MATTHEW, CLIMATE AND EVOLUTION 181 



species is an absolute replica of the older environment of the race. In 

 many cases, it must be profoundly modified by its invasion of new regions, 

 and there are many features in the evolution of a race which appear to 

 be only partly, if at all, dependent on environmental change. But to 

 assume that the present habitat of the most generalized members of a 

 group, or the region where it is now most abundant, is the center from 

 which its migrations took place in former times appears to me wholly 

 illogical and, if applied to the higher animals as it has been to fishes 

 and invertebrates, it would lead to results absolutely at variance with 

 the known facts of the geologic record. 



EEVIEW OP THE EVOLUTION OF VERTEBRATE LIFE 



To my mind, this hypothesis of the evolution of land life in adaptation 

 to recurrent periods of aridity supplies a satisfactory background of 

 cause for the whole evolution of the higher vertebrates. 



We may set aside earlier periods of aridity and continental extension 

 signalized by the development of invertebrate land types, whose early 

 terrestrial adaptation is wholly hypothetical, since the kno^vTi portion of 

 their history is so small and so remote from their origin that we cannot 

 project it backwards with any sort of exactness. As Barrell has pointed 

 out, the arid period of the late Devonian coincides with the probable time 

 of the first adaptation of vertebrates to terrestrial life. In the arid period 

 of the Permian, we see the conditions more clearly prevalent which 

 favored a much more extensive development of land life, and this period 

 marks the rise and early differentiation of the Eeptilia. That reptiles 

 first differentiated from amphibia as a dry-land adaptation seems to be 

 obvious; that the period of their rise corresponded with the greatest ex- 

 treme of aridity, continental emergence and glaciation between Cambrian 

 and Quaternary would, I think, be also generally admitted. The domi- 

 nant order of land reptiles up to the close of the Mesozoic was the dino- 

 saurs, preeminently a dry-land adaptation in their inception, since their 

 most marked characteristic lies in their long limbs, bipedal progression 

 and general parallelism in proportions and structure to the large ground- 

 birds of modern times, which are to-day peculiarly inhabitants of arid 

 regions. The relationship and origin of the more specialized, mostly 

 gigantic, dinosaurs of the later Mesozoic can be best explained by regard- 

 ing them as a succession of derivatives from smaller and more lightly 

 constructed upland dinosaurs, mostly unknown to us, the larger and more 

 specialized types being re-adapted to a swamp life and inhabiting the 

 coast marshes whose sediments are still preserved, while the more direct 



