Furthermore, we may conclude that transitional grades
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 of diversified and of changed
habits in the individuals of the same species. When either case
occurs, it would be easy for natural selection to fit the animal, by
some modification of its structure, for its changed habits, or
exclusively for one of its several different habits. But it is
difficult to tell, 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 change 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.
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