84 EVOLUTION OF THE EARTH 



are as it were reflected in the changing picture which the micro- 

 scope is capable of resolving. 



Chemical characteristics of protoplasm. Chemical analysis 

 shows that protoplasm is a colloidal complex comprising chiefly 

 the elements carbon, hydrogen, oxygen, nitrogen, sulphur, 

 phosphorus, potassium, calcium, sodium, chlorine, iron, and 

 magnesium — all of which are commonly found in the inor- 

 ganic world. Indeed, there is no chemical element present 

 which is peculiar to living matter. 



But there are combinations of elements which are distinctly 

 characteristic of protoplasm, not being found in nature except 

 as the result of protoplasmic activity. These chemical com- 

 binations are the proteins, carbohydrates and fats, and of 

 them the proteins are the most significant because they are 

 universally present as a part of all living matter and form 

 quantitatively its chief organic constituent. Proteins invari- 

 ably consist of the elements carbon, oxygen, hydrogen, nitro- 

 gen, sulphur, with sometimes phosphorus or iron; the nitrogen, 

 however, particularly distinguishing them from the other 

 organic compounds of the living complex. Although the 

 stereochemical composition of a protein molecule has not been 

 determined, analyses show that it may be split up into a large 

 number of simpler, though still very complex molecules, and 

 it is evident that its few elements may be represented by hun- 

 dreds and even thousands of atoms. As Underbill has said: 

 **Viewed from the chemical standpoint protein is seen as a huge 

 molecule, complex in structure, labile in character and there- 

 fore prone to chemical change. So large and intricate is the 

 make-up of the molecule that chemists for generations have 

 been baflled in their attempts to gain any adequate conception 

 of its nature. At the present stage of our knowledge it is 

 impossible to form any satisfactory definition of a protein 

 based either on its chemical or physiological properties."^ 



2Underhill, F. P., "The Physiology of the Amino Acids," 1915, published by 

 Yale University Press. 



