THE INSECTS. 129 



learnt to reduce their numbers, in others we are 

 almost inclined to despair. Some economic 

 plants have had to be given up in certain places, 

 to be replaced by others which perhaps in their 

 turn may be abandoned. 



Some impression has been made in temperate 

 climes, but the tropical pests have yet to be con- 

 quered. Man is weaker animal and vegetable 

 life stronger where there is no winter. Every- 

 thing here is burning with energy except man. 



It is only of late years that the study of the 

 interdependence of man on animals and plants 

 animals on plants and plants on animals has been 

 commenced. The problems are innumerable, and 

 perhaps never will be solved. A century ago the 

 farmer looked upon plagues as almost super- 

 natural, now we see that they are a part of the 

 great system or constitution of things. They must 

 not be dealt with too hastily, for already mistakes 

 have been made by treating friends as foes. We 

 are not yet in a position to always tell whether 

 a species may not be useful. It has lately been 

 suggested as at least probable that the fertiliser of 

 cacao is a species of thrips. Hitherto this family 

 has been considered unmitigated pests, to be 

 ruthlessly destroyed wherever found. Wasps are 

 not generally classed with our friends, but some of 

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