GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 105 



formations that rarity precedes extinction; and we know 

 that this has been the progress of events with those 

 animals which have 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 rare before they become extinct — to 

 feel no surprise at the rarity 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 indi- 

 vidual 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 ad- 

 vantage over those with which it comes into competition; 

 and the consequent extinction of the less-favored 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 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 flour- 

 ishing 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, 



