But, whenever the physical or organic
conditions change, to however small an extent, some corresponding change
will be produced in the flora and fauna, since, considering the severe
struggle for existence and the complex relations of the various
organisms, it is hardly possible that the change should not be
beneficial to some species and hurtful to others. The most common
effect, therefore, will be that some species will increase and others
will diminish; and in cases where a species was already small in numbers
a further diminution might lead to extinction. This would afford room
for the increase of other species, and thus a considerable readjustment
of the proportions of the several species might take place. When,
however, the change was of a more important character, directly
affecting the existence of many species so as to render it difficult for
them to maintain themselves without some considerable change in
structure or habits, that change would, in some cases, be brought about
by variation and natural selection, and thus new varieties or new
species might be formed.
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