220 GENERAL ZOOLOGY 



along. Black soil called humus is made up of the usual soil- 

 minerals, mixed with a large amount of decaying vegetable 

 matter and some animal matter. Completely decayed organic 

 matter is that which has returned to its original inorganic, 

 mineral state. The partially decayed organic matter can still 

 be used as tissue-building and energy-producing food ; it is 

 this which the earthworms get in the soil they swallow. 



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

 face at night to void the indigestible contents of the intestine. 

 The coiled, worm-shaped " castings " are familiar to any one 

 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, extending over a period of more than forty 

 years, during which time he also collected facts from all parts 

 of the world and published in a book entitled, The Forma- 

 tion of Vegetable Mould through the Action of Worms, are 

 extremely valuable to us now, in showing how small agencies 

 acting over wide areas, through ages or even years of time, 

 can yield tremendous results. 



If every active earthworm voids its castings at the surface, 

 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 humus acids present at all times in 

 the soil attack and disintegrate the organic matter, thus en- 

 riching the soil with the minerals which the growth of exten- 

 sive crops may have deprived it of. Darwin determined the 

 rate at which objects once upon the surface gradually sink 

 to lower depths. Layers of cinders, chalk, stone, etc., were 

 strewn upon small fields, and trenches were dug from year 

 to year to ascertain the progress of the earthworms' activity. 

 In fields of ordinary fertility, having an average supply of 

 worms, the amount of earth brought to the surface and spread 



