TRANSITIONS OF ORGANIC BEINGS. 157 



logger-headed 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 aptervx ? Yet the structure of each of these oirds 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, so 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 wa3^s, on r,he land and in the water, 

 until their organs of flight had come to a high stage of per- 

 fection, 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 structiwSfr 



