And if we remember that
all such physical changes are slow and gradual in their operation, we
shall see that the amount of variation which we know occurs in every new
generation will be quite sufficient to enable modification and
adaptation to go on at the same rate. Mr. Darwin was rather inclined to
exaggerate the necessary slowness of the action of natural selection;
but with the knowledge we now possess of the great amount and range of
individual variation, there seems no difficulty in an amount of change,
quite equivalent to that which usually distinguishes allied species,
sometimes taking place in less than a century, should any rapid change
of conditions necessitate an equally rapid adaptation. This may often
have occurred, either to immigrants into a new land, or to residents
whose country has been cut off by subsidence from a larger and more
varied area over which they had formerly roamed. When no change of
conditions occurs, species may remain unchanged for very long periods,
and thus produce that appearance of stability of species which is even
now often adduced as an argument against evolution by natural selection,
but which is really quite in harmony with it.
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