accumulation of profitable variations at that age, and by their
inheritance at a corresponding age. If it profit a plant to have its
seeds more and more widely disseminated by the wind, I can see no
greater difficulty in this being effected through natural selection,
than in the cotton-planter increasing and improving by selection the
down in the pods on his cotton-trees. Natural selection may modify and
adapt the larva of an insect to a score of contingencies, wholly
different from those which concern the mature insect. These
modifications will no doubt affect, through the laws of correlation,
the structure of the adult; and probably in the case of those insects
which live only for a few hours, and which never feed, a large part of
their structure is merely the correlated result of successive changes
in the structure of their larv . So, conversely, modifications in the
adult will probably often affect the structure of the larva; but in all
cases natural selection will ensure that modifications consequent on
other modifications at a different period of life, shall not be in the
least degree injurious: for if they became so, they would cause the
extinction of the species.
Natural selection will modify the structure of the
young in relation to the parent, and of the parent in relation to the
young. In social animals it will adapt the structure of each individual
for the benefit of the community; if each in consequence profits by the
selected change. What natural selection cannot do, is to modify the
structure of one species, without giving it any advantage, for the good
of another species; and though statements to this effect may be found
in works of natural history, I cannot find one case which will bear
investigation. A structure used only once in an animal s whole life, if
of high importance to it, might be modified to any extent by natural
selection; for instance, the great jaws possessed by certain insects,
and used exclusively for opening the cocoon or the hard tip to the beak
of nestling birds, used for breaking the egg. It has been asserted,
that of the best short-beaked tumbler-pigeons more perish in the egg
than are able to get out of it; so that fanciers assist in the act of
hatching. 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 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
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