BACTERIA AND PROTOZOA 23 



fertility can cope with. Or they may perish through 

 their own activity, their life clogged by the products 

 they have themselves formed. Widely they differ 

 among themselves. Some of them build up, others 

 of them destroy ; even in the same species there are 

 vast differences. Like communities of men and like 

 single individuals, there are some that are energetic, 

 others that are lazy, others that are tired; as food 

 fails, or is abundant, they are poorly fed or well 

 nourished. They are healthy or sick. To a bad 

 environment or a good one they respond as readily 

 and as notably as the people in our great cities, and 

 with them, too, their heredity has a dominating 

 influence. 



Only within recent years have we realized how in- 

 timately our prosperity is dependent on the bacterial 

 population of the soil. Without bacterial activity 

 it would be of no avail to the farmer to dung his 

 crops; it would be useless for him to attempt to 

 enrich his soil by ploughing in green stuff. The 

 plants and trees that have lived and died wresting 

 Carbon from the air would only cumber the land. 

 All vegetation would be choked, and the earth would 

 become a vast wilderness, unbeautiful and silent, 

 save for the winds and seas and other manifestations 

 of lifeless forces. 



Though at present we are only groping as pioneer 

 discoverers in a new universe, there are some great 

 features that we are able plainly to recognize, dimly, 

 it may be, as astronomers who map the surface of 

 the moon. As the astronomers can speak with 

 certainty of the mountains of the moon, of the con- 



