CHAP. VI.] TRANSITIONS OF ORGANIC BEINGS. 131 



duck (Micropterus of Eyton) ; as fins in the water and as front- 

 legs on the land, like the penguin ; as sails, like the ostrich ; and 

 functionally for no purpose, like the Apteryx ? Yet the structure 

 of each of these birds is good for it, under the conditions of life to 

 which it is exposed, for each has to live by a struggle ; but it is 

 not necessarily the best possible under all possible conditions. It 

 must not be inferred from these remarks that any of the grades of 

 wing-structure here alluded to, which perhaps may all be the 

 result of disuse, indicate the steps by which birds actually 

 acquired their perfect power of flight ; but they serve to show 

 what diversified means of transition are at least possible. 



Seeing that a few members of such water-breathing classes as 

 the Crustacea and Mollusca are adapted to live on the land ; and 

 seeing that we have flying birds and mammals, flying insects of 

 the most diversified types, and formerly had flying reptiles, it is 

 conceivable that flying-fish, which now glide far through the air, 

 slightly rising and turning by the aid of their fluttering fins, might 

 have 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, as faf 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 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 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 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 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 exclusively to one of its 

 several habits. It is, however, difficult to decide, and immaterial 



