132 THE LIVING WORLD. 



the Tertiary (9) we find abundant fauna of .the 

 higher true mammals, indicating that by this time 

 the mammals had been for some time under the 

 influence of an expanding force. From this early 

 Tertiary (9), the development and expansion of 

 the mammals occurred with great rapidity and in 

 a very short time, short at least compared with the 

 long period in which the class seems to have re- 

 mained dormant, the higher mammals had diverged 

 into the modern ones. 



Causes of the Sudden Expansions of Type. 



The question, of course, arises why any group of 

 animals should have remained so long in a compara- 

 tively stationary condition and then have suddenly 

 expanded with such rapidity into a widely diversified 

 fauna. It is hardly ever possible to give a definite 

 answer to this question. A somewhat general answer 

 can, however, be given, which is very suggestive 

 as indicating one of the important laws of animal 

 life. It would seem that the comparatively sud- 

 den expansion of type has occurred when a group 

 of animals in some way begins to occupy a new 

 field of nature. Where conditions remain constant, 

 animal life is held in comparative equilibrium, and 

 therefore is more or less stationary. But any change 

 in conditions will disturb the equilibrium, and the 

 result is always a change in the structure of the 

 animals themselves. Now any change in climate, 

 in temperature, in atmospheric constitution, in con- 

 figuration of the land and sea, etc., will be sure to 

 affect the equilibrium of life. That such changes 



