22 The Study of Animal Life part i 



the ashes of animals into a new life. A strange partner- 

 ship between Bacteria on the one hand and leguminous and 

 cereal plants on the other has recently been discovered. 

 There seems much likelihood that with some plants of 

 the orders just named Bacteria live in normal partner- 

 ship. The legumes and cereals in question do not thrive 

 well without their guests, nay more, it seems as if the 

 Bacteria are able to make the free nitrogen of the air 

 available for their hosts. 



3. Relation of Animals to the Earth. — Bacteria are 

 extremely minute organisms, however, and stories of 

 their industry are apt to sound unreal. But this cannot 

 be said of earthworms. For these can be readily seen 

 and watched, and their trails across the damp footpath, 

 or their castings on the grass of lawn and meadow, are 

 familiar to us all. They are distributed, in some form or 

 other, over most regions of the globe ; and an idea of their 

 abundance may be gained by making a nocturnal expedition 

 with a lantern to any convenient green plot, where they 

 may be seen in great numbers, some crawling about, others, 

 with their tails in their holes, making slow circuits in search 

 of leaves and vegetable debris. Darwin estimated that there 

 are on an average 53,000 earthworms in an acre of garden 

 Tground, that 10 tons of soil per acre pass annually through 

 their bodies, and that they bring up mould to the surface at 

 the rate of 3 inches thickness in fifteen years. Hensen found 

 in his garden 64 large worm-holes in \\\ square feet, and 

 estimated the weight of the daily castings at about 2 

 cwts. in two and a half acres. In the open fields, how- 

 ever, it seems to be only about half as much. But whether 

 we take Darwin's estimate that the earthworms of England 

 pass annually through their bodies about 320,000,000 tons 

 of earth, or the more moderate calculations of Hensen, or 

 our own observations in the garden, we must allow that the 

 soil-making and soil -improving work of these animals is 

 momentous. 



In Yorubaland, on the West African coast, earthworms 

 (Siphonogaster) somewhat different from the common Lum- 

 bricus are exceedingly numerous. From two separate square 



