one and two millions
of species now inhabiting the terraqueous globe; so that if only one of
these were to become extinct annually, and one new one were to be every
year called into being, much more than a million of years might be
required to bring about a complete revolution in organic life.

I am not hazarding at present any hypothesis as to the probable rate of
change; but none will deny that when the _annual_ birth and the _annual_
death of one species on the globe is proposed as a mere speculation,
this at least is to imagine no slight degree of instability in the
animate 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 would, 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
proportion of the whole would belong 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
one thousandth part of the 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 40,000 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 and
animals, assuming the annual birth and death of one species to be the
rate of vicissitude in the animate creation throughout the world.

The observations of naturalists upon living species may, in the course
of future centuries, accumulate positive data, from which an insight
into the laws which go