range of transformation. The oldest known forms occur in the Upper Trias; they attain their most stupendous development in the Upper Jurassic and in the Wealden; and they have died out with the Cretaceous epoch. But already some of their earliest forms had assumed bipedal gait, and the Oolitic Compsognathus had developed almost bird-like hindlimbs.
On the other hand, there are many instances of extremely slow development—facts which raise the difficult question of 'persistent types.' Are these due to a state of perfection which cannot be improved upon? Or are they due to a kind of morphological consolidation (not necessarily specialization) which can no longer yield easily, so that therefore through changes in their surroundings they may come to an end sooner than more plastic groups?
Struthio, the ostrich; Orycteropus, the Cape ant-eater; Tapirus, and many others, existed in the Miocene age practically as
