When there is talk of evolution among laymen it is because of an interest, first, in the evolution of man — our view of Nature is strongly anthropocentric; second, in the evolution of animals — we all recognize something of their kinship with us; and, third, in the evolution of plants they are farthest away from us. By layman I mean anyone not a working biologist.
The working biologists use evolution all the time as a guide in their work, a determinant of their point of view and method of study, a proved and accepted fundamental fact and principle in the science of living things. Hence the biologist in his relation to evolution is as likely to be interested in the evolution of plants, or in a single group of plants, as in the evolution of animals, or even in the evolution of man — though even biologists are human, and it seems an attribute of humanness to have a prime interest in human beings. But now and then one comes across a naturalist who does seem to be more interested in extra-human Nature than in human nature. Just after the great California earthquake of 1906, which gave scientific men an unusual opportunity to study earthquake ways, the sister of a Western scholar confided to me, somewhat bitterly, of her brother: 'He is no longer a man; he is just a geologist.'
But the layman is always a man, and he sees most importantly in evolution something significant in the problem of the whence, how, and whither of man, and something that must be attended to in developing his world philosophy. Also, and of the same kind and degree of importance, something that has a specific delicate relation to religion, or, at any rate, to theology.
To the mind of the public, evolution has recently assumed again a position of seizing interest. The last time it had such an interest was in the immediate years after the publication of Darwin's Origin of Species. The psychological results of the tremendous cataclysm produced by the war have made men take stock of the status of religion among them. Is religion stronger or weaker because of the war? Or did the realized possibility of such a war, such an unchristian behavior on the part of supposedly Christian peoples, reveal an antecedent and continuing feebleness of religion that must be thoughtfully considered and immediately attended to? Whatever the reason, there has been, since the war, a quickening of attention among us in Europe and America to the status of our religion. And in course of this attention evolution has come again to the position of whipping boy for those who take their religion too emotionally and thoughtlessly, hence violently. If religion is weakened it must be the fault of something and somebody. Putting aside swiftly the uncomfortable thought of the possibility of anything being wrong with religion itself and with its expounders, the fundamentalists see in those black beetles, evolution and Darwin, the disturbing and criminal something and somebody who are guilty of this weakening. Have at them! And Mr. Bryan does.
So here is evolution, especially the evolution of man, again on the defensive. Are we, to go through, all over again, the recital of the classic evidences of the actuality and manner of evolution as read from the four great documents of comparative anatomy, embryology, paleontology, and geographical distribution? And then add to these the confirmatory new evidences that the years since Darwin have brought forth? Must we recall again the fundamental identities, with their obviously adaptive modifications, of the organs and organ-systems of man with those of the other vertebrate and especially mammalian animals the convincing recapitulation in man's embryology of successive conditions of heart and blood-cells and lungs and brain and other organs, which conditions are more or less like the adult conditions of these parts in the successively higher vertebrate classes, the fishes, amphibians, reptiles and birds, and finally, mammals; the occurrence of fossils of prehistoric man from the days of early glacial time, a half-million years ago, up to the near-present, showing the gradual changes in skeletal structure, with their unmistakable implications, the straightening legs and reducing jaw and orbital and occipital crests, and the expanding brain cavity, in a word, the changes from beastliness to humanness; and, finally, the distribution of the living races of man in such a way as to tell the same story as the distribution of the animals and plants? Must all this overwhelming testimony that man is an evolutionary product be rehearsed again because Mr. Bryan says that it does n't exist; or that, if it exists, it need not be taken into account by the truly informed, who have in the book of Genesis a complete manual of world and human origin?
If so, biologists are willing to tell it all over again. But it does seem absurd to have to do it. Especially when there are some other matters concerning evolution that have not been so often retold, and rather need telling and discussion. Sixty years of active study since Darwin, of evolutionary phenomena and of technical discussion among specialists, do not leave evolution just where it was when Darwin and his coadjutors had to drop it. For example, Darwin saw in natural selection a satisfying explanation of the origin of species. We do not see this now. We see in natural selection an important factor in the control of evolutionary lines of plant- and animal-development, and a restraining sieve for the too unfit species, but not a sufficient unaided cause of species-transmutation and -adaptation. There is no mere 'survival of the fittest'; there is a survival of all not too unfit.
