398 General Biology 



However, no one can tell the number of years required to lay down 

 the various strata any more than he can tell how many years elapsed 

 to form the intervals between such laying down; and these intervals 

 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 the change in the parent could be inherited by the organ- 

 ism'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 has been forthcoming which could explain how such en- 

 vironmental changes could affect the germplasm and thus be inherited, 

 biologists 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 vertebrate 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 evolution are independent of each other and of environ- 

 mental 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. 



