1930 ] 
The Food of Insects 
9 
are mutually adaptive. The instincts determining food se- 
lection are firmly fixed in the germ-plasm and the insect is 
doomed to feed to the end of its days on beans, cabbage, 
yeast or what-not, unless some fortunate shift or mutation 
of instinct may add pork to the beans, or perchance combine 
hops and malt with the yeast ration. Such persistence over 
long periods during which whole groups of insects and 
plants have evolved in mutual adaptation seem only to be 
explained on the basis of instinctive behavior. This view 
which I have upheld in the past has been recently questioned 
by some, who would place the matter upon a purely physi- 
ological basis, but I cannot see that there is convincing evi- 
dence to support this conclusion or to controvert my own 
contentions that we are dealing with persistent instincts 
rather than with digestive necessities. 
With the foregoing considerations on phytophagous in- 
sects as a basis it is possible to draw certain conclusions of 
a general nature which indicate some of the ways in which 
the development of oligophagy and monophagy has in- 
fluenced the evolution of the higher plants as well as that 
of the insects themselves. Due to its tendency to reduce the 
chances of extinction of plant species whose existence might 
be jeopardized by an abundance of polyphagous insects we 
can see that it has tended to increase the diversity of the 
flora. The development of many mutual adaptations of 
plants with both monophagous and oligophagous insects are 
dependent upon the specific food habits of the insects con- 
cerned and it is thus clear that we must attribute to the 
development of these instincts many of the remarkable 
morphological modifications seen in both plants and in- 
sects. In the present state of our knowledge, at least, there 
is no other causal explanation for their existence. Since 
speciation and the rapidity of evolutionary change in gen- 
eral are highly correlated with adaptive modifications, we 
must attribute to the phenomena of restricted food a highly 
important role in determining the trend of these evolution- 
ary processes. 
A relationship similar to the one just given was early 
noticed by Darwin in connection with anthophilous insects 
and entomophilous flowers and his interpretation of the 
mutual modifications of insects and flowers has long since 
