624 Ijiorganic Composition of the Blood in Vertebrates, etc. 



is, that the sea contained 0"4 per cent, of sodium chloride. If we accept this, 

 it follows that, when the Vertebrate kidney began to evolve, the sodium 

 chloride in the sea had increased only 0"3 per cent., and since then as much 

 as 2"1 per cent. This would place the origin of the Eovertebrate form at a 

 date not earlier than the second eighth, and not later than the first fourth 

 of the whole geological period. 



We know, from the results of the analyses given above, that the magnesium 

 in the blood of Liimdus lags behind that of the ocean, which is ever growing 

 in amount. The ratio between it and the sodium in mammal's blood may be 

 put approximately 1 : 100. The ocean, then, in Eovertebrate time would 

 have a higher magnesium content in relation to sodium, approximately 

 l"50-2'00 : 100, or one-eighth to one-sixth of the magnesium concentration 

 of the ocean of to-day. These estimates also would place the origin of the- 

 Vertebrate kidney at a time somewhere between the beginning of the second 

 eighth and that of the second sixth of the whole geological period. 



These are speculations which are advanced with reserve. They may he- 

 accepted absolutely, or rejected wholly, only when we are in possession 

 of the results of analyses of the blood plasma in all the representative- 

 Vertebrate classes, as well as of the blood of the higher types of Inver- 

 tebrates. 



Enough, however, has been advanced here to make it extremely probable 

 that the inorganic composition of the blood plasma of Vertebrates is an 

 heirloom of life in the primeval ocean. 



