THE KAHrUWOEM 171 



be kept under favorable conditions, each half maj^ develop 

 its missing organs, so that two complete worms will result. 

 The anterior half of one worm may be attached to the hinder 

 end of a second worm bj' the cut edges, owing to the fact that 

 the cut edges grow together. This operation is called grafting. 

 Economics. — Earthworms are, to a certain extent, inju- 

 rious to vegetation, since they eat tender seedlings and roots; 

 but, on the other hand, they are almost indispensable to agri- 

 culture. Their burrows permit rain to percolate deep into the 

 ground, instead of running off on the surface. They keep the 

 soil loose, facilitating the penetration of the roots of plants. 

 The earth that passes through their bodies is ejected on the 

 surface of the ground near the openings of their burrows, and 

 is called a " casting." By means of castings the deeper-lying 

 earth is lirought to the surface, and the surface layer of rich 

 earth, called " vegetable mold," is in this way increased in 

 thickness by additions to its upper surface. The thickness of 

 the layer of mold which the castings of one year, if uniformly 

 spread out, would make has been estimated by Darwin to be 

 in England aI)out two-tenths of an inch. Most of these 

 castings are merely taken from the deeper-lying mold, but 

 they are emiched by the intestinal secretions in passing 

 through the body of the worm. These intestinal secretions 

 are said to have the power of slowly dissolving sand and 

 thus of turning it into soil. Darwin says: "It is a mar- 

 vellous reflection that the whole of the superficial mold over 

 any smooth expanse has passed, and will again pass, every few 

 years, through the bodies of worms. The plow is one of the 

 most ancient and most vahiable of man's inventions, but long 

 before he existed the land was, in fact, regularly plowed, 

 and still continues to be thus plowed, by earthworms." 