But this does not mean returning whole-heartedly to an acceptance of Lamarck's proffered explanation of species-transmutation as caused by adaptive individual modifications and the inheritance and cumulation of these 'acquired characters.' Nor does it mean accepting exclusively the mutations explanation of species origin, despite the general agreement that mutations (rather large, immediately heritable variations) do occur and do make some new, plant and animal forms. Nor, finally, does it mean seeing in, the. Mendelian juggling and recombining of unit characters in the ease of hybridizations a sufficient explanation of new species and adaptive specialization.
What it does mean is that, despite the much additional that has been learned confirmatory of the actuality of evolution, and the new wealth of knowledge that has been gained about the manner and mechanism of some of the principal basic factors of evolution, notably heredity and variation, biologists to-day are less agreed among themselves, or, better put, are more agnostic concerning the causal explanation of evolution now than they were just after Darwin and Huxley had made evolution a household word and natural selection its widely accepted explanation. Of course, natural selection, or Darwinism, never was a unanimously accepted evolution explanation. There were always Lamarckians; but after Weismann, the post-Darwinian champion of Darwinism who out-Darwined Darwin in his insistence on the All-macht of natural selection, had made his fight on the inheritance of acquired characters, Lamarckism went largely into eclipse. Yet there have always been Lamarckians since Lamarck, and are to-day, although there are but few who adhere to Lamarck's own naive form of Lamarckism, with its assumption of the direct inheritance, in photographic replica,' of bodily modifications acquired during the, lifetime of the individual.
The Lamarckians of to-day are Neo-Lamarckians, with various forms of explanation of how parental modifications may cumulate in successive generations by heredity. And new examples of such claimed inherited modifications are every now and then put forward. Among the more recent and important of these claims are those of Kammerer of Vienna; whose accounts of his experiments in inducing changes by environmental influence in the mode of reproduction of various salamanders and in the color of various amphibians and reptiles, with a claimed definite hereditary transmission of these changes in later untreated generations, excited much attention at the last meeting of the British Association for the Advancement of Science. Also those of the Americans, Guyer and Smith, who have reported the positive inheritance of certain eye-defects induced in rabbits by a toxic serum, and whose unusually carefully conducted experiments and elimination of alternative explanations give their claims a very serious importance. And, finally, those of Pavlov the great Russian physiologist, whose white mice, trained to come to their food by the ringing of a bell, produced young who learned their lesson much more quickly, and in turn produced young still more quickly responsive to the signal. However many carelessly claimed instances of modification of species-character by an inheritance of acquired characters can be proved to be uncertain, and thus to be useless as evidence for the Lamarckian explanation of evolution, any single one that cannot be, otherwise explained will have the gravest consequence in the search for the actual causes of evolution.
As a matter of fact, despite the inability of the Lamarckians, or of biologists in general, to offer any indubitable cases of Lamarckian inheritance (unless the most recently advanced cases are of this character), and despite. the heavy weight thrown against the Larmarckian explanation of evolution by almost all that has been learned in the recent years about the physical basis of heredity — in spite of all this, many reputable and thoughtful biologists remain convinced in their own minds that any satisfactory causal explanation of evolution, especially adaptive evolution, must contain as an important fundamental element some form of the Lamarckian assumption. There must be more than just chance variation in successive generations out of which adaptive modification and specialization are to arrive. Almost all the paleontologists believe, on the basis of their knowledge of animal and plant series extending through long periods of time, in some form of orthogenesis, or determinate variation. There must be something, they believe, that drives evolution on in more or less fixed lines, even though these lines lead, as they have led in the case of the crinoids, the ammonites, the great dinosaurs, and various other highly specialized lines, to over-specialization and extinction. Now unless the paleontologists accept some mystic inherent driving factor, such as the Èlan vital, or other, to explain this phenomenon, they must find in environmental influence and the impression on heredity of the changes caused by it, the explanation of the initiation, if not the maintenance, of this evolutionary movement.
Another fact of much significance in connection with adaptive evolution — a fact which I first stressed sixteen years ago in my Darwinism To-day — and which points strongly, to my mind, to some as yet unexplained means of introducing the acquirements of the individual into the heredity of the species, is that many of the inherited species-adaptations of plants and animals are in quite the same direction as the adaptive changes acquired in their lifetime by individuals of closely related species, which may well be the original species from which the newer more adapted ones have branched off. For example, in streams into which hot springs flow individuals of species occurring in the streams may, in the search for place and food, find themselves in warmer and warmer water, and may adapt themselves physiologically, sometimes with slight but recognizable structural changes, to high temperature conditions.