Page - 371 - in The Origin of Species

EXTINCTION 371 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 sur- prise 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 conse- quent 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 neigh- bourhood; 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 natu- rally 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 spe- cies 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 gener- ally 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, i.e., the species of the same genus, will be the most liable to extermination. Thus, as I believe, a number of new species descended from one species, that is a new genus, comes to supplant an old genus, belong- ing to the same family. But it must often have happened 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