VII.] NATURAL CHECKS. 191 



conditions. The pressure may be suddenly removed, 

 when they break forth in riot. Thus for over ten 

 years the pear tree psylla was known to exist in the 

 neighborhood of Cornell University before it came to 

 be a serious foe. And now, after a year or two of 

 great activity, the species again appears to have 

 received a check. The gypsy moth is another illus- 

 tration of the long period of incubation of some 

 invasions. It was not until twenty years after its 

 introduction into Massachusetts that it began to attract 

 attention as an injurious insect. It could not have 

 required all these years for its multiplication into a 

 sufficient horde to arrest attention, for a mathematical 

 calculation will show that this could have occurred in 

 much less time if the species were allowed to propa- 

 gate to the full extent of its capabilities. 



After a time the check will come. The potato 

 beetle has already passed its zenith. The codlin-moth 

 and the curculio have lost much of their fury in the 

 east. The enemies of insects increase as well as the 

 competitors. Parasites, finding innumerable insects 

 upon which to prey, increase with great rapidity, until 

 they devour their own means of support. They, in 

 turn, succumb, and the defeated host rallies ; so the 

 alternate warfare goes on forever. Witness how the 

 tent -caterpillars come and go. The reign of destruc- 

 tion of this insect is apt to be brief, sometimes last- 

 ing only a year or two. Other insects hold their own 

 for a longer time. This period of activity is somewhat 

 characteristic of the different species. 



We are taught, by these considerations, that we 

 should not become disheartened with the sudden influx 

 of enemies. These invasions are not peculiar to modern 



