There can be little doubt, however, of the most perfect adaptive arrangements prevailing through all the geological epochs, some of which have been plausibly conjectured. As reptiles, for instance, differ from birds and mammals, in having a lower and simpler structure of the lungs and heart, and therefore a less active performance of the respiratory functions, they become less dependent on the atmosphere or oxygen for existence. “Hence,” says Professor Owen, “from their extraordinary prevalence in the secondary periods, under varied modifications of size and structure, severally adapting them to the performance of those tasks in the economy of organic nature which are now assigned to the warm-blooded and quick-breathing classes, the physiologist is led to conjecture that the atmosphere had not undergone those changes, which the consolidation and concentration of certain of its elements in subsequent additions to the earth’s crust may have occasioned during the long lapse of ages during which the extinction of so large a proportion of the reptilian class took place. And if the chemist, by wide and extended views of his science in relation to geology, and mineralogy, should demonstrate, as the botanist from considerations of the peculiar features of the extinct flora has been led to suspect, that the atmosphere of this globe formerly contained more carbon and less oxygen than at present, then the anatomist might, _à priori_, have concluded that the highest classes of animals suited to the respiration of such a medium must have been the cold-blooded fishes and reptiles. And beside, the probability of such a condition of the zoological series being connected with the chemical modifications of the air, the terrestrial reptiles, from the inferior energy of their muscular contractions, and still more from the greater irritability of the fibers and power of continuing their actions, would constitute the highest organized species, best adapted to exist under greater atmospheric pressure than operates on the surface of the earth at the present time.”