18 MAN AND ANIMALS 
to prevent any unnecessary disturbance of the settled order of the life of his 
region which will convert relatively stationary species into widely oscillating 
ones; second, to destroy or render stationary all the oscillating species injurious 
to him; or, failing in this, to restrict their oscillations within the narrowest limits 
possible. For example, remembering that every species oscillates to some 
extent and is held to relatively constant numbers by the joint action of several 
restraining forces, we see that the removal or weakening of any check or barrier 
is sufficient to widen and intensify this dangerous oscillation, and may even 
convert a perfectly harmless species into a frightful pest. 
Forbes mentions that cottony scale, a common pest in our parks, 
was rare in natural conditions. The close setting of trees has favored. 
its increase. Close setting is nearly always a factor which has to be 
considered. 
How do pests arise? The recent rise of the wheat aphis may be 
taken asanexample. The spring of 1907 was very warm in the southern 
part of the wheat belt, and the grain aphis, which is said to reproduce 
freely at temperatures from too° F. to below freezing, was accordingly 
able to reproduce without interruption from its parasites and enemies, 
which do not become active at such low temperatures as occurred. 
When the weather grew warmer and the enemies appeared the aphids 
were so numerous that the work of the enemies was hardly appreciable. 
But since they too, like the aphids, are rapid reproducers, with such 
favorable conditions they were able to increase rapidly. With their great 
increase the aphids decreased and soon their numbers were far too great 
for the available aphid food. The enemies therefore decreased because 
of the absence of sufficient food, and this portion of the society was 
accordingly restored to an approximate equilibrium. It is to be under- 
stood that such an oscillation in the society is far-reaching in its effects. 
It has been noted that such oscillations affect the whole community. 
The birds and mammals find certain kinds of food abundant and accord- 
ingly eat things different from what they do under different conditions. 
Such fluctuations in the animal communities are constantly going on. 
The whole process may be summarized as follows: 
. Weather conditions unfavorable to enemies and favorable to plant pest. 
. Increase in pest. 
Increase in enemies. 
. Decrease in pest. 
. Decrease in enemies. 
. Balance. 
Aun & WwW NH 
