamarck. The modern idea of descent, as a system of branching due to divergence in those species descended from the same parent species, was expounded luminously by Lamarck, yet Darwin discovered it independently for himself. He says:

But at that time (1844) I overlooked one problem of great importance; and it is astonishing to me. . . how I could have overlooked it and its solution. This problem is the tendency in organic beings descended from the same stock to diverge greatly in character as they become modified. That they have diverged greatly is obvious from the manner in which species of all kinds can be classed under genera, genera under families, families under suborders, and so forth, and I can remember the very spot in the road where to my joy the solution occurred to me.

It is this same view that Lamarck had fully expounded thirty-five years before.

We have now arrived at the period just before the publication of Darwin's famous book. It is sometimes said that the time was ripe for the reception of the ideas formulated by Darwin—it was in the air, as we say—but if so, it must have been so attenuated as to be invisible to eyes as sharp as Huxley's and the other famous naturalists of that time. Huxley says that within the ranks of the biologists he met with but one who had a word to say for evolution. Outside these ranks the only person known to him "whose knowledge and capacity compelled respect" and who advocated evolution was Herbert Spencer. "Many and prolonged were the battles they fought" on this topic, but Huxley maintained his agnostic position. He states:

I took my stand upon two grounds; firstly that up to that time the evidence in favor of transmutation was wholly insufficient; and secondly that no suggestion respecting the causes of the transmutation assumed which had been made was in any way adequate to explain the phenomena. Looking back at the state of knowledge at that time I really do not see that any other conclusion was justified.

This frank statement of Huxley not only gives us an insight into the position of one of the most progressive zoologists of that time, but what is of more importance it implies also why the "Origin of Species" convinced him of the doctrine of evolution.

We have now sufficiently traced the possible influences of the times on Darwin. Before we proceed to study the influence of Darwin on his time, let us for a moment pause to consider what influence Darwin's own surroundings had in shaping his views. His voyage in the Beagle had brought him in contact with the question of geographical distribution. He read Malthus in 1838 and this gave him his first idea of the survival of the fittest; and, as his son and biographer states, this date marks "the turning point in the formation of his theory," so that by 1844 he formulated "a surprisingly complete presentation of the argument afterwards familiar to us in the 'Origin of Species.'" His extensive study of variation under domestication furnished him with the experimental evidence that went so far towards making his study of variation of far-reaching and profound importance. Indeed, in this one essential respect, Darwin was far ahead of all of his contemporaries, and, if you will pardon the anachronism, far ahead of his successors. It is only in recent years that zoologists and botanists have begun once more to work the rich mine of materials at their very doors. The paper of Wallace on "Natural Selection" in 1858, the reception to the "Origin of Species" in 1859, the storm of disapproval it met on the one hand, the staunch and able friends it made on the other, need only be recalled in passing.

We come now to the influence that Darwin's work has had on modern zoology. That influence is due not alone to the "Origin of Species" that gave to the world an abstract only of his views, but equally to his other works, especially, I think, the "Variation of Animals and Plants under Domestication," and the "Descent of Man." After Darwin and largely as an outgrowth of the wide interest his views aroused in all branches of zoology we find activity going on in many lines of work. One group of workers, the systematists, have kept nearer, I think, to the older traditions. They have been concerned with three of the most important matters that have a direct influence on the "Origin of Species "—the intensive study of species and varieties, the geographical and geological distribution of animals, and the influence of the environment in modifying species. Their results have supplied the most extensive contributions, perhaps, that have been made to the theory of species-formation and transmutation. They seem to me, however, to have paid less attention to another, equally important, field, that of the adaptation of animals to their environment, and the causes that have been effective in bringing about this adaptation. To physiology we look in vain for an answer to this question, that is perhaps a physiological problem, for while physiology has advanced to a wonderful degree our knowledge of the complicated adjustments within the body, the origin in time of these adjustments and their relation to the outer world has excited less interest.

The morphologists, or philosophical anatomists, form the second great group of students whose activity is a direct outgrowth of Darwinism. The determination of the relationships of the great classes of animals on the principle of descent has occupied much of their time. Two other important fields of labor have also fallen to their share. The study of development or embryology has been almost exclusively pursued by morphologists, inspired in large part by the theory of recapitulation.

The older form of the doctrine, that in the development of the individual the past history of the race is repeated, has been revived—a doctrine much in vogue in the early part of the last century, which has continued to have its followers despite the different interpretation that von Baer gave to the same facts. Whatever interpretation we choose at the present time, the presence of structures like gill-slits in the human embryo, directly comparable to those in the fish, has had an important influence in disentangling the relationship of living animals to their remote ancestors.

