TRANSITIONS OF ORGANIC BEINGS 187 



'fectly winged animals. If this had been cfTcctcd, who would 

 have 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 par- 

 ticular habit, as the wings of a bird for flight, we should bear 

 in mind that animals displaying early transitional 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 selec- 

 tion. 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 num- 

 bers 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 state of perfection, 

 so as to have given them a decided advantage over other ani- 

 mals in the battle for life. Hence the chance of discovering 

 species with transitional grades of structure in a fossil con- 

 dition 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 both of diversified 

 and of changed habits in the individuals of the same species. 

 In either case it would be easy for natural selection to adapt 

 the structure of the animal to its changed habits, or exclu- 

 sively to one of its several habits. It is, however, difficult 

 to decide, and immaterial for us, whether habits generally 

 change first and structure afterwards; or whether slight 

 modifications of structure lead to changed habits ; both prob- 

 ably often occurring almost simultaneously. Of cases of 

 changed habits it will sufiice merely to allude to that of the 

 many British insects which now feed on exotic plants, or ex- 

 clusively on artificial substances. Of diversified habits innu- 

 merable instances could be given : I have often watched a 

 tyrant flycatcher (Saurophagus sulphuratus) in South Amer- 



