ZOOGENESIS 



different divisions within the phyla which are quite 

 comparable to the changes in the balance between the 

 different divisions seen as we pass from age to age in 

 the fossil record. 



It is doubtful whether at any time in the past there 

 has ever been such a great diversity of climatic condi- 

 tions as is found at the present day. The present 

 tropics are probably hotter than the climate in any 

 past age since life began. This is suggested by the 

 fact that animal types which from their geological 

 history we recognize as ancient if they are of restricted 

 distribution are mostly found not in the hot tropics, 

 but in more or less warm temperate regions of equable 

 temperature and humidity, at moderate elevations in 

 the tropics, or at moderate depths in the tropical seas. 



Somewhere or other on the earth today we probably 

 find duplicated all, or nearly all, of the climates of 

 the past so far as temperature and humidity are con- 

 cerned. But we cannot be sure of duplication when 

 it comes to the very important question of light or 

 the strength and frequency of winds. 



If we may judge the past from what we know of 

 conditions at the present time we are in a position to 

 understand the constancy of the major groups or 

 phyla, and also the constant changes from age to age 

 that took place within the phyla. 



[12.9] 



