98 EXTINCTION. [Chap. XI. 



runner 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-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 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 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 species 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 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, 



