ANIMALS IN THE SOIL 47 



wc have the earthworm, helpful rather than harmful, which 

 is perpetually wriggling its way through the soil, eating its way 

 tlirough it and extracting what organic matter it can get from 

 the swallowed earth; and passing the indigestible remains in 

 the form of worm castings on to the surface of the land. Darwin 

 has estimated that these dried-up casts will raise the level of 

 an acre of ground in England by one-tenth of an inch each 

 year. 



The soil, as we have seen, is simply teeming with life, 

 while manured soil teems more. An investigation carried on 

 at Rothamsted on two plots of ground, one of which had 

 received no manure since the accession of Queen Victoria in 

 1837, and the other of which had received 14 tons of farmyard 

 manure per acre, per annum, since 1843, showed that there were 

 normally three times as many animals in the latter as there 

 were in the former. In round numbers there were 15,100,000 

 animals per acre, of which 7,720,000 were insects, in the 

 manured plot. The corresponding numbers in the unmanured 

 acre were 4,950,000, of which 2,470,000 were insects. The 

 greatest number, both of insects and other invertebrates, 

 occurred in the upper three inches of the soil; but there 

 were exceptions to this. Most of the injurious insects, such 

 as the wire-worm, Elater, and the larvae of the daddy-long- 

 legs, Tipula, were not affected materially by the manurial 

 treatment of the plot. The invertebrata concerned consisted 

 of larvae and imagos of all the principal groups of insects, 

 many species of centipedes and millipedes and a certain 

 number of spiders and of mites, many worms and round- 

 worms and certain terrestrial Crustacea, such as the wood- 

 louse ; and a number of snails were also found. 



There are many other burrowing animals, such as the marmot 

 and the prairie dogs, the rabbit, the fox and the badger, but 

 the most important of these in our country is the mole. This 

 burrower displaces a very large quantity of earth. It works 

 very hard, has a tremendous appetite, eats all manner of 

 insect grubs, and will consume its own weight of earthworms 

 in the course of twenty-four hours. It seems to work rather 

 spasmodically, as the earth castings are ejected at periods of 

 three hours. 



