FLYING ANIMALS. 37 



membrane of a Pterodactyle corresponds either with 

 the one marked 4 or that marked 5 in Fig. 13 (probably 

 the latter), and it may therefore be said that while a 

 Pterodactyle flies with one finger, a Bat flies with its 

 whole hand. Equally marked is the difference between 

 the wing of a Bat and that of a Bird ; the latter having 

 only the first three fingers of the Bat's wing developed, 

 and all of these being strangely modified from the 

 ordinary form, while the chief elongation has taken 

 place in the bones of the arm and fore-arm, instead of 

 in those of the fingers, and flight is effected by the aid 

 of feathers instead of by a membrane. 



This completes our survey of the various modes of 

 flight obtaining in the animal kingdom. In it we have 

 indicated the difference between spurious and true 

 flight, have shown how the former is but an extreme 

 development of the long leaps taken by arboreal 

 animals, and have suggested how it may have gradually 

 passed onwards into true flight. We have also seen 

 how the wings of the Invertebrate animals differ in 

 toto from those of the Vertebrates; while among the 

 Vertebrates true flight has been independently deve- 

 loped in three distinct groups Pterodactyles, Birds, 

 and Bats on totally different structural lines; the 

 latter instance thus affording us an excellent example 

 of the way in which different groups of animals may 

 be variously modified to occupy the same position in 

 the realm of nature. The supersession of the Ptero- 

 dactyles by the Birds as the lords of the air is in 

 accordance with what we have observed elsewhere, 



