186 ORIGIN OF SPECIES 



brane stretches from the comers of the jaw to the tail, and 

 includes the limbs with the elongated fingers. This flank- 

 membrane is furnished with an extensor muscle. Although 

 no graduated links of structure, fitted for gliding through the 

 air, now connect the Galeopithecus with the other Insec- 

 tivora, yet there is no difficulty to 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 connected fingers and fore-arm of the Galeopi- 

 thecus might have been greatly lengthened by natural selec- 

 tion; 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 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, fly- 

 ing 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 ^x^to per- 



