Evolution and Adaptation

by Thomas Hunt Morgan

[3]. This paragraph is a free translation of Fleischmann’s text.

A study of the bony structure of the limb shows that typically it consists of a single proximal bone (the humerus in the upper arm, the femur in the thigh), followed by two bones running parallel to each other (the radius and ulna in the arm and the tibia and fibula in the shank); these are succeeded in the arm by the two series of carpal bones, and in the leg by the two series of tarsal bones, and these are followed in each by five longer bones (the metacarpals and metatarsals), and these again by the series of long bones that lie in the fingers and toes. Despite the manifold variety of forms, Fleischmann admits that both the hind- and the fore-limbs are constructed on the same plan throughout the vertebrates. Even forms like the camel, in which there are fewer terminal bones, may be brought into the same category by supposing a reduction of the bones to have taken place, so that three of the digits have been lost. In the leg of the pig and of the reindeer, even a greater reduction may be supposed to have taken place. Fleischmann points out that these facts were supposed to be in full harmony with the theory of descent.

The analysis of the origin of the foot of the horse gave even better evidence, it was claimed, in favor of the theory. The foot consists of a single series of bones corresponding to the middle finger and toe. When, as sometimes happens, individual horses are found in which in addition to the single middle finger two smaller lateral fingers with small hoofs appear, the followers of the descent theory rejoiced to be able to bring this forward as a confirmation of their doctrine. The occurrence was explained as a sporadic return to an ancestral form. The naïve exposition of the laws of inheritance that were supposed to control such phenomena was accepted without question. And when finally a large number of fossil remains were found by paleontologists,—remains showing a gradual increase in the middle finger, and a decrease in size of the lateral fingers,—it was supposed that the proof was complete; and anatomists even went so far as to hold that the original ancestor of the horse was a five-fingered animal.

This same law of type of structure was found to extend to the entire vertebrate series, and the only plausible explanation appeared to be that adopted by Darwin and his followers, namely, that the resemblance is the result of the blood-relationship of the different forms. But a simple comparison of the skeleton of the limbs if carried out without theoretical prejudice would show, Fleischmann thinks, that there is only a common style, or plan of structure, for the vertebrates. This anatomical result has about the same value as the knowledge of the different styles of historical architecture—that, for instance, all large churches of the Gothic period have certain general principles in common. The believers in the theory of descent have, however, he thinks, gone beyond the facts, and have concluded that the common plan in animals is the consequence of a common descent. “I cannot see the necessity for such a conclusion, and I certainly should unhesitatingly deny that the common plan of the Gothic churches depended on a common architect. The illustration is, however, not perfect, because the influence of the mediæval school of stone-cutters on its wandering apprentices is well known.”

Fleischmann adds that if the descent theory is true we should expect to find that if a common plan of structure is present in one set of organs, as the limbs, it should be present in all other organs as well, but he does not add that this is generally the case.

The weakness of Fleischmann’s argument is so apparent that we need not attempt an elaborate refutation. When he says there is no absolute proof that the common plan of structure must be the result of blood-relationship, he is not bringing a fatal argument against the theory of descent, for no one but an enthusiast sees anything more in the explanation than a very probable theory that appears to account for the facts. To demand an absolute proof for the theory is to ask for more than any reasonable advocate of the descent theory claims for it. As I have tried to show in the preceding pages, the evidence in favor of the theory of descent is not absolutely demonstrative, but the theory is the most satisfactory one that has as yet been advanced to account for the facts. Fleischmann’s reference to the common plan of structure of the Gothic churches is not very fortunate for his purpose, since he admits himself that this may be the result of a common tradition handed down from man to man, a sort of continuity that is not very dissimilar in principle from that implied in the descent theory; in the latter the continuity of substance taking the place of the tradition in the other. Had the plan for each, or even for many of the churches, originated independently in the mind of each architect, then the similarity in style would have to be accounted for by a different sort of principle from that involved in the theory of descent; but as a matter of fact the historical evidence makes it probable that similar types of architecture are largely the result of imitation and tradition. Certain variations may have been added by each architect, but it is just the similarity of type or plan that is generally supposed to be the outcome of a common tradition.

Fleischmann’s attempt in the following chapter to belittle Gegenbaur’s theory of the origin of the five-fingered type of hand from a fin, like that of a fish, need not detain us, since this theory is obviously only a special application which like any other may be wrong, without in the least injuring the general principle of descent. That all phylogenetic questions are hazardous and difficult is only too obvious to any one familiar with the literature of the last thirty years.

Fleischmann devotes a long chapter to the geological evidences in connection with the evolution of the horse, and attempts to throw ridicule on the conclusions of the paleontologists by emphasizing the differences of opinion that have been advanced in regard to the descent of this form. After pointing out that the horse, and its few living relatives, the ass and the zebra, are unique in the mammalian series in possessing a single digit, he shows that by the discovery of the fossil horses the group has been simply enlarged, and now includes horses with one, three, and five toes. The discovery of the fossil forms was interpreted by the advocates of the descent theory as a demonstration of the theory. The series was arranged by paleontologists so that the five-toed form came first, then those with three and one toe, the last represented by the living horses. But the matter was not so simple, Fleischmann points out, as it appeared to be to the earlier writers, for example to Haeckel, Huxley, Leidy, Cope, Marsh. Different authors came to express different opinions in regard to the genealogical connection between the fossil forms. Several writers have tried to show that the present genus, Equus, has not had a single line of descent, but have supposed that the European horses and the original American horses had different lines of ancestry, which may have united only far back in the genus Epihippus. Fleischmann points out that the arrangement of the series is open to the criticism that it is arbitrary, and that we could equally well make up an analogous series beginning with the five-fingered hand of man, then that of the dog with the thumb incompletely developed, then the four-fingered hind-foot of the pig without a big toe and with a weak second and fifth digit, then the foot of the camel with only two toes, and lastly the foot of the horse with only one toe. It sounds strange that Fleischmann should make such a trivial reply as this, and deliberately ignore the all-important evidence with which he is, of course, as is every zoologist, perfectly conversant. Not only are there a hundred other points of agreement in the horse series, but also the geological sequence of the strata, in which some at least of the series have been found, shows that the arrangement is not arbitrary, as he implies.

Fleischmann then proceeds to point out that when the evidence from other parts of the anatomy is taken into account, it becomes evident that all the known fossil remains of horses cannot be arranged in a single line, but that there are at least three families or groups recognizable. Many of these forms are known only from fragments of their skeletons—a few teeth, for instance, in the case of Merohippus, which on this evidence alone has been placed at the uniting point of two series. At present about eight different species of living horses are recognized by zoologists, and paleontological evidence shows only that many other species have been in existence, and that even three- and one-toed forms lived together at the same time.

Fleischmann also enters a protest against the ordinary arrangement of the fossil genera Eo-, Oro-, Meso-, Merohippus in a series, for these names stand not for single species, but for groups containing no less than six species under Protohippus, fourteen under Equus, twelve under Mesohippus, and twenty under Hipparion. Fleischmann concludes: “The descent of the horses has not been made out with the precision of an accurate proof, and it will require a great deal of work before we get an exact and thorough knowledge of the fossil forms. What a striking contrast is found on examination between the actual facts and the crude hopes of the apostles of the descent theory!...”