DIFFICULTIES ON THEORY 165 



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 of 

 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. Of diversified habits innumerable instances 

 could be given : I have often watched a tyrant fly- 

 catcher (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 king- 

 fisher at a fish. In our own country the larger 

 titmouse (Parus major) may be seen climbing branches, 

 almost like a creeper ; it often, 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 sometimes see individuals of a species following 

 habits widely different from those of their own species 

 and of the other species of the same genus, we might 

 expect, on my theory, that such individuals would 



