Agassiz refused to be convinced, yet he, like Cuvier and Linnæus in earlier days, helped collect the evidence which compelled the acceptance of an idea of evolution. Huxley and Haeckel became the great champions of the newer interpretation against its philosophical antagonists. Herbert Spencer, already an evolutionist, joyfully welcomed the new evidence, and in his Synthetic Philosophy was the first to make universal application of the new principle. In a few years the battle was over so far as the scientific world was concerned, and for half a century every man of any repute as a scientist has been an evolutionist. This contest had its amusing as well as tragic features and among the former may be listed the refusal of Whewell, author of a great History of the Inductive Sciences, to allow a copy of The Origin of Species to be put in the library of Trinity College, Cambridge. DARWINISM In scientific circles Darwinism refers to the theories of Darwin, not to the question of evolution. There is much confusion on this point in the general public. As Kellogg says: To too many readers Darwinism is synonymous with organic evolution or the theory of descent. The word is not to be so used or considered. Darwinism, primarily, is a most ingenious, most plausible, and, according to one's belief, most effective or most inadequate, causomechanical explanation of adaptation and species-transforming. The fact is that the name Darwinism has been pretty consistently applied by biologists only to those theories practically original with Darwin which offer a mechanical explanation of the accepted fact of descent. Of these Darwinism theories the primary and all important one is that of natural selection. Included with this in Darwinism are the now nearly wholly discredited theories of sexual selection and of the pangenesis of gemmules. The fair truth is that the Darwinian selection theories, considered with regard to their claimed capacity to be an independently sufficient mechanical explanation of descent, stand today seriously discredited in the ... biological world. On the other hand it is also fair truth to say that no replacing hypothesis or theory of species-forming has been offered by the opponents of selection which has met with any general or even considerable acceptance by naturalists.10 Pangenesis In judging the theories of Darwin it must be remembered that he started with the common notions of his day from which he could not escape and that his primary interest lay in the question of survival. Variation he saw all about him in the world. He believed that the development, or degeneration, of parts of the body were in some way influential in determining the characters of the next generation. He made a guess that from the affected part some messenger as it were, a bit of matter which he called a "gemmule," was sent to the reproductive organs to be incorporated in the germ cells from which the offspring were to come. When the study of cell life developed it was quickly learned that the germ cells are not formed in this fashion and the idea of pangenesis was discarded at once. Sexual Selection Darwin was familiar with the contests of the males over the possession of the females and the many intricate courtships of the animal world. He hazarded a guess that here was a sort of sexual selection, the females either belonging to the successful males or picking out those of greatest beauty or possessed of greatest dancing ability. This is an afterthought on Darwin's part and is in reality an interpretation of animal conduct in human terms. For all that we know the females cannot appreciate the beauty of the males or may consider their performances as silly and in bad taste. We know so little of the reasons for the choice of mates among humans that we must hesitate to explain animal conduct. Hence sexual selection has not won general acceptance. Natural Selection The theory of natural selection has been far more successful. It has been noted that this did not originate with Darwin solely. Darwin was very familiar with the rapid variation of domestic plants and animals. Here he saw that man by process of artificial selection determined what types should be preserved and what types prevented from reproducing. This suggested that in the natural world a similar process might take place, in as much as the high birth rate always assured an abundance of forms from which selection might be made. Darwin saw that any given type of life is neatly adjusted to the conditions under which it lives. Any variation therefore is likely to spell disaster. But conditions may change or there may be a shift to a new environment. Then the variant type may have an advantage and will survive while the old type dies out. Here we have what is called natural selection. This is a brilliant generalization supported by a multitude of cases for which, as Kellogg says, no other explanation has won acceptance. It does not account for variation but it does seek to explain survival. A few cases will serve as illustrations. Protective Coloration With silk threads Cesnola tethered forty-five green praying mantises to green herbage, and sixty-five of the brown variety to withered plants. He watched them for seventeen days and all survived unnoticed by birds. But when he put twenty-five green ones among brown herbage all were killed by birds in eleven days, while of forty-five brown ones on green grass, only ten survived at the end of seventeen days. Here we have definite proof of a selective death-rate, definite proof of the selective value of the protective coloration.11 Poulton and Saunders fastened 600 pupe of the tortoise shell butterfly (Vanessa urtica) to nettles, tree-trunks, fences, walls, and so on. At Oxford there was a mortality of 93 per cent, pointing to an extremely high elimination-rate, and the only pupæ that survived were on nettles, where they were least conspicuous. At St. Helena, in the Isle of Wight the elimination was 92 per cent on fences where the pupæ were conspicuous, as against 57 per cent among nettles where they were inconspicuous.12 .. Dr. C. B. Davenport placed 300 chickens in an open field. Eighty per cent were white or black and hence conspicuous; twenty per cent were spotted and hence inconspicuous. In a short time twenty-four were killed by crows but only one of the killed was spotted.13 There are other cases where the results are not in accord with those just given. ... ... The large tomato worm occurs in two colors, being generally green, almost exactly matching the tomato leaves and stems on which it lives, and more rarely brown and very conspicuous. These caterpillars may be observed to be not infrequently eaten by robins and cuckoos and pecked to death and partially eaten by chipping sparrows. Last summer 34 of these caterpillars were counted on a row of tomato plants. Of these 32 were of the green and only 2 of the brown phase. Later in the summer the number of these caterpillars was observed to be gradually diminishing until in early September, when they had attained their full growth, the green ones had been reduced to 18, while both of the brown ones remained. Furthermore, it became apparent that no less than 16 of the green caterpillars were parasitized by an ichneumon fly to whose attacks they eventually succumbed, with the net result that in spite of being protectively colored, out of 32 green caterpillars at the beginning of the season only two a number just equalling that of the unprotected brown caterpillars-survived to pupate with the latter.14 Obviously protective coloration is not the only factor determining survival and in the case just cited may have had nothing to do with survival. Mimicry, the close resemblance of one variety to another, the moth which resembles a dry leaf when at rest, has been stressed by some students. Many physical characters seem to have no significance. No one suggests that the passenger pigeon was killed off because it had twelve feathers in its tail, while the very similar mourning dove survived because it had fourteen. On the other hand, the habit of the former in flying and nesting in enormous flocks as compared to the relatively solitary life of the latter may have been a factor. But can we account for this social life on physical grounds? There seems to be no selective elimination when the tornado kills all in its path save such as chance to be protected, nor in the killing by lightning of any person who chances to be in a particular place, nor in the destruction caused by the drying up of a stream. But, if perchance, some individuals survive the drought because of a greater resisting power there may be real selection. A little difference in the thickness of the skin might save cattle in a tick-infested region. While it is possible to exaggerate some feature which catches our eye, or to be mistaken, there seems to be such a thing as natural selection, and the explanation bids fair to last at least until some genius like Darwin comes along with a lot of new evidence which forces revision. EVIDENCES FOR EVOLUTION Regardless, then, of what happens to the Darwinian explanations, for this is unimportant, the naturalists believe in evolution more firmly than ever and much more evidence is at hand than Darwin could offer eighty years ago. This evidence comes from many fields. Paleontology The study of ancient life. With the possible exception of one Lingula (brachiopod) all living animals are different from the oldest fossils. Yet our minds can explain present forms only in terms of descent from the older. The entire history is not clear and may never be, and cannot be read in any one section of earth, but each decade increases our in |