230 Amphibia 



to similar conditions of life, and thus, although descended 

 from different ancestors, come to resemble one another to 

 a greater or less degree. Where the structure was originally 

 fundamentally different the resemblances are only superficial 

 and the essential differences are easily distinguished, as for 

 example, when we compare a bat and a bird, or a whale and 

 a fish. When on the other hand, the forms in question are 

 originally somewhat closely related, and belong to the same 

 group, adaptive convergence may produce a much greater sim- 

 ilarity of structure, so that it is often difficult to distinguish 

 such similarity from true affinity or community of descent. 

 This is well illustrated by the Anura adapted to arboreal life, 

 commonly known as tree-frogs. The arboreal species all agree 

 in the possession of adhesive discs on the toes, which are neces- 

 sary to enable them to cling to the foliage, but when we ex- 

 amine the internal anatomy we find that they often differ 

 greatly from one another, and show affinities to different groups 

 and to other species which may be aquatic, terrestrial, or 

 burrowing. Although the discs have a similar structure and 

 act in the same way, the structure of the terminal claws which 

 support the discs differs considerably, as we have already seen, 

 and the internal anatomy, such as the structure of the pectoral 

 girdle, may be very different in forms equally arboreal. Thus 

 the Hylidae belong to the Arcifera and are therefore more 

 closely related to the toads than to the true frogs, Hylodes be- 

 longs to another group, the Cystignathidae, and the Dendro- 

 batinae are arboreal modifications of the true frogs or Ranidae. 

 According to Dr. Gadow this last subfamily itself is not a 

 natural group, that is, not descended from common ancestors, 

 but a convergent group. They have been separated from the 

 other Ranidae on account of the absence of teeth, but the fact 

 that some occur in South America, and others in Madagascar 

 and Africa, suggests that they are merely arboreal adaptations 

 of the Ranidae living in those different parts of the world. As 

 Dr. Gadow puts it, it seems that the tropical forests must have 

 tree-frogs, and these are manufactured in each region from the 

 material that happens to be handy, that is to say, from the 

 Anura that were already living in the region. Tree-frogs are 

 therefore not all related together, but those of each region are 

 most closely related to the other Anura of that region. 



