PHYLOGENY AND CLASSIFICATION. 91. 



As a rule, evolution leads to specialisation and differ- 

 entiation along the ever diverging and forking branches 

 of the phylogenetic tree. Usually the more diverse 

 organisms become the more successful they are in the 

 struggle for existence, since they interfere less with each 

 other owing to their adoption of different modes of life 

 and different feeding habits. On a given plot of ground 

 more individual plants can live if they belong to several 



FIG. 2. Convergent evolution. Salamandra (1), a normal Urodele Am- 

 phibian ; and Siphonops (2), a legless Amphibian ; Agama (4), a nor- 

 mal Lacertilian ; and Amphisbaena (5), a legless Lacertilian. (2) 

 and (5) being adapted to a burrowing life have come to resemble 

 the Earth-worm (3) and each other. 



species adapted in different ways, than if they belong to 

 one species only. But occasionally evolution leads to 

 convergence in function and structure. Organisms and 

 their parts may then come to resemble each other, an- 

 alogies are developed. Thus a burrowing snake like 

 Typhlops, a burrowing lizard like Amphisbaena, or 

 amphibian like Siphonops, acquire a resemblance to an 

 earthworm (Fig. 2) ; or again, among the Mammalia, the 

 " flying " squirrel, Pteromys, and the " flying " phalanger, 



