THE WEB OF LIFE 57 



The monograph that Gilbert White wished for 

 in 1777 was pubhshed by Darwin in 1881, the 

 year before he died — " the completion," he said, 

 " of a short paper read before the Geological 

 Society more than forty years ago." With his 

 characteristic thoroughness and patience he worked 

 out the part that earthworms have played in the 

 history of the earth, and proved that they deserve 

 to be called the most useful animals. By their 

 burrowmg they loosen the earth, making way 

 for the plant rootlets and the raindrops ; by 

 bruising the soil in their gizzards, they reduce 

 the particles to more useful, powdery form ; by 

 burying the surface with castings brought up from 

 beneath, they have been for untold ages ploughers 

 before the plough, and by burying leaves they 

 have made a great part of the vegetable mould 

 over the whole earth. In illustration of the last 

 point, we may notice that we recently found thir- 

 teen midribs of the leaves of the rowan, or mountain- 

 ash, radiating round one hole like the spokes 

 of a wheel ; the withering leaflets had been carried 

 down, and two were sticking up at the mouth of 

 the burrow : that meant 91 leaflets to one hole. 

 Darwin showed that there often are 50,000 (and 

 there may be 500,000) earthworms in an acre ; 

 that they often pass ten tons of soil per acre 

 per annum through their bodies ; and that they 

 often cover the surface at the rate of three inches 

 in fifteen years. Though our British worms only 

 pass out about 20 oz. of earth in a year, the weights 

 thrown up in a year on two separate square yards 

 which Darwin watched were respectively 6"75 lb. 

 and 8*387 lb., which correspond to 14| and 18 

 tons per acre per annum. 



