PALEONTOLOGY 397 



no doubt were often much longer than the time it took to form the 

 strata. 



Intense cold or heat, resulting from a climatic change, undoubtedly 

 killed many organisms which were unable to adapt themselves to the 

 changing conditions of the past, while mountain ranges becoming ele- 

 vated cut off the moisture-supply of others who went the same way. 



The glacial period is considered synonymous with the permian, and 

 represents the extreme of cold, while the tropical period, the extreme of 

 heat, is represented by coal beds (Fig. 245). 



The mechanics of adaptation of living organisms to new climatic 

 and environmental changes has given rise to much speculation. 



Lamarck thought that the organism was directly affected by any 

 change in environment and that this change then affected the germ- 

 plasm so that it could be inherited by the organism's offspring, and thus 

 result in a permanent racial change. 



Others taught that both somatoplasm and germplasm are simulta- 

 neously affected. This theory is known as that of parallel induction. 



Darwin, like Lamarck, believed that small environmental changes 

 became large ones as they were successively inherited. In fact, this was 

 held by nearly all the early workers since the time of Darwin ; but, as 

 no evidence was forthcoming which could explain how such environ- 

 mental changes could affect the germplasm and thus be inherited, biolo- 

 gists are inclined to hold with Professor H. H. Newman, that "external 

 factors accelerate or retard processes that were already under way in 

 the germplasm, so that the response appears to be something new in 

 kind when it is only the result of a sudden acceleration of a character 

 evolution already under way. Whatever be the underlying mechanism 

 involved in adaptive changes, there is no hope of explaining adaptations 

 on the Darwinian basis, through the selection of the best out of a vast 

 area of purely fortuitous variations ; for if the historical study of verte- 

 brate evolution reveals one thing more clearly than any other, it is that 

 evolutionary changes are ordinarily progressive, and determinate in 

 character, and that in many respects these ordinary processes of evolu- 

 tion are independent of each other and of environmental changes." 



This means that we need not hold that animals always adapt them- 

 selves to their environment, but that they can migrate to environments 

 which are best suited to them. And there is ample evidence to show 

 that such migrations took place quite often. Some of these are shown 

 by the land-bridges (over which animals passed) now destroyed, which 

 connected islands and continents with each other. The animals were 

 then shut off from their original home by the destruction of the bridges. 

 Such animals are said to be geographically isolated. 



Not only have animals migrated, but as already stated, the climate 

 itself migrated. This is shown by the fact that the marine and glacial 



