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SCIENCE. 



[N. S. Vol. VIII. No. 192; 



gradual modificatioa of forms resulting ia 

 new types, the gradual process of extinction 

 of useless and senile forms, and the modifi- 

 cation and renewal of those which became 

 adapted to the new geographical conditions. 



It should be borne in mind that this ex- 

 tension of the low coasts of the continents 

 began in Ordovician times, but the remark- 

 able expansion of our continent after the 

 Appalachian 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 wit- 

 nessed. Moreover, it seems evident that 

 the variation which took place at this 

 epoch was by no means fortuitous, but de- 

 termined 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 N"ew Zealand. 



Already on Permian soil reptiles were 

 not infrequent. They were generalized 

 composite forms comprising the Progano- 

 sauria, the forerunners of the Hatteria of 

 New Zealand, and the Theriodontia, 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 dryer, perhaps more elevated, 

 areas than the stegocephalous amphibians ; 

 and these conditions were fulfilled in Trias- 

 sic and Jurassic times, when the reptilian 

 orders multiplied, all the orders of the class 



having been differentiated in the Mesozoic 

 era, if not before. 



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 wav- 

 ing 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 more elevated 

 and dryer coast the incoming and expan- 

 sion of reptilian life were fostered ; with 

 still higher plains and hills, besides the in- 

 creasing abundance of flowers and other 

 seed-bearing plants and of the insects which 

 visit them, existence for birds became pos- 

 sible, and with them that of a few scattered 

 mammals of small size and generalized 

 structure, with similar insectivorous habits. 



During the age of reptiles, when they 

 swarmed in every jungle, throughout the 

 forests and over the plains, competition 

 rose so high that some of them were forced 

 to take flight, and bat-like, provided with 

 membranous wings, the pterodactyls lived 

 in a medium before untried by any verte- 

 brate, and finally there appeared in the 

 Ornithostoma of the Cretaceous a colossal 

 flying reptile, its wings spreading twice as 

 much as any known bird, with a head four 

 feet in length ; its long toothless jaws clos- 

 ing on swarms of insects or perhaps small 

 fry of its own type. But the experiment 

 in point of numbers or capacity for ex- 

 tended flight did not succeed. Another 

 type assayed the problem with better suc- 

 cess. There appeared feathered and event- 

 ually toothless vertebrates with the fore 



