PARACHUTE FLIGHT 91 



homology nor with general structural convergence 

 or homoplasy. 



The case of Pteromys and Galeopithecus, 

 quoted above, may be regarded as an example 

 of the homoplastic modification of homogenetic 

 structures in different orders, i.e., ordinal con- 

 vergence ; but the homoplasy, in the literal sense, 

 is not complete, since there are important differ- 

 ences of structural detail, the digits of Galeopithecus 

 being webbed, 1 and the wrist of Pteromys carry- 

 ing an accessory cartilaginous rod which gives 

 material support to the parachute. 



The degrees of convergence are endless and 

 can only be classified in a broad way ; it is, we 

 may repeat, a fundamental phenomenon and con- 

 sequently of frequent occurrence, though not 

 always equally demonstrative. Very striking are 

 those cases where special features which are 

 exceptional within a certain limited systematic 

 range are repeated independently, as the phos- 

 phorescent and electric organs of fishes. The 

 late Professor Howes, in one of his addresses, 

 quoted as a critical example of convergence the 

 fact that the Mesozoic reptiles yielded terrestrial, 

 aquatic, and aerial forms, just as did the Tertiary 

 mammals which replaced the former in order of 

 dominance. 



Amongst animals which are more or less 



1 Flower and Lydekker, "Mammals Living and Extinct," 

 London, 1891, Fig. on p. 615. 



