One species of animal has been preserved by length of
neck, which enabled it to reach high-growing fruits and leaves; another
by a thicker skin, which made it difficult for enemies to devour;
another by a colour which made it easier to hide. One plant has been
preserved by a bright flower which attracted insects to carry its pollen
to other flowers of its kind; another by a sweet fruit which attracted
birds to scatter its seed. Meanwhile other animals and plants that had
not these advantages perished for the lack of them. The result would be
to maintain, and perpetually, though with exceeding slowness, more and
more to adapt to the conditions of their life, those species whose
peculiarities gave them some advantage in the great struggle for
existence.
Here again we have the working of known laws of life, capable of
accounting for what we see. And the high probability cannot be denied
that by evolution of this kind the present races of living creatures
have been formed. And to these arguments the strongest corroboration is
given by the frequent occurrence, both in plants and animals, of useless
parts which still remain as indications of organs that once were useful
and have long become useless. Animals that now live permanently in the
dark have abortive eyes which cannot see, but indicate an ancestor with
eyes that could see.
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