9 o SPECIAL CONVERGENCE 



sense of the term, although it is frequently quoted 

 as a stereotyped example of homoplasy. Perhaps 

 it would be more logical to express it in other 

 language for which a suitable technical term does 

 not seem to be available, unless we may borrow 

 a word from another division of the subject and 

 call it a heterotypic modification of homogenetic 

 structures, the common starting-point being the 

 pentadactyle limb of terrestrial vertebrates. 



Birds and bats are directly comparable in terms 

 of the theory of convergence, not only because 

 they are alone amongst vertebrate animals in 

 their capacity for rapid and changing flight, but 

 also because they are both warm-blooded. Fly- 

 ing fishes bear the same relation to birds as they 

 do to bats, whatever that relation may be held 

 to be. In my opinion they are in no way com- 

 parable, inasmuch as the flight of fishes is of the 

 nature of a parachute flight and so belongs to a 

 different category. Although well enough twenty 

 years ago, it would not be admissible to-day to 

 figure a flying fish and a bat upon the same page 

 for the purpose of illustrating the principle of 

 homoplastic wing-formation, except in respect of 

 the one point of the hypertrophy of the pectoral 

 rays and digits. It would be more appropriate 

 to present a flying fish in conjunction with a 

 flying lizard, or even a flying squirrel, the nature 

 of the relation between them being one of general 

 functional convergence, neither associated with 



