Chap. VI, 
TRANSITIONAL HABITS. 
183 
sitional grades of the structure will seldom continue to 
exist to the present day, for they will have been sup¬ 
planted by the very process of perfection through natural 
selection. Furthermore, we may conclude that transi¬ 
tional 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 fiight 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 fiight 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 struc¬ 
ture 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. Of diver¬ 
sified habits innumerable instances could be given: I 
have often watched a tyrant fiycatcher (Saurophagus 
sulphuratus) in South America, hovering over one spot 
