WING FLIGHT 89 



expansion of the pectoral fins, an exceptional 

 modification which has been acquired inde- 

 pendently within the limits of two very distinct 

 families. 



It is remarkable to find such strictly homo- 

 logous organs as the pectoral fins of Teleostean 

 fishes modified in a virtually identical manner 

 to perform a special and exceptional function, 

 whose transformation is nevertheless not homo- 

 genetic but homoplastic. This typical example 

 serves to illustrate one of the most interesting 

 manifestations of convergence, namely, the homo- 

 plastic modification of homogenetic structures. 



The comparison between the wings of birds 

 and of bats offers different conditions. They are 

 equally efficient as organs of sustained aerial 

 flight ; they show complete functional equiva- 

 lence ; the fore-limbs are homologous ; but the 

 transformation has proceeded along divergent 

 lines, an expanded wing-membrane or patagium 

 with reduced epidermal appendages (hairs) in the 

 one case ; a rudimentary patagium and enlarged 

 epidermal appendages (feathers) in the other; with 

 corresponding differences affecting the skeleton of 

 the wing-supporting limbs, hypertrophied digits 

 in the bat, atrophied digits in the bird. Unlike 

 the flying fishes, the modification has not taken 

 place in an identical manner, but in an almost 

 diametrically opposite manner ; it cannot there- 

 fore be described as homoplastic in the strict 



