We see the same principle underlying the scientific doctrine of
evolution. People often think loosely that the idea of evolution,
in the case, let us say, of a bird like a heron, with his
immobility, his long legs, his pointed beak, his muscular neck, is
that such characteristics have been evolved through long ages by
birds that have had to get their food in swamps and shallow lakes,
and were thus gradually equipped for food-getting through long ages
of practice. But of course no particular bird is thus modified by
circumstances. A pigeon transferred to a fen would not develop the
characteristics of the heron; it would simply die for lack of food.
It is rather that certain minute variations take place, for unknown
reasons, in every species; and the bird which happened to be
hatched out in a fenland with a rather sharper beak or rather
longer legs than his fellows, would have his power of obtaining
food slightly increased, and would thus be more likely to
perpetuate in his offspring that particular advantage of form. This
principle working through endless centuries would tend slowly to
develop the stock that was better equipped for life under such
circumstances, and to eliminate those less suited to the locality;
and thus the fittest would tend to survive.
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