372 ORIGIN OF SPECIES 



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 inferior- 

 ity 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 sec- 

 ondary period, we must remember what has been already 

 said on the probable wide intervals of time between our con- 

 secutive formations; and in these intervals there may have 

 been much slow extermination. Moreover, when, by sud- 

 den 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 par- 

 take 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 accord 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 that we understand the many 

 complex contingencies on which the existence of each spe- 

 cies depends. If we forget for an instant that each species 

 tends to increase inordinately, and that some check is always 

 in action, yet seldom perceived 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 naturalised in 



