682 THE POPULAR SCIENCE MONTHLY. 



faith. The case of icerya and vedalia, as I have frequently 

 pointed out^ was exceptional and one which can not easily be re- 

 peated. 



One of the numerous phases of the vedalia experiment is that 

 the wide newspaper circulation of the facts not always most ac- 

 curately set forth has brought me communications from all 

 parts of the world asking for supplies of the renowned little lad}^- 

 bird for use against injurious insects of every kind and descrip- 

 tion, the inquiries being made, of course, under a misapprehen- 

 sion of the facts. 



While this California experience thus affords one of the most 

 striking illustrations of what may be accomplished under excep- 

 tional circumstances by the second method of utilizing beneficial 

 insects, we can hardly expect to succeed in accomplishing much 

 good in this direction without a full knowledge of all the ascer- 

 tainable facts in the case and a due appreciation of the pro- 

 founder laws of Nature, and particularly of the interrelations of 

 organisms. Year in and year out, with the conditions of life un- 

 changed by man's actions, the relations between the plant-feeder 

 and the predaceous and parasitic species of its own class remain 

 substantially the same, whatever the fluctuations between them 

 for any given year. This is a necessary result in the economy of 

 Nature ; for the ascendency of one or the other of the opposing 

 forces involves a corresponding fluctuation on the decreasing side, 

 and there is a necessary relation between the plant-feeder and its 

 enemies, which normally must be to the slight advantage of the 

 former, and only exceptionally to the great advantage of the lat- 

 ter. This law is recognized by all close students of Nature, and 

 has often been illustrated and insisted upon by entomologists in 

 particular, as the most graphic exemplifications of it occur in in- 

 sect life, in which fecundity is such that the balance is regained 

 with marvelous rapidity, even after approximate annihilation of 

 any particular species. But it is doubtful whether another equally 

 logical deduction from the prevalence of this law has been suffi- 

 ciently recognized by us, and this is that our artificial insecticide 

 methods have little or no effect upon the multiplication of an in- 

 jurious species except for the particular occasion which calls 

 them forth, and that occasions often arise when it were wiser to 

 refrain from the use of such insecticides and to leave the field to 

 the parasitic and predaceous forms. 



It is generally when a particular injurious insect has reached 

 the zenith of its increase and has accomplished its greatest harm 

 that the farmer is led to bestir himself to suppress it ; and yet it 

 is equally true that it is just at this time that Nature is about to 

 relieve him in striking the balance by checks which are violent 

 and effective in proportion to the exceptional increase of and con- 



