622 Prof. Macallum. Inorganic Composition of the [June 23, 



against the elimination of waste nitrogenous matter, and this function, after 

 the lapse of a long period of time, became fixed in the Ganoid kidney, with 

 the result that, when the Teleosts arose, their kidneys had developed two 

 very fixed functions, instead of one as in those of the Elasniobranchs, and 

 these two have been transmitted with undiminished or slightly lessened force 

 to their descendants, whether of fresh water or marine habitat, through the. 

 millions of years which have elapsed since the J urassic. Thus, in the blood 

 of the cod there is no accumulation of urea beyond the limit that is found in 

 the blood of the higher Vertebrates. 



It is easy to understand how a uniformity of composition of the internal 

 medium of animals is a powerful factor in influencing the course of evolution. 

 The capacity of the organism to make and keep its own internal media 

 uniform gives an enormous advantage to it, for it can change its habitat and 

 adapt itself to a new environment without affecting the stable conditions 

 under which its own tissues and organs do their best work. 



This independence of external mediji is much more characteristic of 

 Vertebrates than of any Invertebrates, and, in fact, it may be regarded as a. 

 special feature of Vertebrate life. It is, indeed, difficult to conceive how 

 Vertebrates could have arisen and undergone the extraordinary development 

 and adaptation to either terrestrial or aquatic life which they have expe- 

 rienced in geological time if their internal medium had not been maintained 

 constant. 



The establishment of that constant internal medium would therefore appear 

 to have been the first step in the evolution of Vertebrates from an Inver- 

 tebrate form. That, on the otlier hand, postulates that the kidney, developed 

 to regulate and keep constant the internal or circulatory fluid, was essentially 

 the first typically Vertebrate organ, and therefore of origin more ancient than 

 that of the Vertebrate brain and spinal cord. 



That to-day the earliest appearance in the Vertebrate embryo of structures 

 which are subsequently to develop into the kidney is after the neural groove 

 arises, constitutes, apparently, an objection to tliis view which can, however, 

 be met. The distinctive parts of the ovary and testis of the Invertebrate 

 form out of whicli tlie Protovertebratc developed undoubtedly gave rise to 

 the distinctive parts of the ovary and testis in the Vertebrate. The repro- 

 ductive cells of tlio ovary and testis are, therefore, of origin perhaps as remote 

 in time as tlic origin of the Mctazoa. In the Vertebrate embryo, however, 

 the distinctivcdy reproductive ('](;in(Mits mnke their appearance at a dat(^ later 

 than tliat at whicli the ncur;i,l tube ai'iHOS, and tliis retardation is, without 

 lioulit, (lu(^ to the clfect exercised l)y the jxjstponement of the; time when the 

 Koxual function begins to operatic in tlu; individual of a s])ecies. It may thus 



