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.
No doubt many instincts of very difficult explanation could be opposed to
the theory of natural selection--cases, in which we cannot see how an
instinct could have originated; cases, in which no intermediate gradations
are known to exist; cases of instincts of such trifling importance, that
they could hardly have been acted on by natural selection; cases of
instincts almost identically the same in animals so remote in the scale of
nature that we cannot account for their similarity by inheritance from a
common progenitor, and consequently must believe that they were
independently acquired through natural selection.
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