822 
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 which 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 palaeontological 
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 
