Page:Quarterly Journal of the Geological Society of London, vol. 34.djvu/510

428 428 prof. r. owen on the modifying influence

Discussion.

Professor Seeley gathered that the chief object of Prof. Owen's paper was to show that the modern Crocodiles have been evolved from those of the Secondary rocks, and in consequence of the altered conditions of their struggle for existence which resulted from the development of mammalian life in Tertiary as compared with earlier ages. While admitting the evolution, he thought it had no necessary dependence upon the incoming of mammalian prey. Thus the removal backward of the palato-nares in the modern Crocodiles was a condition that would be gradually and inevitably developed, even if mammals had not existed. In the living Crocodile these nares are surrounded by the pterygoid bones, and to the dorsal surface of those bones enormous muscles are attached, which form great hemispherical masses on each side of the head, and are attached to the inner side of the lower jaw. But in the Secondary Crocodiles this pterygoid region of the palate makes a nearer approach to the condition of the bones and palato-nares in certain lizards. And since in the lizards the internal pterygoid muscle has no such development, owing to the larger size of the temporal muscle, he considered that the gradual increase in size of the pterygoid muscles, consequent on the lateral action of the jaws in tearing food, was the cause of the osseous differences of the palate in this region in the two types, because the tension of the muscles acting on the pterygoid bones at their suture with the palatines would cause those bones to elongate, and as the muscles grew more powerful the palato-nares would thus be carried further and further backward with the bones which embrace them ; and similarly, by lateral tension, the pterygoid bones would grow in the line of the median suture and so widen, while they reduced the size of the aperture of the palato-nares. This physiological explanation seemed to be independent of the condition of prey being mammalian.

Dr. Meryon was sorry that the author had not gone to the full length in regard to evolution, and, referring to the backward posi- tion of the palato-nares, said that this was a provision adapted to their holding their prey under water to drown it. He also referred at some length to the influence of the nervous system in inducing changes.

Mr. Htjlke observed that with respect to Prof. Owen's idea that « warm-blooded animals were not preyed on by the Mesosuchian Crocodiles, it could not be doubted that such did actually exist contemporaneously with them, and that they might become an easy prey when fording rivers or lakes, as at the present time. He thought that the inference drawn from the strong scutal armour of the Mesosuchia, that these lived in presence of stronger and larger animals, and so stronger armour was necessary for their defence, was not justified by the facts of to-day. The Jacquari of Tropical America, as an example, is similarly armoured, yet has no larger and stronger adversaries. In referring to the suggestion that the backward posi- tion of the palato-nares was an adaptation to exclude the entrance of