instincts.

All spiders have the same spinning apparatus, but one kind weaves
radiating webs, another irregular ones, while a third makes none at
all, but lives in holes, whose walls it overspins, and whose entrance
it closes with a door. Almost all birds have a like organisation for
the construction of their nests (a beak and feet), but how infinitely
do their nests vary in appearance, mode of construction, attachment
to surrounding objects (they stand, are glued on, hang, &c.),
selection of site (caves, holes, corners, forks of trees, shrubs, the
ground), and excellence of workmanship; how often, too, are they not
varied in the species of a single genus, as of parus. Many birds,
moreover, build no nest at all. The difference in the songs of birds
are in like manner independent of the special construction of their
voice apparatus, nor do the modes of nest construction that obtain
among ants and bees depend upon their bodily organisation.
Organisation, as a general rule, only renders the bird capable of
singing, as giving it an apparatus with which to sing at all, but it
has nothing to do with the specific character of the execution . . .
The nursing, defence, and education of offspring cannot be considered
as in any way more dependent upon bodily organisation; nor yet the
sites which insects choose for the laying of their eggs; nor, again,
the selection of deposits of spawn, of their own species, by male
fish for impregnation. The rabbit burrows, the hare does not, though
both have the same burrowing apparatus. The hare, however, has less
need of a subterranean place of refuge by reason of its greater
swiftness. Some birds, with excellent powers of flight, are
nevertheless stationary in their habits, as the secretary falcon and
certain other birds of prey; while even such moderate fliers as
quails are sometimes known to make very distant migrations.