A VIEW IN PERSPECTIVE. 135 



Now in the explanation thus given we must of 

 course deal somewhat with hypothesis. We do not 

 know positively that it was the occurrence of the 

 new variation in the reproductive habits that gave the 

 stimulus to the mammals, and thus caused the rapid 

 divergence in the Tertiary period. We know, how- 

 ever, that the mammals did at that time receive a 

 new impulse from some source, and did immediately 

 expand to fill the new field of nature open to them, 

 and we have pretty good evidence, moreover, that at 

 the same time the above-mentioned change in the 

 reproductive organs occurred. From all of this we 

 learn to picture to ourselves the animal kingdom as 

 constantly searching after new fields for expansion. 

 The different types of animals are constantly migra- 

 ting here and there, constantly subjected to new 

 conditions, and therefore constantly undergoing 

 change. Generally the severe competition with 

 nature and with enemies keeps them in restraint 

 by cutting off all sportive branches, and allowing 

 only the central strongest forms to continue to exist. 

 But occasionally a change in the conditions produced 

 by geological forces gives certain classes the advan- 

 tage over others, or changes in the configuration of 

 the land produce new fields for expansion. Or, 

 again, some one of the sportive branches proves 

 to have inherent qualities of great vigor, and in this 

 way offers a new field of organic type. In any of 

 these cases the opportunity is seized 

 ately, and shows itself in rapid expj 

 tainly a fact that the multipli< 

 greatest in the early life of a gei 



