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 larvae. 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.
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