350 EXTINCTION. 



The theory of natural selection is grounded on the belief 

 that each new variety and ultimately each new species, is 

 produced and maintained by having some advantage over 

 those with which it comes into competition; and the con- 

 sequent extinction of the less-favored forms almost inevita- 

 bly follows. It is the same with our domestic productions; 

 when a new and slightly improved variety has been raised, 

 it at first supplants the less improved varieties in the same 

 neighborhood; when much improved it is transported far 

 and near, like our short-horn cattle, and takes the place of 

 other breeds in other countries. Thus the appearance of 

 new forms and the disappearance of old forms, both those 

 naturally and those artificially produced, are bound 

 together. In flourishing groups, the number of new spe- 

 cific forms which have been produced within a given time 

 has at some periods probably been greater than the number 

 of the old specific forms which have been exterminated; 

 but we know that species have not gone on indefinitely in- 

 creasing, at least during the later geological epochs, so 

 that, looking to later times, we may believe that the pro- 

 duction of new forms has caused the extinction of about 

 the same number of old forms. 



The competition will generally be most severe, as for- 

 merly explained and illustrated by examples, between the 

 forms which are most like 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 species, 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 belonging to some one group has seized on the place 

 occupied by a species belonging to a distinct group, and 

 thus have caused its extermination. If many allied forms 

 be developed from the successful intruder, many will have 

 to yield their places; and it will generally be the allied 

 forms, which will suSer from some inherited inferiority in 

 common. But whether it be species belonging to' the 

 same or to a distinct class, which have yielded their places 



