186 NATURE STUDY 



rolling or by the action of the wind or by rain, so that 

 the worms are thus constantly levelling the surface of the 

 earth. 



Worms prepare the ground excellently for seedlings. 

 They sift the soil, so that no stones larger than they can 

 swallow are left in it. They mix it, as does a gardener 

 who wishes to prepare fine soil for his choicest plants. 

 They cover up bones, etc., thus bringing them in a more 

 or less decayed state within reach of the roots. They 

 cover seeds which lie upon the surface, thus giving the 

 radicle a leverage ; and, last of all, their burrows facili- 

 tate the penetration of the roots. 



Says Darwin : " The plough is one of the most ancient 

 and most valuable of man's inventions ; but, long before 

 he existed, the land was, in fact, regularly ploughed by 

 earth worms. 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 creatures." 



Birds : 



Stray robins and bluebirds remain with us in Phila- 

 delphia all winter. In the latter part of February those 

 who have gone farther south begin to straggle up, stay- 

 ing with us for a short time, but usually going further 

 north. In March, the robins, bluebirds, and crow black- 

 birds become quite common. The red-winged black- 

 bird and the meadow lark are also to be found in 

 their favorite haunts. 



All of these birds eat insects, a story plainly revealed 

 by their long, narrow bills. They are, of course, all 

 perchers. 



