Chap. VI.] TRANSITIONS OF OEGANIC BEINGS. 219 



ever imagined that in an early transitional state they 

 had been the inhabitants 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 tran- 

 sitional 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. Fur- 

 thermore, 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 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 hav- 

 ing existed in lesser numbers, than in the case of species 

 with fully developed structures. 



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

 sifled 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 changed habits, or exclusively to one of its several 

 habits. It is, however, difficult to decide, and im- 



