10(5 TRANSITIONS OF ORGANIC BEINGS. 



which was formerly ranked among bats, but is now believed 

 to belong to the Insectivora. An extremely wide flank- 

 membrane stretches from the corners of the jaw to the tail 

 and inckides 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 Galeopi- 

 thecus with the other Insectivora, yet there is no dif^ 

 ficiilty in supposing that such links formerly existed, 

 and that each was developed in the same manner 

 as with the less perfectly gUding squirrels; each grade 

 of structure having been useful to its possessor. Nor 

 can I see any insuperable difficulty in further believ- 

 ing that the membrane connected 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 hand, 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 necessarially the best possible under all possible con- 

 ditions. 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 reptilesj it is conceivable that flying- 



