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; and these modifications may affect, through
correlation, the structure of the adult. So, conversely, modifications in
the adult may affect the structure of the larva; but in all cases natural
selection will ensure that they shall not be injurious: for if they were
so, the species would become extinct.
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 whole
community; if the community 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 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, used exclusively for opening the cocoon--or the hard tip
to the beak of unhatched birds, used for breaking the eggs. It has been
asserted, that of the best short-beaked tumbler-pigeons a greater number
perish in the egg than are able to get out of it; so that fanciers assist
in the act of hatching.
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