i28o LIFE : OUTLINES OF GENERAL BIOLOGY 



zoologist to combat these injurious animals, both directly and by 

 encouraging natural checks. He has also to advise against operations 

 that upset the Balance of Nature and against careless importations 

 and transportations. 



Seventh, there are animal enemies which injure man neither 

 directly, nor through his stock and crops, but by getting at 

 his stores or permanent products. Termites are very destruc- 

 tive in warm countries; rats and mice spoil much more than 

 they eat; weevils and their relatives destroy stored corn; boring 

 beetles eat away the rafters; ship- worms and boring crustaceans 

 do much harm to wooden piers and the like. All these have to be 

 coped with. 



But an eighth group consists of animals that are man's indirect 

 friends, by keeping a check on the fifth, sixth, and seventh groups, 

 and must therefore be conserved and encouraged. Above all, birds 

 keep down insects, and so help agriculture, abate malaria, and so 

 on. Thus owls keep down the voles; lapwings feed on wireworms 

 and leather-jackets; hedgehogs crunch slugs, and toads are also 

 friends to gardeners; ichneumon-flies lay their eggs in caterpillars; 

 spiders catch many injurious insects; ladybirds levy toll on the 

 green-flies; water-wagtails are fond of the small water- snails that 

 harbour the larval stages of the liver-fluke; and so on through a very 

 long list. Yet in truth the chain is endless, since there are species 

 inimical to this eighth group, and so indirectly to man ; and these 

 in turn have their enemies 



Similarly for plants, there are wild species directly used for food 

 and drink; those that furnish valuable products like textiles and 

 drugs; those that have been cultivated; those that help man, as 

 forests do in improving the climate. On the other hand, there are 

 inimical plants like the "poison-ivy", and many bacteria; the weeds 

 that become pests and the moulds that attack crops ; the fungi that 

 destroy stores and dry-rot wood. 



The central idea of economic biology is that the circle of human 

 life intersects or is intersected by many other circles; and these 

 intersections, which are often changing, have to be controlled in 

 man's interests — these being generously and farsightedly interpreted. 

 Man is part of a web of life, in the weaving of which he increasingly 

 shares ; and the success of his weaving depends on his understanding, 

 which in this particular case is expressed in Economic Biology, 

 which we have here to leave to volumes of its own. 



There is reason for encouragement in the multiplication of societies, 

 laboratories, and journals of Economic Zoology and Botany, not 

 forgetting Economic Biology in the stricter sense; and while the 

 practical gains are already great, there has also been a welcome 

 enrichment of the so-called "pure sciences" at the hands of those 

 who have been largely pre-occupied with concrete problems. 



