Even the quite young aphides behaved in
this manner, showing that the action was instinctive, and not the
result of experience. But as the excretion is extremely viscid, it is
probably a convenience to the aphides to have it removed; and
therefore probably the aphides do not instinctively excrete for the
sole good of the ants. Although I do not believe that any animal in
the world performs an action for the exclusive good of another of a
distinct species, yet each species tries to take advantage of the
instincts of others, as each takes advantage of the weaker bodily
structure of others. So again, in some few cases, 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 have been
here 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 nests of the same species in the
northern and southern United States.
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