98 EXTINCTION. [Chap. XL 



same as to admit that sickness in the individual is the 

 forerunner of death — to feel no surprise 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 competi- 

 tion; and the consequent extinction of the less-fa- 

 voured 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 neighbourhood; 

 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 to- 

 gether. 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 ex- 

 terminated; but we know that species have not gone on 

 indefinitely increasing, at least during the later geo- 

 logical 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 hke each other in all respects. 

 Hence the improved and modified descendants of a spe- 

 cies will generally cause the extermination of the parent- 

 species; and if many new forms have been developed 



