Page - 372 - in The Origin of Species

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