10 
Every now and then, however, we are brought face to face with a 
disturbed balance of a more general nature, the most striking examples 
of which are in the new insect enemies which come from time to time, 
and which in new situations and under strange surrounding conditions 
multiply excessively and assume an economic importance which for the 
moment obscures all older like troubles. So universally true is it that 
the newcomer will prove more destructive than the old familiar pests, 
that this is looked upon as a fixed principle, like the law of gravity. 
A good illustration of this is seen in the recent very valuable book by 
Mr. E. E. Green on the insects of Ceylon. Referring to the insects 
occurring on that island, he takes the position that excessive damage 
by an insect is a proof of its foreign origin. He says that all the 
troublesome insects of Ceylon are introduced species, and that not a 
single undoubtedly native species has attracted any notice as a pest. 
Out of twenty-six species of Chionaspis, for example, occurring on the 
island, three only are injurious, and these are introduced species. In 
this country our experiences have been more varied, and we probably 
would not go to the same extreme, but the history of insect introductions 
and migrations here and elsewhere certainly indicates the truth of the 
general principle. 
A good deal of the effort in applied entomology in recent years has 
been directed at evils of this sort, namely, to prevent the introduction 
of insect enemies from foreign lands, and to quickly exterminate or con- 
trol them where they have gained a foothold. 
~ In connection with this field of work it seems to me that the proper 
appreciation of certain underlying principles affecting the behavior of 
such introduced enemies will show that we are exhibiting much more 
anxiety and alarm than the occasion warrants, and that we are making 
vain attempts to accomplish the impossible and prevent the inevitable. 
This belief has been impressed upon the writer by the history of the 
new injurious insects coming into prominence from time to time, and 
the patent failure of our efforts at extermination or quarantine. 
The underlying influences referred to seem to me not to have been 
sufficiently considered, and offer to my mind an explanation of the 
excessive fecundity and destructiveness of insects and other animals 
transferred to new fields. In other words, I believe there are other and 
more important, though perhaps more obscure, agencies at work than 
mere favorable climate and absence of natural enemies to account for 
the role played by such introduced animals. 
In the case of insects there are two points of view: First, from the 
side of the host; and second, and much the more important, from the 
side of the parasite. 
Taking up the first point of view, the plant, when confronted by a 
new and unfamiliar enemy, is helpless, having no fund of inherited 
powers of resistance directly applicable to fall back upon. The strug- 
gle for existence and survival of the most fit under the old and normal 
