356 BIOLOGY: GENERAL AND MEDICAL 



plant whose juices furnish one of the best insecticides 

 known? Can it be referred to habit? Are we justified 

 in supposing that at some time when both insects and 

 plants were evolving toward the forms in which we now 

 know them, and the plant perhaps contained less nico- 

 tine, a symbiosis was formed which, continuing until the 

 present time, affords these interesting examples of free- 

 dom from intoxication? It would be of interest if this 

 were so, but it may be an entirely erroneous assumption, 

 for the insects may suddenly have taken to the tobacco 

 plants and for other reasons have remained unharmed. 



In considering these topics we must not forget that even 

 lowly animals are provided with organs of special sense 

 and that disagreeable impressions received from other- 

 wise desirable foods may keep them away. Upon such 

 grounds may the careful avoidance of certain apparently 

 useful foods be partly accounted for. It may have been 

 that at some period of famine, the repugnance to the odor 

 or taste giving place to necessity, the caterpillar of the 

 sphinx was driven to the tobacco as the only available 

 food, to which it became accustomed and upon which it 

 has remained. 



If it could be shown that habituation was able to 

 effect the tolerance shown to the poisons upon which 

 certain animals feed and was at the foundation of the 

 physiologico-chemical difference between them and 

 other animals not possessing such tolerance, it would 

 aid us in our study of the problems of immunity and 

 infection. Habituation plays a large part in what is 

 called acquired immunity. Need the reader be re- 

 minded that men commonly habituate themselves to to- 

 bacco and opium and acquire a tolerance to these poisons 

 far beyond that generally shared by their kind? 



Unfortunately, the phenomena of infection and im- 

 munity are incapable of reduction to general principles 

 in the present state of knowledge. 



In looking over the field we first find the condition 

 known as Natural Immunity in which, for no reason that 



