] 07 *^® mouse that never drinks 



is as mysterious as anything very well eould be. Dipo 

 "evolved" the capacity to produce water of metabolism 

 sufficiently to supply his needs and they are quite as great 

 as those of many other desert creatures. It is obviously a 

 gift with great "survival value." But why didn't all the 

 creatures, some of which certainly sometimes perish of 

 thirst, "evolve" it too? What inborn capacity, if that is 

 what it was, made him potentially independent of an ex- 

 ternal water supply? 



Moreover, while Dipodomys was evolving his unusually 

 effective gift for manufacturing water internally, he had 

 also to evolve at the same time a certain special efficiency 

 in an organ which is part of the standard equipment of all 

 except the very simplest animals, and actually has at least 

 an analogue even in the one-celled protozoa: namely the 

 kidney. One authority on this organ. Dr. Homer W. Smith, 

 Professor of Physiology at the New York University Col- 

 lege of Medicine, enthusiastically maintains that the whole 

 story of evolution can be told in terms of the evolution of 

 the kidney. He has also the highest admiration for Dipo's 

 renal equipment. 



Most of us, it seems, do the kidney, whether our own or 

 that of other animals, less than justice. We think of it as 

 merely an organ of excretion, but it is actually far more 

 than that. It is responsible for that "internal environment" 

 in which all our vital organs and the very cells themselves 

 live, isolated from the air which would be fatal. Ever since 

 the days of the first vertebrates it has had a very difficult 

 job to accomplish. So far as the invertebrate animals which 

 still live in the sea are concerned, they have the same salt 

 content as that of the sea itseK, and all their rudimentary 



