BACTERIA AND PROTOZOA 25 



mapping the moon, and in computing her orbit, the 

 position of the bacteriologist who is grappling with 

 botanical and agricultural problems is far less 

 enviable. He must be compared rather with the 

 men who are making a study of Mars. There are 

 certain broad facts of which he can speak with 

 certainty, but as regards others he is beset with 

 controversy, struggling as it were to put a correct 

 interpretation on the vague markings that he speaks 

 of provisionally as the canal system of Mars. There 

 is no dispute now as to the prominent part that 

 bacteria play in relation to soil fertility. It is a 

 commonplace that in an ounce of rich loam soil there 

 may be as many as 150,000,000 bacteria, while in 

 ground polluted with sewage the number may reach 

 the fantastic total of 3,000,000,000. Much, too, of 

 the work that the bacteria accomplish and of their 

 habits is known, proved and accepted. Warmth 

 and a moderate amount of moisture promote their 

 growth; cold and excessive moisture or excessive 

 drought arrest their development. There are some 

 that can thrive in the presence of air, others that can 

 only do their work if air is rigidly excluded. The 

 function of some of them is to break down complex 

 substances to simpler bodies or even to elements; 

 the function of others is to build up substances of 

 high potential energy from inert constituents. With- 

 out their activity the world would rapidly arrive at 

 a deadlock. The material of plants and animals that 

 had reached and passed their prime would cumber 

 the ground, and the fabric of their tissues would 

 not, as now, become available as the raw substances 



