INSECT BEHAVIOR 317 



Even the despotic power of habit may be overborne by individual 

 adaptability. Among caterpillars that have exhausted their customary 

 food, there are often a few that will adopt a new food plant and survive, 

 leaving their more conservative fellows to starve. 



As Darwin himself held, the doctrine of natural selection is applicable 

 to instincts as well as structures. All reflex acts are to some extent 

 variable. Disadvantageous reflexes or combinations of reflexes elimi- 

 nate themselves, while advantageous ones persist and accumulate. 



Indeed, structures and instincts must frequently have evolved hand 

 in hand. The remarkable protective resemblance of the Kallima butter- 



FIG. 297.- Ammophila urnaria using a stone to pound down the earth over her nest. 

 Greatly enlarged. After PECKHAM, from Bull. Wisconsin Geol. and Nat. Hist. Survey. 



fly would be useless, did not the insect .instinctively rest among dead 

 leaves of the appropriate kind. 



Origin of Instinct. There are two leading theories as to the origin 

 of instinct. Lamarck, Romanes and their followers have regarded in- 

 stinct as inherited habit; have supposed that instincts have originated 

 by the relegation to the reflex type of actions that at first were rational, 

 and that instincts represent the accumulated results of ancestral experi- 

 ence. This habit theory, however, has little to support it, and assumes 

 the inheritance of acquired characters which has not been proved. 



The selection theory of Darwin, Weismann, Morgan and others has 

 much in its favor. It regards reflex acts as primitive, as the raw 

 material from which natural selection, as the chief factor, has effected 

 those combinations that are termed instincts. 



