been supplanted by their
successors, which were gradually rendered more perfect through natural
selection. Furthermore, we may conclude that transitional states between
structures fitted for very different habits of life will rarely have
been developed at an early period in great numbers and under many
subordinate forms. Thus, to return to our imaginary illustration of
the flying-fish, it does not seem probable that fishes capable of true
flight would have been developed under many subordinate forms, for
taking prey of many kinds in many ways, on the land and in the water,
until their organs of flight had come to a high stage of perfection,
so as to have given them a decided advantage over other animals in
the battle for life. Hence the chance of discovering species with
transitional grades of structure in a fossil condition will always be
less, from their having existed in lesser numbers, than in the case of
species with fully developed structures.

I will now give two or three instances, both of diversified and of
changed habits, in the individuals of the same species. In either case
it would be easy for natural selection to adapt the structure of the
animal to its changed habits, or exclusively to one of its several
habits. It is, however, difficult to decide and immaterial for us,
whether habits generally change first and structure afterwards; or
whether slight modifications of structure lead to changed habits; both
probably often occurring almost simultaneously. Of cases of changed
habits it will suffice merely to allude to that of the many British
insects which now feed on exotic plants, or exclusively on artificial
substances. Of diversified habits innumerable instances could be given:
I have often watched a tyrant flycatcher (Saurophagus sulphuratus) in
South America, hovering over one spot and then proceeding to another,
like a kestrel, and at other times standing stationary on the margin of
water, and then dashing into it like a kingfisher at a fish. In our own
country the larger titmouse (Parus major) may be seen climbing branches,
almost like a creeper; it sometimes, like a shrike, kills small birds by
blows on the head; and I have many times seen and heard it hammering the
seeds of the yew on a branch, and thus breaking them like a nuthatch. In
North America the black bear was seen by Hearne swimming for hours with
widely open mouth, thus catching, almost like a whale, insects in the
water.

As we someti