te creation. If we divide the surface of the earth into twenty
regions of equal area, one of these might comprehend a space of land and
water about equal in dimensions to Europe, and might contain a twentieth
part of the million of species which may be assumed to exist in the
animal kingdom. In this region one species only could, according to the
rate of mortality before assumed, perish in twenty years, or only five
out of fifty thousand in the course of a century. But as a considerable
portion of the whole world belongs to the aquatic classes, with which
we have a very imperfect acquaintance, we must exclude them from our
consideration, and, if they constitute half of the entire number, then
one species only might be lost in forty years among the terrestrial
tribes. Now the mammalia, whether terrestrial or aquatic, bear so small
a proportion to other classes of animals, forming less, perhaps, than
a thousandth part of a whole, that, if the longevity of species in the
different orders were equal, a vast period must elapse before it would
come to the turn of this conspicuous class to lose one of their number.
If one species only of the whole animal kingdom died out in forty years,
no more than one mammifer might disappear in forty thousand years, in a
region of the dimensions of Europe.

"It is easy, therefore, to see that in a small portion of such an area,
in countries, for example, of the size of England and France, periods
of much greater duration must elapse before it would be possible
to authenticate the first appearance of one of the larger plants or
animals, assuming the annual birth and death of one species to be the
rate of vicissitude in the animal creation throughout the world."(3)

In a word, then, said Lyell, it becomes clear that the numberless
species that have been exterminated in the past have died out one by
one, just as individuals of a species die, not in vast shoals; if
whole populations have passed away, it has been not by instantaneous
extermination, but by the elimination of a species now here, now there,
much as one generation succeeds another in the life history of any
single species. The causes which have brought about such gradual
exterminations, and in the long lapse of ages have resulted in rotations
of population, are the same natural causes that are still in operation.
Species have died out in the past as they are dying out in the present,
under influence of changed surroundings, such a