GREAT STEPS IN ORGANIC EVOLUTION 831 



Sandstone Period in the history of the earth, perhaps 300,000,000 

 years ago, one of the greatest steps in evolution was taken, for 

 certain pioneer amphibians clambered out of the pools and began 

 the colonisation of the dry land. Of the earliest of these we can say 

 little, for only their footprints are left; but their successors in the 

 next great period, the Carboniferous, can be reconstructed from their 

 fossil remains, and it is safe to say that they were successful, promise- 

 ful, and often large in size. Amphibians as big as donkeys frequented 

 the dense and damp Carboniferous forests, whose remains we caU 

 Coal Measures. To an extinct order of pioneer Amphibians, known 

 as Stegocephali, the great class of Reptiles must be traced; and this 

 momentous emergence probably occurred in Carboniferous times. 

 In any case, before the close of the next geological period — the 

 Permian — many of the orders of Reptiles were already established. 

 Yet it is to the subsequent Mesozoic Era (Triassic, Jurassic, and 

 Cretaceous) that the term "Age of Reptiles" is usually and deservedly 

 applied, and among the terrestiial Reptiles the Dinosaurs were the 

 most dominant. It should be noted that just as the mammals of 

 to-day include aquatic types like whales, aerial types like bats, 

 amphibious types like otters, and so forth, so in Mesozoic times 

 there was a similar diversity among Reptiles. The struggle for 

 existence was keen and the will to live was strong, so there were 

 many tentatives as regards haunts and habits. Besides the terrestrial 

 types that crawled, or walked, or ran, there were aquatic, aerial, 

 and amphibious forms. In addition, there were probably arboreal 

 climbers and subterranean burrowers, but animals of the trees and 

 of the underworld are not readily preserved as fossils. 



With a pedigree dating back to the Carboniferous, and gathering 

 strength to the Permian, the great order of Dinosaurs began in the 

 Middle Triassic, perhaps in Europe, where their earliest remains 

 are found. They spread as conquerors all over the world, dominating 

 the earth for millions of years, and then waned away at the close 

 of Cretaceous time. Can we form any picture of the kmd of world 

 they lived in ? 



Contrary to an earlier view, the general opinion of modern geolo- 

 gists is that the position of continental and oceanic areas has been 

 much the same throughout geological time. But there is little doubt 

 that the continents were once much larger and the seas much 

 smaller than they are to-day; and there is no doubt that there have 

 been from time to time vast bucklings of the earth's surface, great 

 elevations here and depressions there, besides drastic changes of 

 climate with far-reaching influences on plants and animals. Towards 

 the end of the Carboniferous time there were enormous uplifts, such 

 as that now represented on a reduced scale by the Appalachian 

 Mountains in North America; and associated with this elevation of 

 vast areas there came aridity. A widespread drought was probably 



