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Show 320 GEOLOGICAL SUCCESSION. CHAP. X. been exterminated either locally or wholly, through man's agency. I ~ay repeat wha~ I published in 1845, namely, that to admit that speCies generally b~come rare before they become extinct-to feel no surpnse at the rarity of a species, and yet to marvel greatl~ when it ceases to exist, is much the same as to admit that sickness in the individual is the forerunner of deathto 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 with which it comes into competition; and the consequent extinction of 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 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 greate.r than that of the old forms which have been exterminated ; 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 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. CnAr. X. EXTINCTION. 321 Hen~e t~e improved and modified descendants of a speCies will generally cause the extermination of th parent-species ; and if many new forms have been de~ velo~_ed ~rom any one species, the nearest allies of that speCies: z. e. the species of the same genus, will be the most liable to extermination. Thus, as I believe, a ~umber of new species descended from one species, that ~s a new genus, comes to supplant an old genus, belongIng to the same family. But it must often have happened th~t a new species belongi~g to some one group will have smz~d. on the place occupied by a species belonging to a di~t1nct group, and thus caused its extermination ; and. If many allied forms be developed from the success~ ul I~truder, many will have to yield their places ; and It will generally be allied forms, which will suffer from ~orne inherited inferiority in common. But whet~ e~ It be species belonging to the same or to a dis~Inct class, which yield their places to other species whiCh have been modified and improved, a few of the sufferers may often. Ion? be preserved, from being fitted t~ some pecu~Iar hne of life, or from inhabiting some distant and Isolated station, where they have esca~ed seve~e c.ompetition. For instance, a single speCies of Tngonia, a great genus of shells in the secondary formations, survives in the Australian seas · and a few members of the great and almost extinct group of Ganoid fishes still inhabit our fresh waters. Therefore the utter extinction of a group is generally: as we have seen, a slower process than its productwn. With respect to the apparently sudden extermination of whole families or orders, as of Trilobites at the close of the palooozoic period and of Ammonites at the close of the secondary period, we must remember what has been already said on the probable wide intervals of time p 3 |