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.