The Evolution of the Kidney 59 



bed in order to supply the tubules with a feeble, irrigating 

 stream. 



At this point you are probably wondering if the title of 

 this discourse is not misrepresenting, since so much of it has 

 been devoted to the lower vertebrates and so little of it to 

 the mammals or to man. I would defend this apparent un- 

 fairness by pointing out that all the mammals together con- 

 stitute but a small fraction of the vertebrates, and man him- 

 self but a single mammalian species. The geological age of 

 truly human forms is at most 1,000,000 years, a slight inter- 

 val indeed out of the 500 to 600 million years which we must 

 apprehend if we are to see the human organism in the proper 

 perspective. But apart from this aspect of the problem I 

 must confess that at this point in the story of the evolution 

 of the kidney there is a serious hiatus in our knowledge, 

 namely, the circumstances surrounding the evolution of the 

 first mammalian forms. 



The mammals have added the only important patent to 

 the kidney since Devonian time : the capacity to excrete urine 

 that is markedly hypertonic, or osmotically more concen- 

 trated than the blood. As pointed out in an earlier paragraph, 

 the elaboration of this hypertonic urine is probably effected 

 by the unique, intermediate thin segment which is present in 

 the tubule of all mammalian forms. 



We must inquire, how did this capacity to excrete a hy- 

 pertonic urine come to be evolved? And we may go on to 

 ask, since the mammals were evolved from reptilian forms, 

 why do they not excrete uric acid like the reptiles and the 

 birds? And since the mammals do not generally live in fresh 

 water, since in fact some mammals, such as the kangaroo rat, 

 can live indefinitely upon dry oatmeal, while others, such as 

 the whales and seals, can live indefinitely in the sea without 



