INTRODUCTION. 17 



rebel, yet every step renders him liable to 

 greater and greater penalties ; he cannot 

 afford to pause, much less to stop. Having 

 gained some power and advantage by wil- 

 fully abrogating the laws of his mother and 

 being threatened on every hand by dangers 

 and disasters hitherto restrained, his only 

 hope is to control the sources of these 

 dangers, which already make him wince. 

 The remedy is perhaps to be found in the 

 careful study of the conditions under which 

 pests and diseases occur rather than the 

 pests themselves. 



While quite ready to admit that the germs 

 of disease are carried by animals, this is by 

 no means everything. A score of persons 

 may be exposed to the same influences but 

 only two or three contract the disease. What 

 is the cause of this immunity ? We all know 

 some of the causes, such as dirt, improper 

 food, impure water or air and climate. Is it 

 not true that when an epidemic appears it 



