3Ji FROM FISH TO PHILOSOPHER 



been no centralization of the nervous system such as 

 ultimately produced the brain, and the earth would 

 never have known the phenomenon of consciousness, at 

 least of an order superior to that of the lobster, scorpion, 

 or butterfly. 



But without an internal environment of relatively con- 

 stant composition in which complicated nerves, muscles, 

 and glands could attain a high degree of elaboration 

 and function in security, it is unlikely that the fresh- 

 water fishes, with their elaborate sensory and motor 

 equipment, would ever have been evolved. It was the 

 evolution of the kidney that suppHed the vertebrates 

 with this stable body fluid. 



The most important constituents in the body fluid of all 

 vertebrates are water and sodium chloride— ordinary 

 table salt, the primary salt of the sea and of all plants 

 and animals. One cannot, in connection with Hving or- 

 ganisms, think of water without salt or salt without 

 water. All marine invertebrates are isomotic with their 

 environment— in other words, their body fluids have the 

 same salt content, and hence the same osmotic pressure, 

 as the sea water in which they Hve. The maintenance of 

 salt and water balance is therefore a relatively simple 

 matter. Excess salt is partly, if not entirely, excreted 

 through the respiratory epitheliimi; the problem of ex- 

 creting water per se does not exist. The kidney is a glan- 

 dular organ of greater or lesser complexity and highly 

 varied in structure in different animals; it is concerned 

 chiefly with the excretion of such waste products as can- 

 not escape from the body by simple diffusion. It may be 

 assumed that such was the situation in the marine an- 

 cestor of the vertebrates. In the evolution of the verte- 

 brates, however, the kidney came into a new and im- 

 portant role: it became the chief route for the excretion 

 of water from the body, an operation that had to be 

 carried out without loss of salt, and thus it came to be 

 responsible for the regulation of the composition of the 



