dislike to the ants: if the latter be not present they are at last
compelled to eject their excretion. But as the excretion is extremely
viscid, it is no doubt a convenience to the aphides to have it removed;
therefore probably they do not excrete solely for the good of the ants.
Although there is no evidence that any animal performs an action for the
exclusive good of another species, yet each tries to take advantage of the
instincts of others, as each takes advantage of the weaker bodily structure
of other species. So again certain instincts cannot be considered as
absolutely perfect; but as details on this and other such points are not
indispensable, they may be here passed over.

As some degree of variation in instincts under a state of nature, and the
inheritance of such variations, are indispensable for the action of natural
selection, as many instances as possible ought to be given; but want of
space prevents me. I can only assert that instincts certainly do vary--for
instance, the migratory instinct, both in extent and direction, and in its
total loss. So it is with the nests of birds, which vary partly in
dependence on the situations chosen, and on the nature and temperature of
the country inhabited, but often from causes wholly unknown to us. Audubon
has given several remarkable cases of differences in the nests of the same
species in the northern and southern United States. Why, it has been
asked, if instinct be variable, has it not granted to the bee "the ability
to use some other material when wax was deficient?" But what other natural
material could bees use? They will work, as I have seen, with wax hardened
with vermilion or softened with lard. Andrew Knight observed that his
bees, instead of laboriously collecting propolis, used a cement of wax and
turpentine, with which he had covered decorticated trees. It has lately
been shown that bees, instead of searching for pollen, will gladly use a
very different substance, namely, oatmeal. Fear of any particular enemy is
certainly an instinctive quality, as may be seen in nestling birds, though