n the different parts of the organic and inorganic worlds, we may next
proceed to examine the results which may be anticipated from the
fluctuations now continually in progress in the state of the earth's
surface, and in the geographical distribution of its living productions.

CHAPTER XLI.

EXTINCTION OF SPECIES.--CHANGES IN THE STATIONS OF ANIMALS.

Extension of the range of one species alters the condition of many
others--The first appearance of a new species causes the chief
disturbance--Changes known to have resulted from the advance of
human population--Whether man increases the productive powers of the
earth--Indigenous quadrupeds and birds extirpated in Great
Britain--Extinction of the dodo--Rapid propagation of domestic
quadrupeds in America--Power of exterminating species no prerogative
of man--Concluding remarks.

We have seen that the stations of animals and plants depend not merely
on the influence of external agents in the inanimate world, and the
relations of that influence to the structure and habits of each species,
but also on the state of the contemporary living beings which inhabit
the same part of the globe. In other words, the possibility of the
existence of a certain species in a given place, or of its thriving more
or less therein, is determined not merely by temperature, humidity,
soil, elevation, and other circumstances of the like kind; but also by
the existence or non-existence, the abundance or scarcity, of a
particular assemblage of other plants and animals in the same region.

If it be shown that both these classes of circumstances, whether
relating to the animate or inanimate creation, are perpetually changing,
it will follow that species are subject to incessant vicissitudes; and
if the result of these mutations, in the course of ages, be so great as
materially to affect the general condition of _stations_, it will follow
that the successive destruction of species must now be part of the
regular and constant order of nature.

_Extension of the range of one species alters the condition of the
others._--It will be desirable, first, to consider the effects which
every extension of the numbers or geographical range of one species must
produce on the condition of others inhabiting the same regions. When the
necessary consequences of such extensions have been fully explained, the
reader will be prepared to appreciate the important influence which
s