TRANSITIONS OF ORGANIC BEINGS. 16? 



fish, which now glide far through the air, slightly rising 

 and turning by the aid of their fluttering lins, might liave 

 been modified into perfectly winged animals. If this had 

 been effected, 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, so far as we know, to escape being de- 

 voured 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 selclom 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 con- 

 clude 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 sub- 

 ordinate forms. Thus, to return to our imaginary illus- 

 tration 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 perfec- 

 tion, 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 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 selec- 

 tion 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 immaterial for us, whether 

 habits generally change first and structure afterward; or 

 whether slight modifications of structure lead to changed 

 habits; both probably often occurring almost simulUne- 

 ouslv. Of cases of changed habits it will suffice mei-eiy to 

 allude to that of the many British insects which now teed 

 on exotic plants, or exclusively on artificial hubbtancee. 



