The Soil 17 



rents in the air keep the materials composing it always 

 stirred up so that the composition is always nearly the 

 same. But for the winds and the natural upward and 

 downward currents of the air there would be an accumu- 

 lation of carbon dioxide in the lower levels just as we find 

 it deep in wells and mines, causing the choke damp so 

 fatal to animal life. 



There are still other lowly soil builders. 

 Subsoiling The late Charles Darwin, the most careful 

 Earthworms ^^^ methodical of students, wrote a book 

 on the work of the earthworm. These 

 earthworms are found in all parts of the world, and have 

 in their various genera an enormous range. They are 

 found in the most distant and isolated islands, in far away 

 Iceland, in the West Indies, in the Coral Islands of the 

 Pacific and in the Falkland Islands of the Antarctic Ocean. 

 Their wide distribution, Darwin says, is hard to explain, 

 since they are killed by salt water, and could not have 

 drifted to these islands. But in every place they inhabit 

 they are continually engaged in throwing up little mounds 

 or castings. No one in the country who has observed 

 anything can fail to have observed these castings of the 

 earthworms. In humid chmates they are found in the 

 greatest abundance, and always in the more moist locali- 

 ties of drier chmates. Every boy who goes fishing knows 

 well that he must look for worms in a moist and fertile 

 piece of ground. An observer in India says that after the 

 water is drawn off from the rice fields the whole soil soon 

 becomes covered with the castings of the worms, and on 

 the lawns they are continually compelled to roll down the 

 castings piled up fully four inches high. Even on the 



