AN EARTHWORM 249 



It is one of the habits of earthworms to come to the sur- 

 face at night to void the indigestible contents of the intes- 

 tine. The coiled, worm-shaped " castings" are familiar to 

 anyone who has noticed the ground where the grass is thin, 

 even by the sides of much-used pavements in cities. The 

 soil in the castings is usually brought from depths varying 

 from a very few inches to as great a depth as five feet. 

 Observations made by Darwin extended over a period of 

 more than forty years. During that time he also collected 

 facts from all parts of the world and published a book en- 

 titled The Formation of Vegetable Mould through the Action 

 of Worms. His observations are extremely valuable to us 

 now, for they show how small agencies acting over wide 

 areas, through ages or even years of time, can yield tre- 

 mendous results. 



If every active earthworm voids its castings at the sur- 

 face, fallen leaves, sticks, and other bits of organic objects 

 will in a short time be covered with a thin coating of earth 

 brought up from beneath. The acids present at all times in 

 the soil attack and disintegrate the organic matter, thus 

 enriching the soil with the minerals of which the growth of 

 extensive crops may have deprived it. Darwin determined 

 the rate at which objects once upon the surface gradually 

 sink to lower depths. Layers of cinders, chalk, stone, and 

 the like were strewn upon small fields, and trenches were 

 dug from year to year to ascertain the progress of the earth- 

 worms' activity. In fields of ordinary fertility, having an 

 average supply of worms, the amount of earth brought to 

 the surface and spread out more or less evenly is one fifth 

 of an inch a year. In the course of comparatively few years 

 this is sufficient to conceal from sight objects of considerable 

 size. Indeed, Darwin witnessed a sterile, stony field with 

 flints "as large as a child's head" transformed into a fertile, 



