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.
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