ADAPTATION TO ENVIRGNMENT WAEDLAW 405 



milk, SO that the osmotic pressure of the solution actually absorbed 

 differs materially from that of the body fluids of the young animal. 



When the composition of milk is examined, it is found that 

 although the proportions of the various constituents show consider- 

 able variations among different animals, and even among individuals 

 of the same species (Wardlaw, 1915, 1917, 1926), the same con- 

 stituents are present in each. The concentration of each constituent 

 appears to be adapted to the needs of each species. It will be remem- 

 bered that in considering the relation of the most primitive living 

 organisms to their environment, some of the evidence was mentioned 

 which seemed to point to a close relation between the proportions 

 of certain elements of inorganic compounds present in the surround- 

 ing medium, and the proportions of these elements in the body of the 

 organism itself. 



A comparison between the proportions of the various elements in 

 the inorganic, or more strictly speaking, the incombustible portions 

 of milk and the proportions of the same elements in the bodies of 

 the young organisms which consume the milk, also shows a remark- 

 able correspondence. These proportions are not those of the circu- 

 lating fluids of the animals. An explanation of this correspondence 

 is not to be sought, therefore, like that between the osmotic pressure 

 of milk and body fluids, in an incidental transference of certain 

 properties from the circulating fluid of the maternal organism to 

 its offspring. The correspondence seems to be due to a definite 

 adaptation of this part of the environment to the needs of the young 

 animal. 



A similar correspondence between those organic constituents of the 

 milk, which are used as building materials, and the composition of 

 the young organism has not been found. This is partly, no doubt, 

 because it would be a matter of very great difficulty to estimate 

 separately the various units into which the proteins of milk are 

 broken up in the course of digestion. But may it not be due, in part, 

 to that more primitive and intimate relation between the inorganic 

 constituents of a living organism and its nutrient medium to Avhich 

 the evidence discussed earlier seems to point? The most primitive 

 living things must have had practically no relation to organic com- 

 pounds in their inorganic environment. The oi'ganic compounds of 

 their own cells they synthesized themselves. The relation between 

 the organic constituents of the organism and those of its environment 

 can only have become of importance at a much later stage in the 

 development of living things, and is likely to be less intimate than 

 that with the inorganic constituents. If this be so, it gives further 

 support to the supposition that certain of the fundamental properties 

 of living matter of to-day are perpetuations of conditions which 

 existed when life was at its beginning. 



