some
direct effect. It is also necessary to bear in mind that, owing to
the law of correlation, when one part varies and the variations are
accumulated through natural selection, other modifications, often of the
most unexpected nature, will ensue.

As we see that those variations which, under domestication, appear at
any particular period of life, tend to reappear in the offspring at the
same period; for instance, in the shape, size and flavour of the seeds
of the many varieties of our culinary and agricultural plants; in the
caterpillar and cocoon stages of the varieties of the silkworm; in the
eggs of poultry, and in the colour of the down of their chickens; in the
horns of our sheep and cattle when nearly adult; so in a state of nature
natural selection will be enabled to act on and modify organic beings at
any age, by the accumulation of variations profitable 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; 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