302 THE ORIGIN OF SPECIES 



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 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 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 

 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 ex- 

 plained 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 exter- 

 mination 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 exter- 

 mination. Thus, as I believe, a number of new species descended 

 from one species, that is, a new genus, comes to supplant an old 

 genus, belonging to the same family. But it must often have hap- 

 pened 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 allied forms be 

 developed from the successful intruder, many will have to yield 

 their places; and it will generally be the allied forms, which will 

 suffer from some inherited inferiority in common. But whether it 

 be species belonging to the same or to a distinct class, which have 

 yielded their places to other modified and improved species, a few 



