THE ORIGIN OF LIFE 29 



are absolutely dependent on being supplied with 

 complex food materials already elaborated by the 

 activity of plant or animal life. The Amoeba feeds 

 on minute plants, such as Diatoms, on Bacteria, and 

 to some extent on other animals, and it could not 

 possibly have existed before a plentiful supply of 

 plant life had covered the earth. The same is the 

 case with all other Protozoa. It is therefore futile 

 to look for the beginnings of life among the car- 

 nivorous and herbivorous Protozoa, or anything 

 nearly related to them, as their whole structure and 

 way of life is arranged on the assumption that living 

 material already exists to furnish them with food. 



It seems equally futile to regard the Bacteria as 

 representing the primal forms in which life appeared. 

 It is true that these organisms stand in a much 

 closer relation to inorganic nature than do the Pro- 

 tozoa, since they are able to obtain their nitrogen 

 not only from inorganic compounds but also in some 

 cases from the pure gas in a free condition, and we 

 may remember the remarkable powers possessed by 

 the sulphur and iron bacteria of obtaining energy 

 by the oxidation of simple inorganic iron and sulphur 

 compounds. But the Bacteria are helpless in another 

 direction; since they are without chlorophyll they 

 cannot obtain their necessary carbon in an inorganic 

 form 1 , and hence they are obliged to seek this element 



] This has been disputed in a few doubtful cases. 



