198 GEOLOGICAL DISTRIBLTION. 



doubtless have effected a foothold in some neighbouring region, 

 and struggle on as best they can under what might be less favour- 

 able conditions for existence. Ultimately the race will be thinned 

 out and extinction of the species follow. A parallel case of sudden 

 appearance and much less sudden disappearance is afforded by the 

 two common species of house-rat, the black and the brown, both 

 of which, through introduction, have become more or less cos- 

 mopolitan in their range. Everywhere where the latter has suc- 

 ceeded in obtaining a foothold the former is gradually, but steadily, 

 fading away, relinquishing piece by piece the territoiy of which it 

 was at one time in full possession. But it still lingers on, and, 

 no doubt, will still continue to so linger for some time in the future, 

 a relic of a once formidable race. Yet its own march of conquest 

 was probably no less rapid than that of its more successful com- 

 petitor, the brown or Norway rat, which appears to have been 

 unknown west of the Volga Eiver prior to about the middle of the 

 last century, and which has since spread so extensively as to render 

 it one of the commonest pests of both continental and insular 

 Europe. The species was first observed on the Pacific coast of the 

 United States subsequent to 1850, but it is scarcely less common at 

 the present time in California than anywhere else. Tlie relation of 

 natural selection to extinction and persistence is clearly stated 

 by Darwin thus ("Origin of Species") : "The competition will 

 generally be most severe, as formerly explained and illustrated by 

 examples, between the forms which are most like to each other in 

 all respects. Hence the improved and modified descendants of a 

 species will generally cause the extermination of the parent species ; 

 and if many new forms have been developed from any one species, 

 the nearest allies of that .tipecies, i. e., the species of the same genus, 

 will be the most liable to extermination. Thus, as I believe, a 

 number of new species descended from one species, that is, a new 

 genus, comes to supplant an old genus belonging to the same 

 family. But it must often have happened that a new species be- 

 longing to some one group will have seized on the place occupied 

 by a species belonging to a distinct group, and thus cause its exter- 

 mination ; and, if many allied forms be developed from the success- 

 ful intruder, many will have to yield their places, and it will gen- 

 erally be allied forms, which will suffer from some inherited inferi- 

 ority in common. But, whether it be species belonging to the same 



