642 



THE AMERICAN NATURALIST 



[Vol. L 



In this way we become aware of the need for information 

 to clear np the question which inevitably arises. 



It is commonly known that the blood and body fluids of 

 animals possess a certain osmotic pressure. Life proc- 

 esses are constantly dependent on the passage of ma- 

 terials in and out of cells and differences in the osmotic 

 pressure of substances within and without the cell are 

 held to be one cause of this mutual movement. Varia- 

 tions in the osmotic pressure of the blood and body fluids 

 of animals are not so generally known. In the case of 

 severe hemorrhage it is a common practice to replace the 

 lost blood by a physiological salt solution which has the 

 same osmotic pressure as that of the blood. Formerly a 

 0.7 per cent, saline solution was used. This is isotonic 

 with amphibian blood. The reason for this was that the 

 fact was first discovered in a study of frog's blood. The 

 saline solution (based on amphibian studies) of the physi- 

 ological laboratories was considered proper for use in 

 hospitals as well. Later it was found that a 0.9 per cent, 

 saline solution represents more nearly the composition of 

 human blood and this solution is in use at present. 



But why does human blood have an osmotic pressure 

 equivalent to that of a 0.9 per cent, saline solution? In 

 order to answer this question we must examine all avail- 

 able data as to the osmotic pressure of the blood and body 

 fluids of animals in general. When this is done it appears 

 in many cases at least as though the osmotic pressure of 

 the blood and body fluids were merely a direct adaptation 

 to the environment. But in other cases this is not so 

 clearly apparent, in fact the osmotic pressure possessed 

 by certain forms shows no evident adaptation to the en- 

 vironment at all. The terrestrial vertebrates illustrate 

 this last condition. It is only when we view the entire 

 question from the standpoint of evolution that the main 

 features of the puzzle become apparent. 



It might be well to explain at this point the meaning of 

 osmotic pressure. One gram molecule of hydrogen gas 

 at atmospheric pressure occupies 22.4 liters space, and to 

 confine this gas in a space of one liter would require a 