The morphologist has also undertaken the study of heredity, and the relation of heredity to the germ-cells that are the links in the chain of organic life. Few other studies have advanced in recent years at a more rapid pace and few have yielded facts of greater significance, for here lies the key to the origin and nature of variations.

Systematists and morphologists alike have been evolutionists, but it is a curious fact of zoological history that until very recently there has been no body of students whose interests have been directed primarily towards the problems of evolution. This is due, I think, to a general feeling that the data for evolution are rather the by-products of the zoologist's work-shop, than products directly manufactured by him. despite the splendid example of Darwin to the contrary. Is it not strange, therefore, with all the real interest in the theory of evolution, that so few of the immediate followers of Darwin devoted themselves exclusively to a study of that process? As I have said, the systematists have been accumulating a vast amount of valuable material, but their chief interest has, on the whole, been in its classification, only secondarily in its bearing on evolution. The morphologist has been busy in applying the theory of evolution to the explanation of group relationships. The paleontologist has perhaps been more directly concerned with the evolution question than has any other worker.

There is a school that calls itself Lamarckian or Neo-Lamarckian which as far as its name goes should include the followers of Lamarck rather than of Darwin. Yet with few exceptions the Neo-Lamarckians derive their inspiration, I think, directly from Darwin. Darwin held that characters acquired during the life-time of the individual may be transmitted to the offspring. He abhorred what he supposed to be Lamarck's rubbish, that an animal acquired a new part by willing it. We have seen that this is a travesty on Lamarck's real teaching, and that on the whole Darwin's view of acquired characters is almost Lamarck's. Yet the modern Lamarckians get their doctrine direct from Darwin rather than from Lamarck, who propounded it fifty years earlier, as had Erasmus Darwin, still earlier.

I have laid emphasis on the relation of Lamarckism to Darwinism in order to draw attention to the problem of adaptation. The NeoLamarckians have kept this all-important question in the foreground, while others have taken adaptation too much for granted in their attempts to explain the origin of species; for species, in a technical sense, may have little to do with the problem of adaptation. The life of an animal is intimately dependent on its adaptative characters, but its "specific characters" may be largely unimportant for its existence. Systematists and morphologists include broadly the followers of Darwin during the thirty years after the publication of the "Origin of Species." They have advanced to a high degree the principles of their science, and the modern aspect of zoology is largely the outcome of their varied and far-reaching labors.

There is a small group of writers scattered amongst these larger groups that are ranked or rank themselves Neo-Darwinians. I must pause a moment to pay them my tardy respects. They have set themselves up to be the true Darwinians. They seem less concerned with the advancement of the study of evolution than with expounding Darwinism as dogma. Their credulity is more remarkable than their judgment. To imagine a use for an organ is for them equivalent to explaining its origin by natural selection without further inquiry. Any examination, in fact, into the nature of variation, they appear to regard as superfluous, although harmless, but it is heresy to study critically the working out of the theory of natural selection. Such has ever been the procedure of the infertile followers of great leaders. In the present instance the result is the more deplorable, since Darwin's own independence of the traditions of all schools, his careful study of facts, his emancipation from prejudice, are his lasting virtues. The NeoDarwinian, worshipping the letter of the law, forgets its import. Let us salute, and pass.

And now we come to the last twenty years of zoology as influenced by Darwin. This, I believe, is the brightest chapter of Darwinism, for the spirit of Darwin is once more abroad.

Foremost amongst the many debts that modern zoology owes to Darwin is this: he pointed out that in order to understand how evolution takes place, we must study the variations of animals and plants, for here is the material on which rests any solid superstructure. To my mind, the appreciation of this maxim and its application is the distinguishing feature of Darwin's work. Before his time the theory of evolution remained but a general idea, though one of profound significance. After Darwin, the theory of evolution rested its claims for recognition on a definite body of information relating to variations and their inheritance. It is these data that first convinced his greatest contemporaries of the reality of evolution, and finally convinced also the rank and file of thinking men. So extensive were the facts of variation accumulated by Darwin, so penetrating was his analysis of these facts, so keen was his insight, and so wise his judgment as to their meaning, that for thirty years afterwards little of importance in this direction was added. In their amazement at Darwin's accomplishment zoologists forgot that he had opened the door leading into an unexplored territory. During the last twenty years the march forward has once more begun and the reward has been immediate.

