nd small mammifers of S. America, one is strongly
induced to join with the catastrophists. I believe, however, that very
erroneous views are held on this subject. As far as is historically
known, the disappearance of species from any one country has been
slow--the species becoming rarer and rarer, locally extinct, and finally
lost{333}. It may be objected that this has been effected by man's
direct agency, or by his indirect agency in altering the state of the
country; in this latter case, however, it would be difficult to draw any
just distinction between his agency and natural agencies. But we now
know in the later Tertiary deposits, that shells become rarer and rarer
in the successive beds, and finally disappear: it has happened, also,
that shells common in a fossil state, and thought to have been extinct,
have been found to be still living species, but very _rare_ ones{334}.
If the rule is that organisms become extinct by becoming rarer and
rarer, we ought not to view their extinction, even in the case of the
larger quadrupeds, as anything wonderful and out of the common course of
events. For no naturalist thinks it wonderful that one species of a
genus should be rare and another abundant, notwithstanding he be quite
incapable of explaining the causes of the comparative rareness{335}. Why
is one species of willow-wren or hawk or woodpecker common in England,
and another extremely rare: why at the Cape of Good Hope is one species
of rhinoceros or antelope far more abundant than other species? Why
again is the same species much more abundant in one district of a
country than in another district? No doubt there are in each case good
causes: but they are unknown and unperceived by us. May we not then
safely infer that as certain causes are acting _unperceived_ around us,
and are making one species to be common and another exceedingly rare,
that they might equally well cause the final extinction of some species
without being perceived by us? We should always bear in mind that there
is a recurrent struggle for life in every organism, and that in every
country a destroying agency is always counteracting the geometrical
tendency to increase in every species; and yet without our being able to
tell with certainty at what period of life, or at what period of the
year, the destruction falls the heaviest. Ought we then to expect to
trace the steps by which this destroying power, always at work and
scarcely perceived by us, becomes