This "survival of the fittest" is what Darwin termed "natural
selection," because it leads to the same results in nature as are
produced by man's selection among domestic animals and cultivated
plants. Its primary effect will, clearly, be to keep each species in the
most perfect health and vigour, with every part of its organisation in
full harmony with the conditions of its existence. It prevents any
possible deterioration in the organic world, and produces that
appearance of exuberant life and enjoyment, of health and beauty, that
affords us so much pleasure, and which might lead a superficial observer
to suppose that peace and quietude reigned throughout nature.
_The Effect under changed Conditions._
But the very same process which, so long as conditions remain
substantially the same, secures the continuance of each species of
animal or plant in its full perfection, will usually, under changed
conditions, bring about whatever change of structure or habits may be
necessitated by them. The changed conditions to which we refer are such
as we know have occurred throughout all geological time and in every
part of the world.
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