DIFFICULTIES OF THE THEORY 143 



other Insectivora, yet there is no difficulty in supposing that such 

 links formerly existed, and that each was developed in the same 

 manner as with the less perfectly gliding squirrels; each grade of 

 structure having been useful to its possessor. Nor can I see any 

 insuperable difficulty in further believing that the membrane con- 

 nected fingers and forearm of the Galeopithecus might have been 

 greatly lengthened by natural selection; and this, as far as the 

 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 neces- 

 sarily 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 transi- 

 tional 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 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 con- 



