EXISTING ORGANISMS— CLASSIFICATION 27 



is apparently unpalatable because of the bitterness of its food 

 plant. 



Convergence. Resemblance between animals of different 

 groups is usually due to convergence as fundamentally different 

 organisms become adapted to the same conditions of environ- 

 ment. The fishes are aquatic organisms, admirably adapted to 

 the conditions of their environment. Whales, seals and dolphins 

 arc fish-like in many ways, but they show their terrestrial origin 

 in that they must breathe air. Their points of reseml)lance to the 

 fishes are due solely to the fact that life in the water is possible only 

 if certain conditions of form and locomotion can be met, and the 

 physical conditions of the ocean are such as to limit the ways in 

 which they can be met. The tail of the dolphin is somewhat like 

 that of the fish, and serves the same purpose^; likewise the wings of 

 insects and of liirds are superficially similar, and have the same 

 function, although they are fundamentally different structures. 

 This resemblance of unlike structures which comes about 

 through the adaptation of different organisms to similar con- 

 ditions is known as analogy, and is a common corollary of conver- 

 gence. 



Divergence. Relationship may also be obscured by the differ- 

 ences in development of closely related animals. Man is a mammal, 

 the squirrel is a mammal, and the seal is a mammal, but man has 

 assumed the erect posture for terrestrial life, the squirrel lives in 

 trees, and the seal is almost wholly aquatic. The result is a com- 

 plete overshadowing of their fundamental similarity, yet the flip- 

 pers of the seal and the front paws of the squirrel are the same 

 vertebrate structures as the hands of man. The term adaptive 

 radiation is applied to this divergence of related forms, and the 

 related structures which thus assume superficial difi^erences are 

 said to be homologous. 



Accuracy of Classification. Such accuracy is wholly dependent 

 upon the ability of the taxonomist to go beyond the superficial 

 characteristics of the organism and to interpret fundamental con- 

 ditions. This has been accomplished to a marked degree in bring- 

 ing our classification to its present state of reasonable perfection. 

 Studies of evolution have been invaluable in this development in 

 that they have brought about a keener realization of relationship 

 between organisms and the progressive nature of such relation- 

 ship, and now that our classification is in such an excellent state, 