Let us tarry therefore a little in these rich and pleasant fields of discovery and examine in some detail what is being done. The study of variation has been actively pursued in three main directions. The biometricians have applied exact measurements to variation; the ecologists have studied the complex influences of the environment; the experimentalist has put to the test the supposed factors of change. Each of these methods has brought out results of significance.

A careful study of variations within each species has shown that taken as a group many variations conform to the law of probability. Popularly expressed, this means that chance determines variations, or, put more exactly, variations taken as a group and measured, give the same mathematical results that follow when any set of objects become arranged according to the laws of probability. There was a time when chance meant lack of conformity to law. Such a popular interpretation has no scientific standing. The great law of causation is not abrogated, but the outcome is only the result of a large number of small influences whose effects depend on the nature of the material and on the nature of the conditions. It is so important that this fact be clearly understood that I may be pardoned if I call to mind some familiar illustrations. No two leaves on a tree are identical, yet if many are measured, they give the curve of probability. Men are of different heights, yet they range about a mode. Color appears in various shades, yet if standardized, it is found to follow the same laws of chance variation.

What value have these facts for the theory of evolution? If in every generation we find that the same kinds of individuals recur, the results mean stability, not progress. That this state of affairs actually exists in many species living under the same environment during successive generations there can be little doubt. But change the environment and the results also change. Another factor comes to light that is independent of outside conditions. It is what has been called preferential mating. If within a group the males and females of certain kinds tend more often to pair with each other, the collective group becomes modified in one or more directions. In man this factor assumes a special importance, for, as Pearson has shown, there is measurable evidence that such mating occurs.

It has often been urged, and I think with much justification, that the selection of individual, or fluctuating variations could never produce anything new, since they never transgress the limits of their species, even after the most rigorous selection—at least the best evidence that we have at present seems to point in this direction. But a new situation has arisen. There are variations within the limits of Linnean species that are definite and inherited, and there is more than a suspicion that by their presence the possibility is assured of further definite variation in the same direction which may further and further transcend the limits of the first steps. If this point can be established beyond dispute, we shall have met one of the most serious criticisms of the theory of natural selection.

It is not without interest to note in this connection that Darwin often assumed that fluctuating variations are transmitted to the offspring. The idea that they are not was a later development—the result, it is true, of a better knowledge of the law of fortuitous effects, or of probability. But we have discovered the additional fact that some small variations are inherited. Let us call these definite variations, and if these be the material with which evolution is concerned, Darwin's assumption in regard to the nature of variation will be, in part, justified.

These small, definite variations appear to be closely allied to those larger, more visible definite variations that we now call mutations. We owe our modern ideas of such variations mainly to de Vries and to those who have followed in his footsteps. Such sudden changes have been long known and were spoken of by Darwin as saltations—or sports. Darwin knew of cases like the ancon ram, from which a race of shortlegged sheep was produced. He knew that totally black or melanistic mutations and albinos arise in many groups suddenly, and transmit their characters. A black-shouldered or japanned peacock has appeared more than once and perpetuated itself without selection. It would be out of place to-day to discuss this absorbing problem. That extreme mutations may at times have been an element of progress in nature few will deny, especially if we exclude such monstrous forms as those the breeder has used in building up domesticated races of animals.

It is not, however, to these extreme examples of definite variations that I wish especially to draw your attention, but to that group of smaller variations of a similar nature that may at their first appearance fall within the limits of ordinary variability. I now ask you, therefore, to follow me in an attempt to apply this latest discovery to the theory of evolution.

If we trace the ancestors of any living animal—man, for example—we discover that his ancestry goes back not as a single line, nor as a converging system of lines, but as a vast branching network. Each man has had 2 parents, 4 grandparents, 8 great-grandparents, 16 in the fourth generation, 32 in the fifth, 64 in the sixth, 128 in the seventh, 256 in the eighth, 512 in the ninth, 1,024 in the tenth. A few generations further removed we should expect to find that the majority of all the individuals of the species had poured their blood, as we say, into each individual of the future generations. Each of us is the descendant of a large population. The statement is not strictly true, for some lines die out, many lines cross, and caste has narrowed the field, but the statement suffices to show that a species moves along as a horde rather than as the offspring of a few individuals in each generation. The mass serves to keep the species afloat in times of calamity, it may have little else to do directly with its advance. Nevertheless this fundamental fact is too often overlooked in the attempt to explain the origin of new races, varieties and species from single favorable variations.

For advance we must look to those individuals that contribute something new to the species—it is the superman that will add something to the common level of humanity, but the rest keep the race alive until his advent and then carry his kind forward on an advancing wave.

