wax, having succeeded best, and having transmitted
their newly-acquired economical instincts to new swarms, which in their
turn will have had the best chance of succeeding in the struggle for
existence.

OBJECTIONS TO THE THEORY OF NATURAL SELECTION AS APPLIED TO INSTINCTS:
NEUTER AND STERILE INSECTS.

It has been objected to the foregoing view of the origin of instincts
that "the variations of structure and of instinct must have been
simultaneous and accurately adjusted to each other, as a modification
in the one without an immediate corresponding change in the other would
have been fatal." The force of this objection rests entirely on the
assumption that the changes in the instincts and structure are abrupt.
To take as an illustration the case of the larger titmouse, (Parus
major) alluded to in a previous chapter; this bird often holds the seeds
of the yew between its feet on a branch, and hammers with its beak till
it gets at the kernel. Now what special difficulty would there be in
natural selection preserving all the slight individual variations in the
shape of the beak, which were better and better adapted to break open
the seeds, until a beak was formed, as well constructed for this purpose
as that of the nuthatch, at the same time that habit, or compulsion, or
spontaneous variations of taste, led the bird to become more and more of
a seed-eater? In this case the beak is supposed to be slowly modified
by natural selection, subsequently to, but in accordance with, slowly
changing habits or taste; but let the feet of the titmouse vary and grow
larger from correlation with the beak, or from any other unknown cause,
and it is not improbable that such larger feet would lead the bird to
climb more and more until it acquired the remarkable climbing instinct
and power of the nuthatch. In this case a gradual change of structure is
supposed to lead to changed instinctive habits. To take one more case:
few instincts are more remarkable than that which leads the swift of
the Eastern Islands to make its nest wholly of inspissated saliva. Some
birds build their nests of mud, believed to be moistened with saliva;
and one of the swifts of North America makes its nest (as I have
seen) of sticks agglutinated with saliva, and even with flakes of this
substance. Is it then very improbable that the natural selection of
individual swifts, which secreted more and more saliva, should at last
produce a species with instincts leading it