I. Croonian Lecture : On the origin of mammals

Abstract

Since in the last 50 years the belief in the descent of all higher forms of life from earlier types has become universal, the desire to discover the origins of things has grown strong and there is probably no group in whose origin we are more interested than that to which we ourselves belong. The problem is fascinating not only because of our personal interest, but perhaps nearly as much on account of its difficulty. The living mammals differ so very markedly from all other living vertebrates that there is no group to which they seem nearly related, and none which is manifestly ancestral to them. With many marvellous specialisations and many peculiar degenerations, they retain a host of primitive characters. In some features they resemble the lizards; in others more strikingly do they agree with Sphenodon . In the possession of a secondary palate and in some other characters they seem to have some Crocodilian affinity; in developing with a primitive streak and in one or two other points they agree with the birds. In the simple character of the bones of the skull, in the relation of certain blood-vessels, and in certain points in the ontogeny the affinities seem nearer to the amphibians than to the living reptiles; and a few characters even seem to suggest an origin directly from some group of fishes. For the solution of the problem we seem to be restricted to three lines of research—Comparative Anatomy, Embryology and Palæontology—and unfortunately all three are subject to serious limitations.