^^"^ THE NEW EVOLUTION Wf 



So changes in climatic conditions affect not the exist- 

 ence or the occurrence of any of the major groups or 

 phyla, but instead the balance and the details of the 

 types included within the phyla, or in other words 

 the various forms in which the structural com- 

 plex characteristic of the several major groups is 

 manifested. 



We may illustrate this point by a comparison be- 

 tween different portions of the earth's surface. Every 

 major group or phylum is represented in seas where 

 the temperature of the water never rises above the 

 freezing point of fresh water (3 2.° Fahrenheit or 0° Cen- 

 tigrade). Every major group is also represented in 

 the tropics. 



But the representatives of the phyla in the very cold 

 and in the very warm water are in practically all 

 cases very different, just as on land arctic and tropical 

 animals are almost always very different. 



In the warm water of the tropics and in the hot 

 tropical lowlands the number of different types of 

 animals included in each of the various major groups 

 is far greater than it is in the cold regions. But in 

 spite of the enormous number of different kinds of 

 animals found in the tropics, there is not the slightest 

 indication of any tendency to produce new phyla, or 

 inter grades between the phyla. 



In regions intermediate between the two extremes 

 of hot and cold all grades of intermediate conditions 

 are to be found, and it is noteworthy that in compar- 

 ing different intermediate areas on the earth's surface 

 we find a curious disparity in the balance between the 



