The Evolution of the Kidney 63 



This interpretation receives support in the fact that in 

 the bird kidney the tubules are of a mixed type, some resem- 

 bhng the reptihan tubule in lacking a thin segment, some re- 

 sembling the mammalian tubule in possessing such a segment. 

 Functionally the bird kidney is intermediate between the 

 reptiles and the mammals, the bird retaining the uric acid 

 habitus of the former, although it can under certain condi- 

 tions elaborate a distinctly hypertonic urine/^ The similarity 

 to the mammalian kidney in the last respect is probably a 

 case of convergent evolution fostered by the common char- 

 acter of warm-bloodedness, for the birds were evolved from 

 reptiles that were far removed from the mammalian stem. 



When, at the close of the Cretaceous, the dinosaurs became 

 extinct, the mammals began to populate the earth. In the 

 Paleocene the lemuroids took to living in the trees and be- 

 came the Eocene tarsioids who looked forward with both eyes 

 at the same time and depended upon the sense of sight rather 

 than upon smell or hearing. In the Oligocene a tarsioid or 

 lemuroid stock gave rise to the monkeys which in the Mio- 

 cene in turn spawned the Dryopithecine apes that roamed over 

 Europe, Africa, and Asia. Then the rising Himalayas buckled 

 central Asia into an uninhabitable mountain chain, and such 

 of the Dryopithecine apes as survived were driven to abandon 

 the trees and to seek their living in the southern plains. From 

 Asia a Dryopithecine descendent migrated into Africa, to 

 spawn there in the Pliocene such forms as Australopithectis 

 africanus, discovered by Dart,^ and Pie slant hr opus trans- 

 vaalensis and Paranthropus robustus, discovered by Broom^* 

 in 1938, and declared by their discoverers and by Gregory 

 and Hellman^^ to be truly neither ape nor man. (For a gen- 

 eral discussion of the origin of man see 3 6. ) 



The kidney is not identical in structure and function in 