If we could count those individuals that are the pioneers of advance, their number might be very small; in order to survive, they must graft themselves onto the stock. They are the harbingers of the better times to come—the forerunners of progress.

We touch here the crucial point of evolution in its relation to Darwin's principle of natural selection. Darwin says that he did not at first realize the overwhelming influence of the mass in its swamping effects on the individual variant. He made a very important concession to this view in the later editions of the "Origin of Species," and thought it necessary to assume that for a new form to arise it must first appear in a large number of individuals.

But to-day the situation has changed and new facts have come to light—facts that remove the enormous difficulty that Darwin met by what may seem now to have been an unnecessary concession.

An imaginary case will illustrate what I wish to say. Suppose that a species consisted in each generation of a million individuals and let us imagine that a new character—a definite variation—appears in an individual. The individual that bears it will pair with another ordinary individual and transmit its new character to all of its offspring. In order to simplify our case let us imagine that from each pair of individuals four reach maturity. The million of individuals has increased to two millions, but accidents and competition may kill off one million of these, so that the race is again reduced to its standard of one million. If, then, we suppose that two of the new kinds of individuals survive on the average, and pair at random, there will be eight in the next generation (in reality only six of the eight will show this character). If these survive they will transmit their character to twelve of their offspring. Gradually, however, step by step, the new character will be added to the whole race. Thus any new, definite character will gradually appear in all the individuals whether it is useful or not. If it is useful it may sooner implant itself on the race than if it is indifferent; for more individuals may survive that possess it, than of those without it. It will spread faster, but in any case it will come in the long run. Thus we see that it spreads, not because it is advantageous, but because it is a definite variation. Injurious characters will have greater difficulties in inflicting themselves on the race, and if distinctly injurious may never succeed.

While one character is spreading, other definite variations may also be adding themselves to the race. Those individuals that combine the greatest number of useful additions will have the best chance of survival. Slowly the race advances in the direction of the sum of its advantages and adaptation; success, not in one but in several characters, is the true criterion of survival.

To fix our attention on each single advantage and to ascribe to it alone the palm of victory gives an incomplete idea of the progress of evolution, for evolution follows the line of the greatest number of adaptations. Success in every generation cannot be traced to one variation, but to the sum of all mingled advantages.

This interpretation broadens, I think, our general conception of natural selection. We see that it is erroneous to suppose that all the individuals that bear a particular, useful trait owe this trait to their descent from one kind of individual, in the sense that this individual is the sole ancestor of all the later survivors. The first individual is not alone the ancestor of all the individuals that later bear its mark, it is only one of 999,999 ancestors that have contributed to the perpetuation of the race.

In order to simplify the case we have imagined that the new variation has appeared in a single individual. Should it appear in more than one, or arise again and again, its implantation would be thereby hastened, but the principle remains the same.

My contention may be summed up in a sentence. Survival-value is not the only test for the perpetuation of any one useful character; it is the sum total of useful variations that determines progress. The species moves as a group always. Evolution is not a simple but a complex problem. This is the general opinion held by most modern zoologists.

To-day there are three great rival claims that attempt to explain how evolution takes place: (1) that which adopts the theory of natural selection in one or another of its aspects; (2) that which maintains that acquired characters are inherited; (3) that which, trying to penetrate deeper into the mystery of life, ascribes to living matter a purposefulness—an almost conscious response to "the course of nature."

In a few concluding words I shall try to point out the standing of these rival claims.

Darwin himself adopted both the first and the second of these veiws. His whole philosophy stands opposed in principle to the third view. He did not hesitate at times to adopt the theory of the inheritance of acquired characters, whenever the facts seemed more in accordance with that interpretation than with that of natural selection. He strenuously objected that he had never intended to refer the entire process of evolution to natural selection, and later in life affirmed that he had perhaps laid too little stress on the influence of the environment. To-day the doctrine of inherited effects is in disgrace, largely owing to the brilliant attack of the philosopher of Freiburg. Nevertheless it has warm adherents; and not a few of the most cautious zoologists now living have expressed themselves in its favor. It has not lacked able advocates, but it has sadly lacked direct evidence to support it. I can show you an example of how it fails when put to the test. I have here a waltzing mouse that turns round in circles instead of moving forwards. This is a domesticated variety and breeds true, i. e., all of its offspring are waltzers. I next show you a pair of mice that were injected with acetyl-atoxyl to cure them of sleeping sickness. They have artificially acquired the same habit as a result of the injection, and have waltzed for nearly a year. Here are their offspring that show not a trace of the trick.

