CLASSIFICATION ACCORDING TO NUTRITION 47 



found constantly in such numbers as in the soil. They occur both as spores 

 and in active vegetation (e.g. nitrifying bacteria). In the soil, as in water, 

 the presence of organic matter is favourable to their increase, and they arc 

 most numerous in cultivated earth. Garden mould seldom contains less than 

 100,000 germs per c.c., among them being almost always some pathogenic 

 forms such as the bacilli of tetanus and malignant oedema. Putrefactive 

 and pigment bacteria are also generally present as well as fermentative 

 and nitrifying organisms. For the ordinary purposes of hygiene the culti- 

 vation upon nutrient media of the bacteria of earth, water, and the atmo- 

 sphere, and the detection and determination of pathogenic species, are 

 sufficient ; the latter task being, however, by no means easy. Still more 

 difficult and tedious is the systematic investigation of all the species present, 

 necessitating as it does the employment of a number of different culture 

 media varying in composition according to the nature of the species to be 

 cultivated. It is not possible, for instance, to isolate the nitrogen-fixing 

 bacteria of the soil upon a gelatine suitable for the tetanus parasite. In 

 each case the character of the culture media must correspond to the require- 

 ments of the organism to be cultivated. 



It has been customary to divide the bacteria, according to their mode of 

 life, into two great physiological classes, the Saprophytes and the Parasites. 

 Both groups are dependent upon other organisms for their sustenance ; both 

 are unable either to build up their protoplasm from inorganic bodies, or 

 to obtain, from the decomposition of such, the energy necessary for their 

 vital processes. Those bacteria which can live and grow only within the 

 tissues of living organisms are called parasites, those which are satisfied 

 with the secretions or excretions of living, or the substance of dead, organisms, 

 are called saprophytes. 



But this classification no longer corresponds to facts. Being a corollary 

 of two axioms of general physiology, it could be held good only so long 

 as their validity was unimpaired. The first of these axioms was, that of 

 all organisms the green chlorophyll-bearing plants alone* arc able, with 

 the aid of sunlight, to assimilate the carbon dioxide of the atmosphere, and 

 from it to build up carbohydrates. All other living things, that is to say, 

 plants devoid of chlorophyll, bacteria, and all forms of animal life, were 

 supposed to be dependent, for their carbon, indirectly or directly, upon the 

 carbon compounds already formed by green plants. The integrity of this 

 principle has now been destroyed by the discovery of certain bacteria in the 

 soil, which are able, even without sunlight, to appropriate the carbon dioxide 

 of the atmosphere. The second postulate was that no organisms of any kind 

 could fix and utilize the free nitrogen of the air. All organic nitrogen was 

 thought to be derived from the nitrates of the soil ; green plants being 



* The red and the brown seaweeds also. 



