Inter-Relations of Living Creatures 659 



that help man's endeavours. The earthworms have largely made 

 the fertile soil, and the humble-bees pollinate the clover. The fish- 

 eries are after all dependent on the multitudes of minute crusta- 

 ceans in the sea-soup, and these on the microscopic Infusorians 

 and Algaa. (d) But there are other animals that hinder man's 

 operations and baulk his experiments. The poisonous snake bites 

 his heel, and the mosquito infects him with yellow fever. In some 

 places the midges are so rampant that life becomes a burden, and 

 the heaviest cloud of depression and despair that has ever 

 rested on the human race is due to a contemptible threadworm 

 the Hookworm whose larva? enter man's skin from the 

 fouled soil. A knowledge of the life-history of the intruding 

 parasite is now making it possible, however, to check its de- 

 plorable ravages. Long ago in man's history the enemies that 

 counted for most were large creatures, such as lions and 

 tigers, wolves and bears; but nowadays most of man's serious 

 enemies are minute, we may even say microscopic, (e) Besides 

 those animals that directly hinder man there are the multi- 

 tudinous enemies of his flocks and herds, his farms and gardens. 

 The field-voles are sometimes "plagues"; the wood-pigeons 

 devour good seed; the Phylloxera spoils the vineyards; the 

 Colorado beetle ravages the potato crop; the number of in- 

 jurious insects is legion. Man's domesticated animals are 

 attacked by numerous parasites: the horse has its "bots," the 

 cattle their "warbles," the sheep their "sturdie," and the pig has an 

 internal menagerie. (/) Others, again, attack man's permanent 

 products and his stores. In warm countries the white ants or 

 termites make sawdust of everything wooden, and imply a con- 

 siderable check on many of man's operations. Also very serious 

 are the grain-weevils that do much harm in granaries, and rats 

 are worst of all. Bookworms and clothes moths can be readily 

 checked, but it is a more difficult problem to cope with cock- 

 roaches, (g) Finally, there are those very useful animals which 

 help to keep down those mentioned in the last three sections (d, e, 



