eivable that a structure like that of
the pitcher-plant could have been produced by accumulated effects of
function on structure; yet it is conceivable that successive selections
of favourable variations might have produced it; and the like holds of
the no less remarkable appliance of the Venus's Fly-trap, or the still
more astonishing one of that water-plant by which infant-fish are
captured. Though it is impossible to imagine how, by direct influence of
increased use, such dermal appendages as a porcupine's quills could have
been developed; yet, profiting as the members of a species otherwise
defenceless might do by the stiffness of their hairs, rendering them
unpleasant morsels to eat, it is a feasible supposition that from
successive survivals of individuals thus defended in the greatest
degrees, and the consequent growth in successive generations of hairs
into bristles, bristles into spines, spines into quills (for all these
are homologous), this change could have arisen. In like manner, the odd
inflatable bag of the bladder-nosed seal, the curious fishing-rod with
its worm-like appendage carried on the head of the _lophius_ or angler,
the spurs on the wings of certain birds, the weapons of the sword-fish
and saw-fish, the wattles of fowls, and numberless such peculiar
structures, though by no possibility explicable as due to effects of use
or disuse, are explicable as resulting from natural selection operating
in one or other way.

In the second place, while showing us how there have arisen countless
modifications in the forms, structures, and colours of each part, Mr.
Darwin has shown us how, by the establishment of favourable variations,
there may arise new parts. Though the first step in the production of
horns on the heads of various herbivorous animals, may have been the
growth of callosities consequent on the habit of butting--such
callosities thus functionally initiated being afterwards developed in
the most advantageous ways by selection; yet no explanation can be thus
given of the sudden appearance of a duplicate set of horns, as
occasionally happens in sheep: an addition which, where it proved
beneficial, might readily be made a permanent trait by natural
selection. Again, the modifications which follow use and disuse can by
no possibility account for changes in the numbers of vertebrae; but after
recognizing spontaneous, or rather fortuitous, variation as a factor, we
can see that where an additional ve