EXriNGTION. 351 



to other modified and impi-oved species, a few of the suf- 

 ferers 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 Trigo- 

 nia, 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 

 oar 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 

 palseozoic 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; if we must marvel, let it be at our 

 own presumption in imagining for a moment that we un- 

 derstand the many complex contingencies on which the 

 existence of each species depends. If vve forget for an 

 instant that each species tends to increase inordinately, 

 and that some check is always in action, yet seldom per- 

 ceived by us, the whole economy of nature will be utterly 

 obscured. Whenever we can precisely say why this species 

 is more abundant in individuals than that; why this 

 species and not another can be naturalized in a given coun- 

 try; then, and not until then, we may justly feel surprise 

 why we cannot account for the extintion of any particular 

 species or group of species. 



