Now, if nature had to make the beak of a
full-grown pigeon very short for the bird's own advantage, the process of
modification would be very slow, and there would be simultaneously the most
rigorous selection of all the young birds within the egg, which had the
most powerful and hardest beaks, for all with weak beaks would inevitably
perish: or, more delicate and more easily broken shells might be selected,
the thickness of the shell being known to vary like every other structure.
It may be well here to remark that with all beings there must be much
fortuitous destruction, which can have little or no influence on the course
of natural selection. For instance, a vast number of eggs or seeds are
annually devoured, and these could be modified through natural selection
only if they varied in some manner which protected them from their enemies.
Yet many of these eggs or seeds would perhaps, if not destroyed, have
yielded individuals better adapted to their conditions of life than any of
those which happened to survive. So again a vast number of mature animals
and plants, whether or not they be the best adapted to their conditions,
must be annually destroyed by accidental causes, which would not be in the
least degree mitigated by certain changes of structure or constitution
which would in other ways be beneficial to the species. But let the
destruction of the adults be ever so heavy, if the number which can exist
in any district be not wholly kept down by such causes--or again let the
destruction of eggs or seeds be so great that only a hundredth or a
thousandth part are developed--yet of those which do survive, the best
adapted individuals, supposing that there is any variability in a
favourable direction, will tend to propagate their kind in larger numbers
than the less well adapted.
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