INFECTION AND IMMUNITY 339 



available foods. Should one inquire how and why this 

 began, the question leads inevitably into the consideration 

 of the greater problems of evolution, for it is clear that 

 such conditions cannot always have obtained, seeing 

 that there must have been a time when neither the 

 living thing nor its food was in existence. That which 

 is consumed must have come into existence before that 

 which consumes it. It can be shown that the food habit 

 of existing animals sometimes undergoes considerable 

 modification. Thus the Colorado potato beetle, in its 

 native habitat, Colorado, fed upon certain plants of the 

 genus Solanum. These plants were not very common, 

 and the beetles were not very numerous until civiliza- 

 tion arrived and they, by preference, seized upon the 

 potato, a related plant, and have become a great nui- 

 sance to the farmer. Perhaps it is not unusual for 

 animals to try a new means of subsistence; if so, we can 

 only know about it when they are successful, for in case 

 of failure they would die and escape observation. 



When experimental endeavors are made to accustom 

 animals to new kinds of food, they commonly sicken 

 and may die. It is well-known that many things are 

 distinctly poisonous for the higher animals. What 

 is it that determines the poisonous or non-poisonous 

 nature of the food? 



There is scarcely an organic poison known upon which 

 some living thing may not feed with impunity. Tobacco, 

 for example, is intensely poisonous to most mammals, 

 birds, insects, and arachnids, yet the growing plant is so 

 constantly attacked by the caterpillar of a large moth 

 that the farmer must look at his plants every day to see 

 that the leaves are not eaten and made unmarketable. 

 The dried leaves lacking the water of the growing plant 

 contain a relatively greater proportion of the poison, yet 

 constitute the regular food of the larva of a beetle. 

 Manufactured tobacco and cigars are not infrequently 

 ruined by being drilled with holes by them. How do 

 these insects escape injury, grow and thrive upon a 



