Il8 THE EVOLUTION OF MAN. 



single genera (Ceratodus) could alone endure ; these occur, 

 for instance, in the Trias. At the present time there are 

 only three extant genera of this whole class : Protopterus 

 annectens, in the rivers of tropical Africa (White Nile, 

 Niger, Quillimane, etc.) ; Lepidosiren paradoxa, in tropical 

 South America (in the tributaries of the Amazon); and 

 Ceratodus Fosteri, in the swamps of Southern Australia 

 (Plate XII.). 149 This wide distribution of the three isolated 

 descendants of the class is alone sufficient to prove that 

 they are the last remnants of a group which was formerly 

 very widely developed. The whole structure of their 

 bodies shows that the group to which they belong forms tho 

 transition between Fishes and Amphibia. The direct tran- 

 sitional structure between the two classes is so clearly 

 expressed in the whole organization of these curious animals, 

 that zoologists yet dispute whether the Dipneusta are 

 Fishes or Amphibia. Some well-known zoologists still class 

 them among Amphibia, while they are usually placed 

 among Fishes. In fact, the characters of both the classes 

 are so united in the Dipneusta that the answer to the 

 question as to their nature depends entirely upon the mean- 

 ing attached to the terms "Fish" and "Amphibian." In 

 their mode of life they are true Amphibia. During the 

 tropical winter, in the rainy season, they swim in the water 

 like Fishes and inhale water through the gills. During the 

 dry season they burrow in the mud as it dries up, and 

 during that period breathe air through lungs, like Am- 

 phibians and higher Vertebrates. In this two-fold respira- 

 tion they do, it is true, coincide with the lower Amphibia, 

 and stand far above Fishes. Yet, in most other characters 

 they more nearly resemble the latter, and stand below the 



