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 effect, 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 insure
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. 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 he