320 GEOLOGICAL SUCCESSION. Chap. X. 



been exterminated, either locally or wholly, through 

 man's agency. I may repeat what I published in 1845, 

 namely, that to admit that species generally become 

 1 rare before they become extinct — to feel no surprise at 

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

 it ceases to exist, is much the 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 

 unknown 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 Avith which it comes into compe- 

 tition ; and the consequent extinction of 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 natural 

 and artificial, are bound together. In certain flourishing 

 groups, the number of new specific forms which have 

 been produced within a given time is probably greater 

 than that of the old forms which have been extermi- 

 nated ; but we know that the number of species has not 

 gone on indefinitely increasing, at least during the later 

 geological periods, so that looking to later times we 

 may believe that the production of new forms lias 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, 



