LIZAEDS, FROGS AND TOADS 377 



noticed in eastern South America, but to judge from the cir- 

 cumstance that one species lives in east Africa, another in west 

 Africa, a third in Ecuador, and a fourth in Central America, 

 according to Dr. Boulenger,* we may conclude that the genus 

 will probably turn up in Guiana. 



The frogs and toads are divided into two groups, one of which 

 (a very small one) contains those that are devoid of a tongue, 

 while all others possess this useful organ. The tongueless 

 toads are generally looked upon as the most primitive of the 

 tailless amphibians. Three genera of these creatures are 

 known to science, two (Xenopus and Hymenochirus) inhabit- 

 ing Africa south of the Sahara, the other (Pipa) eastern 

 South America. The only member of the last genus is the 

 remarkable Surinam toad (Pipa americana) whose skin on 

 the back of the female is provided with pits in which the 

 eggs are placed and the young undergo their whole metamor- 

 phosis. These tongueless toads (Aglossa) are not known 

 as fossils. We have thus to conjecture the origin of their dis- 

 persal from their present distribution which is highly sug- 

 gestive of a former direct land bridge between Africa and 

 South America. This view is considerably strengthened by 

 the fact that the West African Hymenochirus is in several 

 respects more closely related to Pipa than to Xenopus. 



The toothless frogs (Dendrobatidae) appear at first sight 

 to be typical examples of a group which has obtained its pre- 

 sent range by means of a former land bridge between South 

 America and Africa. They occur only in northern South 

 America, in west Africa and Madagascar, a discontinuous dis- 

 tribution which betokens antiquity, and which is not unknown 

 among other animals and plants. Yet Dr. Gadowf argues 

 that the Old World and New World genera were evolved inde- 

 pendently from toothed frogs, that this is, in fact, a case of 

 convergence. It may be so, but his arguments are by no 

 means convincing. 



I may once more be permitted to draw attention here to 

 the opinion long ago expressed by Dr. Gill J that fishes are 



* Boulenger, G. A., " Synopsis of apodal Batrachians," p. 404. 



t Gadow, H., " Amphibia and Reptiles," p. 272. 



I Gill, T., " Principles of Zoogeography," pp. 29 30. 



