8 
Psyche 
[March 
sect, and injury decreases. In general, therefore, the feeding 
of oligophagous insects does not involve simultaneous fluc- 
tuations in a considerable number of plants, especially if 
those concerned are not dominant forms, and likewise, a 
smaller number of species of insects is affected directly. 
The fluctuations that may occur among associated insects 
are to a greater extent in an inverse ratio rather than in 
a direct one. 
Monophagous species present a very remarkable series 
of phenomena. Aside from any parasites they may support 
or predators that they may feed, their relations to the living 
environment are entirely restricted to contacts through the 
medium of the host plant. They can never become so abun- 
dant as to rise up and destroy it, since for obvious reasons 
their fluctuations in abundance trail very closely those of 
their host plants. Therefore, we may never attribute the 
extinction of any plant, even in a restricted region, exclu- 
sively to the activities of a monophagous insect. Other 
plants are affected to a varying extent, dependent mainly 
upon the rarity or abundance of the host plant. If it be a 
dominant species, its fluctuations increase or decrease the 
struggle for existence among competing plants; if it be a 
rare species, this influence upon a series of other plants 
is negligible. It also affects a number of associated insects 
which feed upon the same food plant. The number of these 
is, of course, extremely variable, but careful studies of plant 
faunae indicate that dominant types of plants support sur- 
prisingly large numbers of vegetarian insects mounting into 
hundreds of species in the case of common types such as 
willows, figs, oaks and maize, although the average for 
plants in general falls far below this mark. Certain plants 
'which produce poisons or violently irritating substances, 
(like our common American poison-ivy), enjoy comparative, 
but by no means complete immunity from injury by phy- 
tophagous insects. 
In regard to their relations with the living environment, 
we may say that monophagous insects live a life apart. Their 
association with plants is such that the vicissitudes of life 
for both members of the pair are greatly reduced on ac- 
count of the lesser number of variable factors that affect 
each. So far as abundance of fluctuation in numbers they 
