44 DARWINISM STATED BY DARWIN HIMSELF. 



converted into humus by having passed through the ali- 

 mentary canals of these two worms. It is believed by 

 some persons that worm-burrows, which often penetrate 

 the ground almost perpendicularly to a depth of five or 

 six feet, materially aid in its drainage ; notwithstanding 

 that the viscid castings piled over the mouths of the bur- 

 rows prevent or check the rain-water directly entering 

 them. They allow the air to penetrate deeply into the 

 ground. They also greatly facilitate the downward pas- 

 sage of roots of moderate size ; and these will be nourished 

 by the humus with which the burrows are lined. Many 

 seeds owe their germination to having been covered by cast- 

 ings ; and others buried to a considerable depth beneath 

 accumulated castings lie dormant, until at some future 

 time they are accidentally uncovered and germinate. 



P 313 When we behold a wide, turf-covered ex- 



panse, we should remember that its smooth- 

 ness, on which so much of its beauty depends, is mainly 

 due to all the inequalities having been slowly leveled by 

 worms. It is a marvelous reflection that the whole of 

 the superficial mold over any such 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 

 valuable of man's inventions ; but long before he existed 

 the land was in fact regularly plowed, and still continues 

 to be thus plowed, by earth-worms. It may be doubted 

 whether there are many other animals which have played 

 so important a part in the history of the world as have' 

 these lowly organized creatures. Some other animals, 

 however, still more lowly organized, namely corals, have 

 done far more conspicuous work in having constructed 

 innumerable reefs and islands in the great oceans ; but 

 these are almost confined to the tropical zones. 



