Chap. VI. TRANSITIONAL HABITS. 183 



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 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 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 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 like a kingfisher at a fish. In our 

 owp country the larger titmouse (Parus major) may be 



