TRANSFORMATION OF ORGANS OF ANIMALS 67 



its function, is obviously in process of development, 

 for it has not only retained all five digits, but, what 

 is functionally a sixth digit, has made its appear- 

 ance (rs.). The phalanges are very well developed 

 (phal.), but, on the other hand, the distal phalanges 

 (ph.) have degenerated and are very short. 



Heterocephalus (fig. 36) is a burrowing rodent, 

 only, instead of being talpoide as is the Bathyergus, 

 i.e. instead of exhibiting a body furnished with 



FIG. 36. Heterocephalus Philippi. (After Oldfleld Thomas.) 



strong short legs, Heterocephalus looks more like 

 an ordinary quadruped. In process of adaptation 

 to an underground life, the hair, especially that on 

 the legs, has disappeared. This is the more re- 

 markable, as in no other case does it occur among 

 mammals unless as an adaptation to aquatic life. 

 Here it is in the atrophy of the roots of the hair 

 that degeneration is manifested. 



One form of hair, however, is still to be found 

 on Heterocephalus, for there are a few bristles on 

 the outer sides of the feet. These serve as brushes 

 to sweep away the sand while burrowing. 



