Darwinism Verified. 19 



formed into deserts, and glaciated valleys thaw 

 ing into sunny lakes ; high table-lands sinking 

 into ocean-floors, and submarine ledges rearing 

 their heads as Alpine ranges ; deep-sea mollusks 

 and crustaceans seeking refuge in shallow waters, 

 while littoral organisms migrate upland to find 

 new food and contend with new enemies ; plant- 

 seeds carried by vagrant birds to unwonted hab 

 itats ; peaceful tribes of ruminants decimated by 

 invading carnivores ; ceaseless conflict, and redis 

 tribution of every possible sort, these are the 

 things we are called upon to contemplate. Re 

 membering, then, how stability of species is main 

 tained only by the rigorous selection of a few in 

 dividuals that are best adapted to a given set of 

 exigencies, we see that, as the combinations of 

 exigencies are altered from time to time, the sta 

 bility of species can in general be but temporary. 

 Now and then we may expect to find very long 

 persistency of type where, in spite of great ter 

 restrial changes, some simple set of conditions 

 most important to the organism remains unal 

 tered ; but in the vast majority of cases such per 

 sistence is impossible. It is seldom that the life 

 of any species extends over more than one geo 

 logical epoch ; often the duration is much shorter 

 than this. 



