THE SCIENCE OF BIOLOGY 435 



what matings to make among domesticated animals in order to 

 derive strains with desirable characters and to eliminate the 

 undesirable. 



Humanity in mass and individually is profoundly interested in 

 its food supply. We have seen how animal food is ultimately de- 

 rived from the metabolism of green plants and how it is dependent 

 on their peculiar ability to utilize inorganic nitrates to form proteins 

 (p. 43). In Nature the source of these nitrates is in the metabolism 

 of certain nitrogen-fixing bacteria. A supplementary source of great 

 importance in the development of modern agriculture is the depos- 

 its of animal and plant materials in certain regions, particularly the 

 nitrate beds of Chile. Artificial methods of fixing the nitrogen of 

 the air in forms available for plant nutrition have been developed 

 and are assuming more and more importance in the world supply 

 of fertilizers. Such methods make it possible to manufacture nitrates 

 not only for fertilizers but also for the making of explosives wher- 

 ever there may be a convenient source of power, for example, near 

 a waterfall or near a coal-mining region. Increasingly, the artificial 

 production of nitrates is dislocating world economic machinery 

 with consequent effects on political and governmental systems. For 

 a civilized country that has at its disposal an adequate supply of 

 nitrates is independent of importations both in war and in peace. 



Evidence is conspicuous on every side that the combined practical 

 applications of Biology to human welfare are exerting a profound 

 effect on civilization. Improvement of the amount and quality of 

 human food, control of bacterial disease, more complete under- 

 standing of human physiology and the physiology of human repro- 

 duction and the physiology of childhood have combined to decrease 

 human mortality among the young. With decrease of infant mor- 

 tality there is an accumulation of more human adults with a cor- 

 responding effect on economic and industrial life. As a consequence 

 of the increase in the number of adults there has begun to appear 

 an increase in the diseases of later life; attention is now being 



