852 LIFE : OUTLINES OF GENERAL BIOLOGY 



ancestral ones, there is little evidence for either view; there are 

 bacteria in Carboniferous times or earlier, yet essentially like our 

 familiar ones. Indeed, we cannot but suppose those of decomposition 

 to be as old as any higher life-forms, since otherwise dead animals 

 and plants would have accumulated beyond space for living ones. 

 Nor have they been mere saprophytic agents of the decomposition 

 of organic forms, as we naturalists mainly think of them; nor yet 

 simply also parasitic, as surgery and medicine have to deal with 

 them; but also, from a long past (perhaps before the advent or 

 abundance of higher organic life) in forms of independent and self- 

 sustaining existence; and this even as mud-forming and thus in 

 time rock-forming agents, and on what by some is considered a 

 colossal scale in the early past, and not without corroboration from 

 such forms and their operations observed in progress to this day. 



To ordinary nature-students, the single general conception 

 suffices of the photosynthetic activity of green plants as fundamental, 

 not only to their own existence, and to that of their plant parasites, 

 and of the fungi and the bacteria of their decomposition, but, also 

 directly or indirectly, to the nutrition of the entire animal world 

 as well. Yet we are increasingly learning, from the bacteriologists, 

 of very different and indeed independent modes of life-sustaining 

 among forms of bacteria; and each of these is quite distinct from 

 photosynthesis, indeed with light not utilised. The most familiar 

 example of such distinctive physiological ways of life, for they are 

 nothing less, is that of the teeming bacterial life within the root- 

 nodules of leguminous plants (and a few others, e.g. alders) with 

 their amazing powers of assimilating nitrogen from their soil 

 atmosphere or soil water. For so far from robbing the plant of its 

 nitrogenous substance, as ordinary bacteria and moulds do when it 

 is dead and they decompose it, this bacterial activity, going on within 

 these nodules, richly endows their host-plants : and so enables us to 

 understand their usually exuberant growth and flowering ; and, above 

 all, that exceptional proteid wealth of their seeds, so superior in 

 nutritive values to those of the cereals as to render them, for vast 

 Oriental populations especially, the established substitute for the 

 costlier milk, eggs, fish, or meat demanded by Western peoples; 

 and thus rendering even the strictest vegetarianism perfectly healthy 

 and practicable. 



Again, the bacteriologists of soils are now familiar with other 

 nitrogen-utilising types, free-living, and quite distinct from those 

 symbiotic with leguminous plants. Upon these agriculture is yet 

 more substantially dependent. They are known as nitrite- and 

 nitrate -bacteria respectively, the former utilising salts of ammonia 

 from the soil, and leaving nitrites, which the next group then oxidise 

 to nitrates, which can then be taken up in solution by the roots of 

 plants. Each form, of course, keeps such nitrogen as it needs, and 



