48 DISTRIBUTION AND ORIGIN 



again the only forms of life able to take it up in this state, other organisms 

 receiving it from them at second hand in the shape of highly complex 

 nitrogenous bodies. But this theory too has had to be abandoned since 

 bacteria have been found, semi-parasitic in the tissues of plants, that can 

 assimilate and store up free nitrogen (see Chap. X). The two groups of 

 bacteria which necessitate a qualification of these two fundamental principles 

 are, in a sense, complementary to one another ; on the one hand, the earth 

 bacteria which get their nitrogen from soluble nitrates, and their carbon 

 dioxide from the air ; on the other, the bacteria living on certain Legumi- 

 nosae which derive their carbon dioxide from the tissues of the plant, and 

 their nitrogen from the air. 



These facts force us to recognize the existence among bacteria of a 

 special group of forms characterized by an extremely primitive metabolism, 

 a physiological humility which shows them to occupy the very lowest rung 

 of the ladder of life *. 



Such organisms as these cannot possibly be called saprophytes, and 

 placed side by side with organisms which, like the putrefactive bacteria, have 

 complicated physiological requirements. A better classification would be 

 to divide the bacteria, according to their mode of life, into three biological 

 groups, prototropJric, metatrophic t and paratropJiic (24). Prototrophic species 

 are those which either require no organic compounds at all for their 

 nutrition (nitrifying bacterium), or which, given the smallest quantity of 

 organic carbon, can derive all their nitrogen from the atmosphere (bacteria 

 of root-nodules). With them may be classed those forms that are able, 

 with a minimal supply of organic matter, to break up specific inorganic 

 bodies and derive energy from the process (sulphur and iron bacteria). In 

 no case are the chemical changes involved clearly understood, and there 

 apparently exist forms exhibiting gradations between the purely proto- 

 trophic and the metatrophic habit. 



The metatrophic bacteria, under which heading most known species 

 must be placed, cannot live unless they have organic substances at their 

 disposal, both nitrogenous and carbonaceous. They flourish in every place 

 where these are accessible, in impure water, on foodstuffs of all kinds, and 

 in all the cavities of the body which are in communication with the exterior 

 and offer nourishment in the form of secretions or particles of food ; the 

 mouth, fot instance, and nose, the alimentary canal, and the vagina. Many 

 of the metatrophic bacteria bring about profound changes in the chemical 

 composition of the media in which they grow: fermentation (zymogenic 

 bacteria), or putrefaction (saprogenic bacteria). Others break up the sub- 

 stratum in a less conspicuous manner, but develop particularly in media 



* It seems more than probable that farther investigation will result in the discovery of other low 

 organisms (protozoa) with similar functions. 



