28 VERTEBRATE ZOOLOGY 



of subsidence and elevation there would be no boundaries between 

 consecutive geologic strata. 



There have also been rhythms of alternating aridity and humidity 

 which have been associated with marked evolutionary changes in land 

 life. The principal periods of mountain uplift are shown in Fig. 7. 

 One of the most significant of such periods was the great Permo- 

 Triassic arid period which saw the earliest true land animals, the 

 reptiles, make their first adaptive radiation and also gave rise to the 

 first mammalian and perhaps avian stocks. 



The elevations of mountain ranges have from time to time vastly 

 altered the land habitats, especially those of the central continental 

 plains, which were rendered arid by being cut off from the moisture 

 of the sea. Finally, periodic eras of cold (glacial epochs) have alter- 

 nated with tropical or semi-tropical eras, and these have naturally 

 exercised a profound effect upon the character of the faunas and 

 floras. In general the coal measures serve as a marker for the trop- 

 ical periods, among which the Upper Carboniferous and Upper Cre- 

 taceous were the most striking. After the warm, moist climate of the 

 Upper Carboniferous there followed rather suddenly the Permian 

 glacial period accompanied by continental elevation and increased 

 aridity. There was no such sudden change at the end of the Creta- 

 ceous; but during that period there were extensive continental ele- 

 vation, considerable increase in aridity, and a period of cold, not 

 severe enough however to be termed glacial. 



A consideration of palseogeography and a knowledge of the cli- 

 matic changes during the vast period of vertebrate evolution is ab- 

 solutely essential for an intelligent understanding of the organic 

 evolutionary processes. 



It seems quite clear that the great geologic changes have been 

 accompanied by equally marked changes in the floras and faunas of 

 the earth. In general the changes in the organic world have been 

 of such a kind as to bring about adjustments of the animals and 

 plants to the changed conditions. What the exact mechanics em- 

 ployed by the organism in making these responses has been, has never 

 been definitely determined. The mechanics of adaptation remains 

 one of our unsolved problems. One theory, that of Lamarck, is that 

 the organism responds directly to the changed environment and that 

 the germ-plasm reflects the somatic response, so that the change be- 

 comes a permanent racial asset. Another theory is that of paral' 



