EFFECT OF PLANT AND ANIMAL LIFE 381 



Certain animals, like the crayfish, have likewise a habit of 

 burrowing in the ground, though as they are wholly subter- 

 ranean or aquatic in their nature, the results are less conspicu- 

 ous to the casual observer. In searching for their food, these 

 animals bore numerous horizontal channels or galleries some- 

 times an inch or so in diameter and extending for many feet, 

 usually ending in an upward shaft reaching to the surface, or 

 the margin of a pond or stream. These form natural drainage 

 channels and allow a more ready access of air, converting what 

 might under other conditions be a heavy, clayey or even marshy 

 soil, unfit for cultivation, into one light and fertile. 



By burrowing through dams and embankments, they have, 

 however, in some instances so weakened these structures as to 

 cause them to give way, and large districts have become inun- 

 dated and rendered unfit for cultivation. 



Probably none of the forms of animal life thus far mentioned 

 produce such wide-spread and beneficial results as have been 

 ascribed by Darwin^ to the common earthworm, the angleworm 

 of the New England disciples of Isaac Walton. These insig- 

 nificant creatures burrow in the moist rich soil, and derive 

 their nourishment from the organic matter it may contain. In 

 order, however, to obtain this comparatively small amount of 

 nutritive matter, they devour the earth without any selective 

 power, and pass it through their alimentary canals, rejecting 

 the non-nutritious portions, which nearly equals in bulk that 

 first taken in. The numerous holes made, while in part perhaps 

 to afford passage to the surface, are mainly excavated in this 

 process of soil eating and actually represent the amount of ma- 

 terial which the worms have passed through their digestive 

 systems. 



Darwin states that in certain parts of England these worms 

 bring to the surface every year, in the form of excreta, more than 

 10 tons per acre of fine dry mould, *'so that the whole superficial 

 bed of vegetable mould passes through their bodies in the course 

 of every few years.'' By collecting and weighing the excretions 

 deposited on a small area during a given time, he found that the 

 rate of accumulation was an inch in every five years. The 



like a field of gigantic potato hills" and he expressed the belief that in 

 Brazil, and the tropics generally, ants are of more geological importance 

 than are earthworms in temperate regions. 

 ^The Formation of Vegetable Mould. 



