THE SCIENCE OF BIOLOGY 433 



vitamins. The consequence is the control of deficiency diseases, 

 pellagra, rickets, and so on, and the development of adequate ra- 

 tions not only for the maintenance of the human body but also for 

 the benefit of domesticated animals. Work did not stop after the 

 vitamins had been identified. Chemical analyses of foods, the chemi- 

 cal structure of the vitamins, and the search for other possible 

 vitamins, continue. Activity increases rather than diminishes and 

 other important results will no doubt be obtained. 



Out of laboratory findings concerning the nature of bacterial life 

 cycles has grown a science of Sanitation, which concerns itself with 

 the eradication of disease-carrying insects, the purification of food 

 and of water supplies, and the measures for control of epidemics. 

 This science has made it possible for people to dwell in congested 

 cities largely free from the devastating epidemics that scourged the 

 cities of the past. Moreover, the nature of the metabolism of bac- 

 teria, what may be their chief source of food and how this food is 

 chemically altered, has received attention. Hope may be expressed 

 that such purely abstract work may in time reveal facts that will 

 afford an opportunity for the control or elimination of pathogenic 

 bacteria by attack on their food supply. 



Studies of the life histories of all animals have included studies 

 of the complicated life histories of parasitic animals. These studies 

 have revealed the most vulnerable points in the life cycles of para- 

 sites and have brought about measures for their control. For ex- 

 ample, knowledge that the infection of the human by hookworm 

 takes place through the penetration of the skin of the feet by the 

 larval forms makes it possible to prevent such infections by the 

 destruction of the infected human excrement and compulsory wear- 

 ing of shoes by inhabitants of infected areas. Similarly, the control 

 of injurious insects is based upon their life histories, their feeding 

 habits, and their sensitivity to various poisons. The biologist is inter- 

 ested in such life histories because they reveal relationships between 



