140 TRANSITIONS OF ORGANIC BEINGS [Chap. VI 



organs of flight are concerned, would have converted the animal 

 into a bat. In certain bats in which the wing-membrane extends 

 from the top of the shoulder to the tail and includes the hind-legs, 

 we perhaps see traces of an apparatus originally fitted for gliding 

 through the air rather than for flight. 



If about a dozen genera of birds were to become extinct, who 

 would have ventured to surmise that birds might have existed 

 ■which used their wings solely as flappers, like the 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 func- 

 tionally 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 

 necesarily 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 con- 

 ceivable 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 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 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 



