322 GEOLOGICAL SUCCESSION, Chap. X. 



between our consecutive formations ; and in these inter- 

 vals 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 a new area, they will have exter- 

 minated in a correspondingly rapid manner many of the 

 old inhabitants ; and the forms which thus yield their 

 places will commonly be allied, for they will partake of 

 some 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 presumption in imagining for a moment 

 that we understand the many complex contingencies, 

 on winch the existence of each species 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 in- 

 dividuals than that ; why this species and not another 

 can be naturalised in a given country ; then, and not 

 till then, we may justly feel surprise why we cannot 

 account for the extinction of this particular species or 

 group of species. 



On the Forms of Life changing almost simultaneously 

 throughout the World. — Scarcely any palseontological 

 discovery is more striking than the fact, that the forms 

 of life change almost simultaneously throughout the 

 world. Thus our European Chalk formation can be 

 recognised in many distant parts of the world, under 

 the most different climates, where not a fragment of the 

 mineral chalk itself can be found; namely, in North 



