THE BALANCE OF LIFE. 65 



Thirdly, the plaut life may be entirely altered. This is by far the 

 most obvious cause and deserves separate discussion. 



Lastly, the interrelations of the fauna are deliberately upset by man 

 in shooting insectivorous birds, in checking' beneficial insects and in 

 making" the conditions unsuitable to useful insect-eating animals. 



The last two are the really important causes that affect insects 

 directly. In cultivated areas, we grow large numbers of the same plant 

 side by side; any insect that can feed upon a cultivated crop finds 

 abundant food, has not to search for it, and can readily lay its eggs in one 

 place. Instead of searching through the jungle for the particular plant 

 she requires, a female moth emerges in a field of that plant, finds a mate 

 at once and can readily lay eggs ; she is not exposed to enemies in her 

 flights to find a mate or in her endeavours to find sufiicient food-plants on 

 which to lay her eggs. Not only does man grow larger areas, but he 

 grows the plants at seasons of the year when food is otherwise scarce ; 

 the crops grown under irrigation in the hot dry weather help many 

 insects over a critical time and so give them an additional opportunity of 

 breeding and multiplying. 



Further, plants grown under somewhat artificial conditions have not 

 the same vigour to resist pests as plants growing wild in the jungle. 

 Few crop' plants are grown where they naturally thrive ; in the jungle 

 there is competition, there is a struggle for life and only strong healthy 

 plants in good vigour can live ; in cultivation plants are kept alive by 

 stimulation, are grown in soil that does not suit them, are " domesticated " 

 and have not the vigour that especially resists the plant parasites. How 

 seldom we see a wild plant attacked by scale insects or plant lice? A 

 wild plant has the vigour to resist, to make itself distasteful and to 

 out arrow the disease. 



In addition to helping the increase of insects by the artificial manner 

 in which he grows his crops, man does so also by checking the birds and 

 other predators which check insects. These include birds, lizards, bats, 

 predatory and parasitic insects and the like. They are discussed in detail 

 elsewhere, but we can see that our artificial conditions upset this part of 

 the balance of life and so give opportunities for the abnormal increase of 

 insects. 



If we consider this question as a whole, we can dimly see that every 

 now and then the checks which are usually operative may temporarily be 

 suspended, so that we get a vast increase in the numbers of some common 

 insect, i.e., of an insect which, if abundant, probably destroys a crop. 

 Then we have an outbreak of a '^pest,'' a perfectly natural phenomenon 

 due to causes which man himself brings about, In nature, and generally 



