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. 



