98 Descriptive Zoology. 



Effect of Earthworms on the Soil. Darwin says that in all regions 

 where there is found a smooth expanse of vegetable mold, which is 

 ^he substance of all black soils, this mold has passed and will pass 

 again, every few years, through the bodies of earthworms. Before the 

 observations and experiments of Darwin the world hardly dreamed 

 what an important part earthworms have played in making the soil what 

 it is, but some previous observers had an inkling of it. The quality 

 of the soil is altered by the digestive process. It is worked over, and 

 the deeper layers are brought to and deposited upon the surface. This 

 inversion of the soil is essentially the same as that of plowing, so, as 

 Thomson says, earthworms were plowers before the plow. The holes 

 also aid circulation of air and water in the soil. To get a clear idea of 

 the effects of these worms on the soil, the student should read Darwin's 

 Vegetable Mould and Earthworms. 



NunSber of Earthworms and Extent of their Work. Darwin esti- 

 mated that in the tillable soil of England there were, on the average, over 

 fifty thousand earthworms to the acre ; that they bring up eighteen tons 

 of soil to the acre ; that they cover the surface at the rate of an inch in 

 five years ; and that thus in long ages they have buried large rocks and 

 ancient buildings. And his conclusion is that "it may be doubted 

 whether there are many other animals which have played so important 

 a part in the history of the world as have these lowly organized ani- 

 mals. 11 In the United States earthworms are not so numerous. 



Harm done by Earthworms. Earthworms do some harm by eating 

 tender seedlings and delicate roots, but this is trifling in amount as 

 compared with the very^reat aid they render to agriculture. 



Repetition of Parts. If a person had grown up without having seen 

 an earthworm, at first sight of one he would probably be impressed with 

 its sameness of structure, nearly all the rings or segments having the 

 same general appearance. Dissection shows that the internal structure 

 is not so very different, the anterior portion having somewhat of a 

 variety in the development of the parts of the digestive, circulatory, and 

 reproductive organs. Back of the middle of the body there is no seg- 

 ment which adds anything new in function to the body, each segment 

 being simply a repetition of what precedes. As a rule, multiplicity of 

 parts, without corresponding variety of structure and function, marks an 

 animal as low in rank. 



Recovery after Mutilation. When an earthworm is cut in two in the 

 middle, the anterior end probably lives in most cases, as it has all the 



