92 ANIMAL LIFE AND HUMAN PROGRESS 



swimming larva cannot survive in drawn water for more than 

 forty-eight hours. Thus a simple preventive expedient is 

 open. 



Major Leiper was led to his discovery by knowing that the 

 young stages of the Trematode worm (Distomum hepaticum), 

 which causes the disease of liver-rot in sheep, are harboured 

 inside the little fresh-water snail called Limnaea truncatula, 

 common in most pools. The more drainage of pasture-land, 

 the fewer pools, the fewer fresh-water snails, the less liver-rot 

 in sheep. And we may make the idea of the web of life 

 picturesque again by noticing that the water wagtail is very 

 fond of the fresh-water snails, so there is a linkage between 

 the preservation of water wagtails national assets in any 

 case and the success of sheep-farming. 



Parasitism is a bit of a knot, perhaps, in the web of life, 

 but perhaps we look at it most reasonably when we see it as a 

 particular mode of a universal tendency to inter-linkage. 

 And it is interesting to note that just as a partnership may 

 sink into parasitism, so a parasite may sometimes make good 

 by becoming a partner. This is probably the case with the 

 mycorhiza fungi on the roots of some trees, and the bacteria 

 in the root-tubercles of leguminous plants. But let us change 

 the subject. 



8. Teachers would find it useful to look up the 1903 

 British Museum Report on Economic Zoology, in which Sir 

 Ray Lankester clearly arranged the chief practical inter- 

 relations between man and animals. There are (1) those 

 captured for food and other products, from rabbit to oyster, 

 from whitebait to whale ; (2) those bred for use, from cattle 

 to bees, from silkworms to turkeys ; (3) those that help, from 

 earthworms to flower- visiting insects ; (4) those that hinder, 

 from snakes to mosquitoes ; (5) those that injure useful 

 animals and plants, such as voles and scale-insects ; (6) those 

 that spoil man's permanent products, such as white ants, 

 clothes moths, book- worms, and (7) those that keep down 

 the last three, from lapwings to lady-birds, from hedgehogs 

 to ichneumon flies. 



The mere statement of this useful classification suggests 



