GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 107 



inhabit our fresh waters. Therefore the utter extinction 

 of a group is generally, as we have seen, a slower proc- 

 ess than its production. 



With respect to the apparently sudden extermination 

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

 of the paleozoic 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 ac- 

 cords 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 

 understand the many complex contingencies on which 

 the existence of each species depends. If we forget for 

 an instant that each species tends to increase inordi- 

 nately, 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 

 naturalized in a given country; then, and not until then, 

 we may justly feel surprise why we cannot account for 



