CHAPTER VIII. INSTINCT.
7. 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 to neglect other materials and to make its nest
exclusively of inspissated saliva? And so in other cases. It must,
however, be admitted that in many instances we cannot conjecture whether it
was instinct or structure which first varied.