INSECT BEHAVIOR 3 61 



and, in a sense, accidental, or purposeless, such novel depart- 

 ures as those of the Polistcs or the Ammophila would seem to 

 denote adaptability. 



Even the despotic power of habit may be overborne by indi- 

 vidual 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 combi- 

 nations of reflexes eliminate 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 butterfly 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 instinct 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 experience. This habit 

 theory, however, has little to support it, and assumes the in- 

 heritance 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 primi- 

 tive, as the raw material from which natural selection, as the 

 chief factor, has effected those combinations that are termed 

 instincts. 



Instincts and Tropisms. We have already emphasized 

 the fact that an instinct is a reflex act or a combination of 

 reflex acts. The same fact may now be stated in these words : 

 an instinct is a tropism or a combination of tropisms. The 

 more important of these tropisms have been considered. 

 Whenever possible it is better to discard the ambiguous term 