Cases like these, and I could cite not a few, show how cautiously we must view the theory that such acquired characters are inherited. The experiments do not disprove the possibility, but until direct evidence is forthcoming, judgment must remain suspended.

It has seemed, therefore, to many modern zoologists that we must face the two alternatives, either natural selection or purposeful response. Natural selection has been likened in recent years to a sieve that lets the non-adapted pass through and conserves the adapted. On the sieve metaphor natural selection produces nothing—it is described as a process of destruction of the unfit. How then can natural selection the destroyer become a factor in a creative process?

I have already tried to indicate how natural selection may assume such a rôle. If definite variations appear, however small or large, that are of some benefit, they may engraft themselves in time on to the species; if other useful definite variations are also adding themselves, if their presence insures some further definite variations in the same directions, advance is certain. In other words the elimination of the unfit has not produced the fit, but it has left the conditions more favorable for further progress in the direction of fitness. This is the interpretation of Darwinism that attracts at present the serious attention oi the most thoughtful and advanced students of evolution.

I hesitate to bring before you in a closing sentence or two the alternative doctrine of purposefulness—a doctrine so fraught with human and superhuman import, for of all theories of creation it undoubtedly makes the strongest emotional appeal to mankind.

We are so conversant with the fact in human affairs that whenever purpose is involved there is an intelligent agent—a mind that designs, a mind that foresees—that our thinking has become tinctured with the idea that wherever there is purpose there is something like mind that has anticipated it. Organic nature is full to the brim of what seems purposeful adaptation—means to ends. Two modern zoologists and a noted philosopher have nailed this banner to their mast-head.

There is one consideration above all others that warns the zoologist against speaking dogmatically about purposefulness, or its absence, in the response of living matter to its environment—his ignorance of the causes of variation. If I have implied that all variation is purely "accidental"; if I have led you to infer that it is entirely fortuitous, I have gone beyond the facts. We must be careful to distinguish between the individual differences that we can safely ascribe to chance, and the small definite variations that arise in the germ. The latter appear to be limited, to be in part determined by the internal nature of the parts affected, and to be constant when they have once appeared, but more than this we dare not affirm. We may believe if we like that the evidence indicates that they are not purposeful, but we can not prove this. If they are not purposeful then the purposefulness of the living world has no direct relation to the origin of useful variations. The origin of an adaptive structure and the purpose it comes to fulfill are only chance combinations. Purposefulness is a very human conception for usefulness. It is usefulness looked at backwards. Hard as it is to imagine, inconceivably hard it may appear to many, that there is no direct relation between the origin of useful variations and the ends they come to serve, yet the modern zoologist takes his stand as a man of science on this ground. He may admit in secret to his father confessor, the metaphysician, that his poor intellect staggers under such a supposition, but he bravely carries forward his work of investigation along the only lines that he has found fruitful.

In the last analysis it is a matter of expediency; or if the word jars, a matter of instinct. Why forsake the gold mine at our feet, because the transmutation of metals is a philosophic possibility?

Whether definite variations are by chance useful, or whether they are purposeful are the contrasting views of modern speculation. The philosophic zoologist of to-day has made his choice. He has chosen undirected variations as furnishing the materials for natural selection. It gives him a working hypothesis that calls in no unknown agencies; it accords with what he observes in nature; it promises the largest rewards. He does not deny, if he is cautious, the possibility that there may be a purposefulness in the sense that organisms may respond adaptively at times to external conditions; for the very basis of his theory rests on the assumption that such variations do occur. But he is inclined to question the assumption that adaptive variations arise because of their adaptiveness. In his experience he finds little evidence for this belief, and he finds much that is opposed to it. He can foresee that to admit it for that all important group of facts, where adjustments arise through the adaptation of individuals to each other—of host to parasite, of hunter to hunted—will land him in a mire of unverifiable speculation. He fears to enter thereby on a field of exploitation of nature that has proved itself so sterile in the past.

We have reached the end of our theme. If I have led you too far into some of the remote corners of zoological thought, I must plead that such thoughts are the legitimate outgrowth of Darwinism.

I have tried to show you the modern zoologist at work on the great theory of evolution. We stand to-day on the foundations laid fifty years ago. Darwin's method is our method, the way he pointed out we follow, not as the advocates of a dogma, not as the disciples of any particular creed, but the avowed adherents of a method of investigation whose inauguration we owe chiefly to Charles Darwin. For it is this spirit of Darwinism, not its formula, that we proclaim as our best heritage.

  1. A lecture on "Darwin's Influence on Zoology," delivered at Columbia University, February 26, 1909.