302 EXTINCTION. Chap. X. 



improved and modified dcsrcudants of a species will generally 

 cause the externiiiiatiou 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 

 he the most liable to extermination. Thus, as I believe, a 

 number of new speci(3s descended from one species, that is, a 

 new genus, comes to sujiplant an old geiuis, belonging to the 

 same family. But it must often have happened that a new 

 species belonging to some one group has seized on the placfa 

 occupied by a species belonging to a distinct group, and thus 

 have caused its extermination. If many allied forms be de- 

 veloped from the successful intruder, many will have to yield 

 tlunr places ; and it Avill generally be the allied forms which 

 will suffer from some inherited inferiority in common. But 

 whetlicr 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 of the sufferers may often be preserved 

 for a long time, from being fitted to some peculiar line of life, 

 or from inhabiting some distant and isolated station, where 

 they will have escaped severe competition. For instance, 

 some species of Trigonia, a great genus of shells in the second- 

 ary formations, survive in the Australian seas; and a few mem- 

 bers of the great and almost extinct group of Ganoid fishes 

 still inhabit our fresh waters. Therefore the utter extinction 

 of a group is generalh', as Ave have seen, a slower process than 

 its production. 



With res^DCct to the apparently svidden extermination of 

 whole families or orders, as of Trilobites at the close of the 

 paleozoic period and of Ammonites at the secondary period, 

 we must remember what has been already said on the probable 

 wide intervals of time between our consecutive formations ; 

 and in these intervals there may have been much slow exter- 

 mination. I\Ioreover, when, by sudden immigration or by un- 

 usually rapid development, many species of a new group have 

 taken possession of an area, many of the older species will have 

 been exterminated in a correspondingly rapid manner; and the 

 fonns Avhich thus yi(^^ld their places vrill commonly be allied, 

 for they Avill partake of the same inferiority in common. 



Thus, as it seems to me, the manner in which single species 

 and Avliole groups of species become extinct accords well with 

 the theory of natural selection. We need not marvel at ex- 

 tinction ; if we must marvel, let it be at our own presumption 

 in imagining for a moment tliat wc understand the many com- 



