656 THE AMERICAN NATURALIST. (VoL. XXXII. 
revolution, rather than the upheaval of the plateau itself, so 
favorably affected plant and animal life that at the dawn of the 
Mesozoic a great acceleration in the process of type-building 
was witnessed. Moreover, it seems evident that the variation 
which took place at this epoch was by no means fortuitous, but 
determined along definite lines caused by the definite expansion 
of the continents, and their resultant topography. 
We have seen that as a result of the folding and upheaval 
of the Appalachians there may have been at the beginning of 
Triassic time, in addition to the tropical lowlands, a somewhat 
cooler upland zone, and possibly even snow-clad mountain 
peaks, with glaciers descending their sides, as we may now 
witness in New Zealand. 
Already on Permian soil reptiles were not infrequent. They 
were generalized composite forms comprising the Proganosauria, 
the forerunners of the Hatteria of New Zealand, and the Therio- 
dontia, from which the mammals are now supposed to have 
been derived. They disappeared at the end of the Triassic, 
together with the labyrinthodonts, from which the reptiles are 
thought to have originated. These reptiles having scaly bodies 
and claws, their habits must have been like those of the lizards 
of to-day, and they were adapted for hotter and drier, perhaps 
more elevated, areas than the stegocephalous amphibians ; and 
these conditions were fulfilled in Triassic and Jurassic times, 
when the reptilian orders multiplied, all the orders of the class 
having been differentiated, or at least were in existence, in the 
Mesozoic era. 
The geographical features throughout the Mesozoic were 
these : more or less dry and broad plains, vast fresh-water 
lakes, uplands clad with coniferous forests afterwards to be 
replaced by forests of deciduous trees; flower-strewn plains 
overgrown with waving grasses, and jungles with rank growths 
of bamboo. We can, without going into detail, well imagine 
that the geographical features of the Mesozoic continents were 
such as to provoke the appearance of the higher classes of 
vertebrates. As the land rose higher and the low swampy 
coastal areas became more limited, this would tend to restrict 
the habitat of the stegocephalous amphibians ; with a slightly 
