278 EXTINCTION. [CHAP. XL 



not gone on indefinitely increasing, at least during the later geo- 

 logical epochs, so that, looking to later times, we may beliave 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 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 

 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, belonging 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 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 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 secondary formations, 

 survive 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 production. 



With respect to the apparently sudden extermination of whole 

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

 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 between our consecutive formations ; and 

 in these intervals there may have been much slow extermination. 

 Moreover, when, by sudden immigration or by unusually 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 forms which thus 

 yield their places will commonly be allied, for they will partake 

 of the same inferiority in common. 



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

 whole groups of species become extinct accords well with the 

 theory of natural selection. We need not marvel at extinction ; 



