22 THE STUDY OF ANIMAL LIFE CHAP. 



animals into a new life. A strange partnership between 

 Bacteria and many kinds of leguminous plants illustrates 

 another thread in the web of life. The Bacteria form 

 nodules on the roots and in some obscure way they make 

 the free nitrogen of the atmosphere available for their hosts. 

 Thus it is that poor soil can be made rich by ploughing 

 in crop after crop of leguminous plants. And this is 

 only one of a score of ways in which Bacteria are bound 

 up in the bundle of life with other creatures. 



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 not found in very wet 

 places, or in very dry soil, or near the spray of the sea, 

 but otherwise they are abundantly represented in most 

 regions of the globe. 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 esti- 

 mated that there are on an average 53,000 earthworms 

 in an acre of garden ground, 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. Heiiseii found in his garden 64 large 

 worm-holes in 14 J square feet, and estimated the weight 

 of the daily castings at about 2 cwt. in two and a half 

 acres. In the open fields, however, 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 



