nd by a successful struggle against the
encroachments of other plants and animals. That minute parasitic plant,
called "the rust" in wheat, has, like the Hessian fly, the locust, and
the aphis, caused famines ere now amongst the "lords of the creation."
The most insignificant and diminutive species, whether in the animal or
vegetable kingdom, have each slaughtered their thousands, as they
disseminated themselves over the globe, as well as the lion, when first
it spread itself over the tropical regions of Africa.

_Concluding remarks._--Although we have as yet considered one class only
of the causes (the organic) by which species may become exterminated,
yet it cannot but appear evident that the continued action of these
alone, throughout myriads of future ages, must work an entire change in
the state of the organic creation, not merely on the continents and
islands, where the power of man is chiefly exerted, but in the great
ocean, where his control is almost unknown. The mind is prepared by the
contemplation of such future revolutions to look for the signs of
others, of an analogous nature, in the monuments of the past. Instead of
being astonished at the proofs there manifested of endless mutations in
the animate world, they will appear to one who has thought profoundly on
the fluctuations now in progress, to afford evidence in favor of the
uniformity of the system, unless, indeed, we are precluded from speaking
of _uniformity_ when we characterize a principle of endless variation.

CHAPTER XLII.

EXTINCTION OF SPECIES.--INFLUENCE OF INORGANIC CAUSES.

Powers of diffusion indispensable, that each species may maintain
its ground--How changes in physical geography affect the
distribution of species--Rate of the change of species due to this
cause cannot be uniform--Every change in the physical geography of
large regions tends to the extinction of species--Effects of a
general alteration of climate on the migration of species--Gradual
refrigeration would cause species in the northern and southern
hemispheres to become distinct--Elevation of temperature the
reverse--Effects on the condition of species which must result from
inorganic changes inconsistent with the theory of transmutation.

_Powers of diffusion indispensable, that each species may maintain its
ground._--Having shown in the last chapter, how considerably the
numerical increase or the extension of the geograp