22 2 EVOLUTION, GENETICS, AND EUGENICS 



stalks of leaves and twigs into it; and, most of all, by throwing up such 

 infinite numbers of lumps of earth called worm-casts, which, being 

 their excrement, is a fine manure for grain and grass. Worms prob- 

 ably provide new soil for hills and slopes where the rain washes the 

 earth away; and they affect slopes probably to avoid being flooded. 

 .... The earth without worms would soon become cold, hard- 

 bound, and void of fermentation, and consequently sterile 



These hints we think proper to throw out, in order to set the inquisitive 

 and discerning at work. A good monograph of worms would afford 

 much entertainment and information at the same tims, and would 

 open a large and new field in natural history." 



The monograph that Gilbert White wished for in 1777 was pub- 

 Hshed 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 burrowing 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 thirteen 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 14I and 18 tons per acre 

 per annum. 



We follow the work further and it becomes evident that the con- 

 stant exposure of the soil bacteria on the surface is bound to be 

 important, on the one hand, in allowing them to be scattered by wind 

 and ram, on the other in exposing them to the beneficent action of the 



