INSECTS INJURIOUS TO AGRICULTURE 417 



rational to believe, on the contrary, that at the end of a longer or 

 shorter period there will be brought about an accommodation between 

 the plants and their enemies, analogous to that which is produced 

 between animals and bacterial, trypanosome or piroplasmic diseases. It 

 is well demonstrated that certain insects like the scale insects inoculate 

 in the sap certain toxic products, and it happens that varieties of 

 plants that have never been attacked before by these insects are at first 

 peculiarly subject to their action, then at the end of a longer or shorter 

 time they begin to acquire a relative immunity — an immunity ac- 

 quired in a way quite different from the formation of anti-toxins may 

 be conceived, that is to say, an immunity attributable to many defensive 

 adaptations of the plant, consisting in modifications of the plant tissue 

 and tending either to render the attack of the insect more difficult, or 

 to diminish the quantity of food which it can get, or again to render 

 the lesions which it produces less dangerous. 



If, for example, the San Jose scale is less injurious in its native 

 country, can it be said that this is the case only because of the para- 

 sites which hold it in check ? Are there not serious reasons for admit- 

 ting that the trees of this country may be formed of varieties adapted 

 to this insect, capable of resisting it? 



Finally, we should also take into consideration the fact that if, dur- 

 ing the first years following its introduction, an injurious species of 

 exotic origin can multiply freely without any parasite to interrupt its 

 multiplication, there often comes a time sooner or later when the 

 parasites of a country living at the expense of the indigenous forms 

 most nearly related to the exotic species progressively adapt themselves 

 to the latter and end in limiting its propagation. This cause, added 

 to the progressive adaptation of plants, appears to be bringing about the 

 actual diminution of the virulence of the San Jose scale in America. 



It results from what precedes that in the problem with which we 

 are occupying ourselves the factors are multiple, and that it will be a 

 great mistake to consider one or the other and not all. 



If, because of the too absolute manner in which it has been formu- 

 lated, the theory of the utilization of beneficial insects must surrender 

 somewhat, it is necessary also to point out the errors and the exaggera- 

 ions to which it has given rise in practise, for they give rise to 

 excessive hopes, provoke serious disillusions, and are a discredit to the 

 whole method. 



In the first place, a grave fault resulting from an excessive con- 

 fidence in the action of parasites consists in advising the suppression 

 of insecticides in a region where it is desired to acclimatize the bene- 

 ficial species. In the great majority of cases, at least in regions where 

 insecticides are employed with success to hold a crop enemy in check, 

 the desire to acclimatize a beneficial species should not cause the gen- 



VOL. LXXII. — 27. 



