INSECT BEHAVIOR 301 
and, in a sense, accidental, or purposeless, such novel depart- 
ures as those of the Polistes 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 
