416 POPULAR SCIENCE MONTHLY 



plication, its orginal country should be searched for parasites living at 

 its expense, and these should be procured and naturalized. This is, if 

 I am not mistaken, the theory of Alex. Craw and his school. 



In objection to this doctrine it should be urged that there exist 

 insects which can be considered as veritable plagues to our crops, and 

 which, however, are undoubtedly indigenous, such, in Europe, as the 

 cockchafer, the apple anthonomus, the pyralis of the vine, the cochylis ; 

 and for America, the Colorado potato beetle, which would have ruined 

 the culture of the potato in the United States if the use of Paris green 

 had not been discovered. 



But other stronger objections may be urged, if not against the prin- 

 ciple of the theory, at least against its too great application and against 

 the exclusive way in which it has been propounded. Admitting that it 

 is incontestible that certain insects can become terrible plagues where 

 they are introduced into a new country because they are not accom- 

 panied there by their natural enemies, it is manifestly going too far 

 to hold that it is always to the absence of the natural enemies of an 

 insect of exotic origin, taking the proportions of a plague, that it 

 owes its virulence. 



We know well, for example, that it is for entirely different causes, 

 depending upon the nature of the affected plants, that the Phylloxera 

 occasioned an unprecedented disaster in Europe ; and it would have been 

 taking a false step at the time of invasion of this insect to undertake 

 long researches to procure its natural enemies. 



An indigenous insect, which has been for a long time practically 

 harmless, can become more dangerous and even arrive at the condition 

 of a plague simply because man, by new crop conditions, in favoring 

 the extension of some plants at the expense of others, and substituting 

 for an extremely varied natural vegetation an immense supply of a 

 single plant, has himself broken the equilibrium of nature and favored 

 to a very large degree the multiplication of the insects that attack his 

 privileged crop. It is in the same order of things that an insect living 

 upon a wild plant becomes adapted to a cultivated plant, and multi- 

 plies excessively at the expense of the cultivated plant whose conditions 

 are particularly favorable to its nutrition. One of the most striking 

 examples of this phenomenon is the case of the Colorado potato beetle, 

 of which we have already spoken. This insect, originally from the 

 Eoeky Mountains, lived solely upon wild Solanum, but about 1855 it 

 invaded the potato fields which began to be cultivated in its country, 

 and then gradually spread into all of the potato fields of the United 

 States and Canada, causing terrible damage. 



Finally, it is not necessary to believe any longer that all exotic 

 enemies, whose appearance is signalized by extreme virulence, will 

 bring disaster unless their natural enemies are introduced. It is 



