On the transmutation theory we should expect to find not only a sequence of forms, beginning with the simplest and culminating with the more complex, but also, in the beginning of each new group, forms more or less intermediate in structure. It is claimed by all paleontologists that such forms are really found. For example, transitional forms between the fishes and the amphibia are found in the group of dipnoans, or lung-fishes, a few of which have survived to the present day. There are many fossil forms that have characters between those of amphibians and reptiles, which if not the immediate ancestors of the reptiles, yet show that at the time when this group is supposed to have arisen intermediate forms were in existence. The famous archæopteryx remains have been already referred to above, and it appears in this case that we have not only an intermediate form, but possibly a transitional one. In the group of mammals we find that the first forms to appear were the marsupials, which are undoubtedly primitive members of the group.
The most convincing evidence of transmutation is found in certain series of forms that appear quite complete. The evolution of the horse series is the most often cited. As this case will be discussed a little later, we need not go into it fully here. It will suffice to point out that a continuous series of forms has been found, that connect the living horses having a single toe through three-toed, with the five-toed horses. Moreover, and this is important, this series shows a transformation not only in one set of structures, but in all other structures. The fossil horses with three toes are found in the higher geological layers, and those with more toes in the deeper layers progressively. In some cases, at least, the fossils have been found in the same part of the world, so that there is less risk of arranging them arbitrarily in a series to fit in with the theory.
EVIDENCE FROM DIRECT OBSERVATION AND EXPERIMENT
Within the period of human history we do not know of a single instance of the transformation of one species into another one, if we apply the most rigid and extreme tests used to distinguish wild species from each other.[[2]] It may be claimed that the theory of descent is lacking, therefore, in the most essential feature that it needs to place the theory on a scientific basis. This must be admitted. On the other hand, the absence of direct observation is not fatal to the hypothesis, for several reasons. In the first place, it is only within the last few hundred years that an accurate record of wild animals and plants has been kept, so that we do not know except for this period whether any new species have appeared. Again, the chance of observing the change might not be very great, especially if the change were sudden. We would simply find a new species, and could not state where it had come from. If, on the other hand, the change were very slow, it might extend over so many years that the period would be beyond the life of an individual man. In only a few cases has it been possible to compare ancient pictures of animals and plants with their prototypes living at the present time, and it has turned out in all cases that they are the same. But these have been almost entirely domesticated forms, where, even if a change had been found, it might have been ascribed to other factors. In other cases, as in the mummified remains of a few Egyptian wild animals (which have also been found to be exactly like the same animals living at the present day), it was pointed out by Geoffroy Saint-Hilaire that, since the conditions of the Egyptian climate are the same to-day as they were two thousand years ago, there is no reason to expect any change would have taken place. But waiving this assumption, we should not forget that the theory of evolution does not postulate that a change must take place in the course of time, but only that it may take place sometimes.
[2]. The transformation of “smaller species,” described by De Vries, will be described in a later chapter.
The position that we have here taken in regard to the lack of evidence as to the transformation of species is, perhaps, extreme, for, as will be shown in some detail in later chapters, there is abundant evidence proving that species have been seen to change greatly when the conditions surrounding them have been changed; but never, as has been stated, so far, or rather in such a way, that an actual new species that is infertile with the original form has been produced. Whether, after all, these changes due to a change in the environment are of the kind that makes new species, is also a question to be discussed later.
The experimental evidence, in favor of the transformation of species, relates almost entirely to domesticated forms, and in this case the conscious agency of man seems, in some cases, to have played an important part; but here, even with the aid of the factor of isolation, it cannot be claimed that a single new species has been produced, although great changes in form have been effected. It is clear, therefore, that we must, at present, rely on other data, less satisfactory in all respects, to establish the probability of the theory of transformation.
MODERN CRITICISM OF THE THEORY OF EVOLUTION
Throughout the whole of the nineteenth century a steady fire of criticism was directed against the theory of evolution; the names of Cuvier and of Louis Agassiz stand out preëminent in this connection, yet the theory has claimed an ever increasing number of adherents, until at the present time it is rare to find a biologist who does not accept in one form or another the general principle involved in the theory. The storm of criticism aroused by the publication of Darwin’s “Origin of Species,” was directed more against the doctrine of evolution than against Darwin’s argument for natural selection. The ground has been gone over so often that there would be little interest in going over it again. It will be more profitable to turn our attention to the latest attack on the theory from the ranks of the zoologists themselves.
Fleischmann, in his recent book, “Die Descendenztheorie,” has made a new assault on the theory of evolution from the three standpoints of paleontology, comparative anatomy, and embryology. His general method is to try to show that the recognized leaders in these different branches of biology have been led to express essentially different views on the same questions, or rather have compromised the doctrine by the examples they have given to illustrate it. Fleischmann is fond of bringing together the antiquated and generally exaggerated views of writers like Haeckel, and contrasting them with more recent views on the same subject, without making sufficient allowances for the advances in knowledge that have taken place. He selects from each field a few specific examples, by means of which he illustrates the weakness, and even, as he believes, the falsity of the deductions drawn for the particular case. For example, the plan of structure of the vertebrates is dealt with in the following way: In this group the limbs, consisting typically of a pair of fore-legs and a pair of hind-legs, appear under the form of cylindrical outgrowths of the body. In the salamander, in the turtle, in the dog, the cylindrical legs, supporting the body and serving to support it above the ground, are used also for progression. The general purpose to which the limbs are put as organs of locomotion has not interfered with an astonishing number of varieties of structure, adapted to different conditions of existence, such as the short legs used for creeping in salamanders, lizards, turtles, crocodiles; the long and thin legs of good runners, as the hoofed animals; the mobile legs of the apes used for climbing; and the parachute legs of some squirrels used for soaring. Even more striking is the great variety of hands and feet, as seen in the flat, hairy foot of the bear; the fore-foot of the armadillos, carrying long, sickle-shaped claws; the digging foot of the mole; the plump foot of the elephant, ending in a broad, flat pad with nails around the border, and without division into fingers; the hand of man and of the apes ending with fine and delicate fingers for grasping. To have discovered a general plan of structure running through such a great variety of forms was proclaimed a triumph of anatomical study.[[3]]