THE WORK OF EARTHWORMS 283 



Darwin's statistics are eloquent, but we cannot give 

 more than a few samples. He showed that there are often 

 50,000 (and there may be 500,000) earthworms in an acre 

 of good soil ; that they often pass 10 tons of soil per acre 

 per annum through their bodies ; and that they often cover 

 the surface at the rate of 3 in. in fifteen years. Though a 

 common British worm only passes out about 20 oz. of earth 

 in a year, the weights deposited in a year on two separate 

 square yards which Darwin watched were respectively 

 675 Ib. and 8-387 lb., which correspond to 14! and 18 tons 

 per acre per annum. A field which was so thickly covered 

 with hard flints that it was known as " the stony field," was 

 left untouched for thirty years, after which, we are told, a 

 horse could gallop from one end to another without ever 

 striking a stone. 



As we follow the work further, additional aspects of 

 importance are revealed. It is plain, for instance, that 

 the constant exposure of the soil-bacteria is bound to 

 have far-reaching effects both for good and ill. On the 

 one hand, it allows the microbes to be scattered by wind 

 and rain ; on the other hand, it exposes them to the action 

 of the sunlight, the most universal, effective, and econ- 

 omical of all germicides. In Yorubaland, on the West 

 Coast of Africa, Mr. Alvan Millson calculated (following 

 Darwin's methods) that about 62,233 tons of subsoil are 

 brought every year to the surface of each square mile, 

 and that every particle of earth, to the depth of two feet, 

 is brought to the surface once in twenty-seven years. It 

 need hardly be added that the district is fertile and healthy. 



Earthworms also play their part in the disintegration 

 of rocks, letting the solvent humus-acids of the soil down 

 to the buried surface. Their castings on the hill-slopes 

 are carried down by wind and rain, and go to swell the 

 alluvium of the distant valleys or the wasted treasures of the 



