lar variations, occurring in cultivated
plants and domestic animals, are capable of being perpetuated and
accumulated by artificial selection, till they have resulted in all the
wonderful varieties of our fruits, flowers, and vegetables, our domestic
animals and household pets, many of which differ from each other far
more in external characters, habits, and instincts than do species in a
state of nature. We have now to inquire whether there is any analogous
process in nature, by which wild animals and plants can be permanently
modified and new races or new species produced.

_Effect of Struggle for Existence under Unchanged Conditions._

Let us first consider what will be the effect of the struggle for
existence upon the animals and plants which we see around us, under
conditions which do not perceptibly vary from year to year or from
century to century. We have seen that every species is exposed to
numerous and varied dangers throughout its entire existence, and that it
is only by means of the exact adaptation of its organisation--including
its instincts and habits--to its surroundings that it is enabled to live
till it produces offspring which may take its place when it ceases to
exist. We have seen also that, of the whole annual increase only a very
small fraction survives; and though the survival in individual cases may
sometimes be due rather to accident than to any real superiority, yet we
cannot doubt that, in the long run, those survive which are best fitted
by their perfect organisation to escape the dangers that surround them.
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 usu