GENERAL CHARACTERS OF LIVING ORGANISMS 31 



other substances (oxygen, salts, water) furnished by the 

 environment. Of these synthesized compounds the 

 most individualized and specific are the proteins. These 

 compounds, characteristically colloidal in their physical 

 properties, constitute, together with certain other 

 materials, chiefly lipoid, the relatively stable, solid, or^ 

 permanent (structural) portion of the protoplasmic 

 complex. 



It is significant that the chief structure-forming 

 compounds should be at the same time those which are 

 chemically the most specific. Specific form and structure 

 are the most obvious peculiarities of the living organism ; 

 hence species are usually distinguished by their structural 

 characters. It is to be remembered, however, that the 

 chemical and physiological characters are equally 

 constant and definite, and must be included in the 

 complete characterization of any species. The essential 

 fact, requiring physiological explanation, is that each 

 individual animal or plant resembles, structurally, 

 chemically, and physiologically other individuals of the 

 same species, while differing from those of other species. 

 As already indicated, the physiological basis of this 

 specificity is to be sought in the specific nature of the 

 chemical processes by which the organism is synthesized. 

 We find in fact that a chemical specificity, corresponding 

 to the specificity of the organism as a whole, is exhibited 

 by its constituent proteins, and apparently by these 

 compounds alone. The other chief biochemical com- 

 pounds (carbohydrates, lipins) are chemically identical 

 in widely differing species, while the proteins vary in 

 their detailed chemical character from species to species. 

 Apparently each native protein has a special composition 



