3^ The Econo77iy of Nature. [■ZOE 



Taking human reasoning as a standpoint, the gradual organic 

 development has different standards; first, the daily growth of plant 

 and animal ; second, a yearly periodicity depending on the solar 

 cycle, and third, secular changes which become perceptible to us 

 only by changes in historical landscapes or the underground 

 remains of plant and animal now superseded by a different vegeta- 

 tion, inhabited by different animals. Rapid changes or disturbances 

 m organic life are exceptions, and even in prehistoric times may 

 not have been so frequent as we are now disposed to think, and 

 probably have more usually taken the form of gradual develop- 

 ments. 



It would lead too far to discuss all the different forms of such dis- 

 turbances that have occurred or may occur. We will only discuss, 

 the one to which considerable practical interest is attached in regard 

 to cultivated land, that is, the multiplication of certain forms of 

 organic life frequently injurious to our plantations, herds, or even 

 ourselves, whether the form multiplied out of proportion be animal 

 or plant. One of the most frequent causes of such disturbance is 

 man himself, but even without his interference, a combination of 

 circumstances produces occasionally such disturbances in districts 

 htde or not at all interfered with by man ; for instance in the Arctic 

 the wanderings of the lemming, a kind of gopher or rat, or, what 

 is of more practical interest, the migrations of different kinds of 

 grasshoppers in our own temperate zone. 



Almost all disturbances of the equilibrium In nature are more or 

 less undesirable, and to prevent their frequent occurrences to- 

 restrict and repair the damages inflicted by them to human inter- 

 ests IS one of the most noble problems of science. To accomplish 

 such object It is necessary to study the phenomenon where It takes 

 Its course without human interference, because the less complicated 

 phenomenon is more likely to reveal to us its causes and show to 

 us the ways and means applied by nature itself to restore the en- 

 dangered balance of power. Thus It Is necessary to study the 

 single case of disturbance first In its isolation, then by comparino- it 



possible ^°"' '"'"' '" ""^''^ ^''"'^" ^^'"'^ '' ^' ''"^" apparenras 

 This study will make us acquainted: first, with the cause or 

 the combmat.on of causes that produced the inordinate multi- 

 plication ol plant or animal, so that it became a pest; second 



