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never creates organisms merely to destroy others that she had pre- 
viously created. Parasites do not exterminate their hosts in any 
instance; their mission is merely to interpose a check to undue 
increase, and it is natural that this should be so, for were the 
host destroyed the parasite itself would perish, unless it were able 
to change its food and prey upon other species. It is by no means 
improbable that in the past certain species have been exterminated by 
their parasites, and, indeed, it is very probable that some such cases are 
in progress now. Many lepidopterous larvae are rarely found free from 
parasites, and the adults are amoug the rarest of our species. Here 
we have instances where the parasite very materially lessens the number 
of the host and allows each year only a very few specimens to escape. 
It is only through the fecundity of the specie that it is enabled to main- 
tain itself at all. These cases are exceptional. Usually the relation of 
the parasite to its host is more moderate. Excessive increase is checked, 
but excessive increase only. There is always a very large propor- 
tion of larvae and usually a comparatively small proportion of para- 
sites. Nature tends to preserve a balance among her creatures, 
and a balance only. Many species which are much subject to parasites 
are abundant each year, and remain equally abundant from year to 
year, varying only very slightly; and these variations are rarely the 
result of an excessive increase of parasites. Nature also works very 
slowly, and she adapts insects as well as other animals to their environ- 
ment only by means that require ages for their completion. Insects 
that are confined to plants which, under natural conditions are not com- 
mon, need few parasites to keep them in check. The great difficulty in 
finding food is in itself a sufficient check, and parasites are not neces- 
sary ; indeed they could not be supported under the circumstances. If, 
by any unnatural condition introduced by man, the supply of food for 
this otherwise rare insect is suddenly increased, it obtains the pos- 
sibility of multiplying rapidly, while the number of parasites does not 
increase proportionally. In the course of time nature may make a 
change and other species may attack this form which has now increased 
abnormally; but this is something that the farmer can not wait for; 
he must have some means of dealing with the insect at once, and he 
must leave the operations of nature to benefit his descendants. The 
spread and increase of the potato beetle, Doryphora 10-lineata, is a case 
in point. Here neither parasites nor natural enemies assist the farmer 
in any noticeable way. He must depend upon his own exertions to save 
his crop. There are, however, many insects which are very commonly 
parasitized, and among them may be mentioned the various species 
of cutworms. It is nothing uncommon to find in an infested field 
that fully one-half, and sometimes as many as three-quarters, of 
the specimens will have eggs of the Tachina flies attached to the 
skin and, probably, others have parasites which are not externally 
visible. Yet the fact that these cutworms are infested by para- 
