218 OUTLINES OF EVOLUTIONARY BIOLOGY 



being scattered in minute granules throughout the cytoplasm 

 (ride Pig. 27). They contain no chlorophyll and the colourless 

 protoplasm is enclosed in a very thin and delicate cell wall. In 

 form they vary greatly, being sometimes spherical, sometimes 

 rod-shaped and sometimes corkscrew-shaped, and frequently 

 they occur united together in chains. Some of them swim about 

 by means of cilia or flagella, others are motionless, and they 

 multiply by simple fission and also by the formation of spores, 

 which have extraordinary powers of resistance and thus serve to 

 secure the wide dispersal of these organisms by air and water. 



A great number of different kinds of these Bacteria are already 

 known to us, and doubtless a vast number still remain to be dis- 

 covered. They occur practically everywhere ; earth, air and water 

 are full of them, and they exercise a most 

 profound influence sometimes injurious 

 and sometimes beneficial upon the lives 

 of other living things. Some of them are 

 the active agents in the putrefaction and 

 fermentation which are rapidly set up in 

 dead organisms, and others are responsible 

 for many of the most grievous ills which 

 living flesh is heir to. Thus most of them 



FIG. 84. - Nitrosomonas live either as saprophytes (upon dead 

 enropa-a, x 600. organic matter) or as parasites (upon 



(From Percival's living organisms) and obtain their supply 

 < ' Agricultural Bacte- * . / 



riology.") o* energy from already formed proteids 



and other complex chemical compounds. 



Some, however, are said to live free in the soil and to be able 

 to derive their food supply, and with it their energy, from purely 

 inorganic substances. These are called nitrifying Bacteria. One 

 form, Nitrosomonas (Fig. 84), converts salts of ammonia into 

 nitrites, and another, Nitrobacter, converts nitrites into nitrates. 

 In this way they perform an important function in preparing the 

 food material in the soil for the use of green plants, but their chief 

 interest for us lies in the fact that they are able, in some manner 

 which we do not understand, to build up their own proteid mole- 

 cules directly from inorganic constituents without the aid of chloro- 

 phyll. They show us that such a thing is at any rate possible, 

 and that therefore the first living things may have been able to 

 maintain their existence without either possessing chlorophyll 

 or having other organisms to feed upon. 



