DIFFICULTIES OF THE THEORY 245 



this had been effected, who would have ever imagined 

 that in an early transitional state they had been the in- 

 habitants of the open ocean, and had used their incipient 

 organs of flight exclusively, as far as we know, to escape 

 being devoured by other fish ? 



When we see any structure highly perfected for any 

 particular habit, as the wings of a bird for flight, we 

 should bear in mind that animals displaying early transi- 

 tional grades of the structure will seldom have survived 

 to the present day, for they will have been supplanted 

 by their successors, which were gradually rendered more 

 perfect through natural selection. Furthermore, we may 

 conclude that transitional states 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 imagi- 

 nary illustration of tiae flying-fish, it does not seem prob- 

 able 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 transi- 

 tional grades of structure in a fossil condition will al- 

 ways be less, from their having existed in lesser num- 

 bers, than in the case of species with fully developed 

 structures. 



I will now give two or three instances both of diver- 

 sified and of changed habits in the individuals of the 

 same species. In either case it would be easy for natu- 

 ral selection to adapt the structure of the animal to its 



