672 GEOLOGY, 



other products of the deformation which extended it, and these form 

 barriers. Desert wastes and other inhospitable tracts, and even glacia- 

 tion, are hable to develop as secondary consequences, and to interpose 

 barriers, and hence the cosmopolitanism of the land-hfe is liable to be 

 less complete than that of the sea-life. 



Restrictive and expansional evolution. — It is obvious from the last 

 discussion that if the picture of the earth's movement above drawn be 

 true, the areas available for particular classes of life may vary greatly 

 from age to age. At times the shallow- water sea-life may be forced to 

 retreat into a very narrow tract on the border of the land, and into chance 

 expansions here and there. In being crowded into this limited tract, 

 perhaps also less adapted for a habitat on account of the change, the 

 life is subjected to severe competition and to hard conditions, and must 

 experience in an intensified degree the effects of the struggle for existence. 

 Whatever of evolutionary potency there may be in such a struggle 

 under such restrictive conditions should be revealed in the modifications 

 of the fauna that ensued. 



On the other hand, when the shallow seas are generally extending 

 themselves upon the land and the land is being base-leveled, and thus 

 adapted to shallow submergence, the shallow- water life enjoys an enlarg- 

 ing realm, and should reveal the effects of evolution under expansional 

 conditions. In affording a comparison between these opposite and 

 alternating phases of restrictional and expansional evolution, geology 

 makes one of its great contributions to the external causes and condi- 

 tions of organic evolution. These will come under repeated consideration 

 in the historical chapters. 



