EXTINCTION 371 



of a species, and yet to marvel greatly when the species 

 ceases to exist, is much the same as to admit that sickness 

 in the individual is the forerunner of death — to feel no sur- 

 prise at sickness, but, when the sick man dies, to wonder and 

 to suspect that he died by some deed of violence. 



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 conse- 

 quent extinction of the less favoured forms almost inevitably 

 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 neigh- 

 bourhood; 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 natu- 

 rally and those artificially produced, are bound together. In 

 flourishing groups, the number of new specific 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 spe- 

 cies have not gone on indefinitely increasing, at least during 

 the later geological epochs, so that, looking to later times, 

 we may believe that the production of new forms has caused 

 the extinction of about the same number of old forms. 



The competition will generally be most severe, as formerly 

 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 gener- 

 ally 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, belong- 

 ing 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 



