Page:Popular Science Monthly Volume 72.djvu/420

, 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 principle 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 accompanied 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 multiplies 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 Rocky 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