support belief that the changes between the types of life have been wrought in that way. The history of fossil animals has not shown transitions of this kind from the lower to higher Vertebrata, but only intermediate, parallel groups of animals, analogous to those which survive, and distinct from them in the same way as surviving groups are distinct from each other. The circumstance that Mammals, Birds, and Reptiles are all known low down in the Secondary epoch of geological time, is favourable to the idea of their history being parallel rather than successive. Such a conception is supported by the theory of elimination of characters from groups of animals as the basis of their differentiation. This loss appears always to be accompanied by a corresponding gain of characters, which is more remarkable in the soft, vital organs than in the skeleton. The gain in higher Vertebrates in the bones is chiefly in the perfection of joints at their extremities; but the gain in brain, lungs, heart, and other soft parts is an elaboration of those structures and an increase in amount of tissue. The resemblances of Ornithosaurs to Mammals are the least conspicuous of their characters. Those seen in the upper arm bone and thigh bone are manifestly not derived from Mammals. They cannot be explained as adaptations of the bones to conditions of existence, because there is no community of habit to be inferred between Pterodactyles and Mammals, in which the bones are in any way comparable. Other fossil animals show that a fundamentally Reptilian structure is capable of developing in the Mammalian direction in the skull, backbone, shoulder-girdle, hip-girdle, and limbs, so as to be uniformly Mammalian in its tendencies. This is proved by tracing the North American Texas fossils named Labyrinthodonts, through the South African Theriodonts, towards the Monotremata and other Mammalia. Just as those animals have obliterated all traces of the Bird from their skeletons, Birds have obliterated the distinctive characters of Mammals. The Ornithosaur has partially obliterated both. With a skull and backbone marked by typical characters of the Reptile, it combines the shoulder-girdle and hip-girdle of a Bird, with characters in the limbs which suggest both those types in combination with Mammals.