5 8 The Great World's Farm 



any kind of vegetable matter, or coarsely upon their own 

 dead comrades, or meat when put in their way. But their 

 chief food consists of half-decayed leaves, enormous quan- 

 tities of which are pulled into their burrows, torn into 

 smaU shreds, and then swallowed and digested; and it is 

 this vegetable matter which changes the color of the earth 

 which the worms swallow with it, and converts it into 

 "mold" — vegetable mold. 



Two w^orms kept in a large pot of sand, well moistened, 

 of course, but consisting only of mineral matter, converted 

 the top layer into vegetable mold four inches deep, 

 simply by the help of the leaves strewed on the surface. 



Seeing only the little dark heaps of soil thrown up by 

 worms on grass-plots and gravel-paths, heaps which are 

 soon washed down again by rain, one has some difficulty 

 in realizing the vast amount brought up in the course of 

 a year. But Mr. Darwin reckoned that near Nice this 

 amounts to from about fourteen to eighteen tons to the 

 acre ; this is supposing them to be as numerous and active 

 over the whole of the field as they were in the one square 

 yard chosen for observation; but it is also supposing them 

 to work for only six months of the year, which he con- 

 sidered a low estimate. The largest amount was brought 

 up on very poor pasture, where leaves were probably 

 scarce, and the worms had to swallow much earth in 

 order to obtain sufficient food. 



On the whole it seems probable that they bring up 

 more than ten tons of soil to the acre in many parts of 

 England year by year, and that the entire mass of mold — 

 the dark surface-soil of every field — passes through their 

 bodies in the course of a few years, and is by these means 

 sifted and rendered extremely fine, besides being thor- 



