BIOLOGY AND HUMAN WELFARE 403 



slaughts of pestilences, and the hope of our immensely more com- 

 plex community life depends upon the development of knowledge 

 of these living agents of disease. 



B. Biology and Agriculture 



True it is that Alan cannot live by bread alone, but it is equally 

 true that the fundamental urge of all living things to secure food 

 and to multiply is, in the final analysis, at the basis of human en- 

 deavor. Alodern agriculture represents the body of knowledge ac- 

 cumulated by mankind during its slow progress toward civiliza- 

 tion, involving increasingly exacting food demands as community 

 life became more and more complex. Agriculture is, of course, 

 dependent upon many fundamental biological sciences — indeed, 

 agriculture is one aspect of applied biology — but merely a few 

 examples must suffice to bring the most significant points before 

 us. 



1. Plant and Animal Food 



As we know, all animals, including Alan, are absolutely de- 

 pendent for their food upon the photosynthetic activities of plants. 

 Green plants must manufacture enough food for themselves, and 

 to spare for the rest of the living world as well. Animals must 

 have ready-made food which, after they have used it, is useless both 

 for animals and for green plants. Here, it will be recalled, the 

 Bacteria and other colorless plants come in and make possible the 

 completion of the biological cycle of the elements in nature: put 

 the materials in a condition in which they are again available to 

 green plants. (Figs. 15, 16, 17.) 



This intimate food interrelationship of all living organisms, which 

 has been demonstrated by interlocking data accumulated by thou- 

 sands of biochemical studies, is not only of profound theoretical 

 interest, but also of incalculable practical importance in all prob- 

 lems of soil fertility, including soil composition and maintenance, 

 crop rotation, etc. To coax greater productivity from the soil — 

 "civilization rests upon the soil" — is a problem of no mean im- 

 portance. One of its most crucial factors is the demand for the 

 economical production of food for plants themselves ; in particular, 

 for an adequate supply of nitrogen in a form readily available for 

 the use of cultivated plants. 



A natural source of nitrogenous plant food is, of course, the 



