Nevertheless, if we consider the subject under another point of
view, it will appear less perplexing. We do not steadily bear in
mind how profoundly ignorant we are of the conditions of existence
of every animal; nor do we always remember that some check is
constantly preventing the too rapid increase of every organised
being left in a state of nature. The supply of food, on an average,
remains constant, yet the tendency in every animal to increase by
propagation is geometrical; and its surprising effects have nowhere
been more astonishingly shown, than in the case of the European
animals run wild during the last few centuries in America. Every
animal in a state of nature regularly breeds; yet in a species long
established, any GREAT increase in numbers is obviously impossible,
and must be checked by some means. We are, nevertheless, seldom
able with certainty to tell in any given species, at what period of
life, or at what period of the year, or whether only at long
intervals, the check falls; or, again, what is the precise nature
of the check. Hence probably it is that we feel so little surprise
at one, of two species closely allied in habits, being rare and the
other abundant in the same district; or, again, that one should be
abundant in one district, and another, filling the same place in
the economy of nature, should be abundant in a neighbouring
district, differing very little in its conditions.
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