268 DARWINISM AND THE PROBLEMS OF LIFE 



shown that the regenerative force is proportionate to the 

 kind of mutilation to which they are most frequently 

 exposed. 



Thus the leeches, which are dreaded and not much 

 exposed to mutilation, have no power of regeneration. 

 The medicinal leech is still very common in France and 

 Hungary, and is a great trouble to bathers, as it gathers 

 in swarms at the first splash. Much less troublesome 

 are the large leeches that are found in the ponds, and 

 known as horse-leeches ; these may be taken in the 

 hand without fear. Their teeth cannot bite through the 

 human skin ; they can only pierce the mucous lining of 

 the nose, the mouth, and other parts. One species of 

 the horse-leech, the Aulastomum gulo, feeds on snails, 

 and does not generally indulge in blood-sucking : the 

 other species, the Hcemopis vorax, may become a great 

 nuisance by getting into the throats of horses and cattle 

 when they bathe, and attaching themselves to the soft 

 parts. But this hcemopis plague is only found in North 

 Africa. 



There is also in our ponds a worm about as thick as 

 a violin-string, and sometimes a foot in length. It looks 

 like a horse-hair, and, as a matter of fact, the rustics 

 have in many places fastened on to it the legend that it 

 is a living horse-hair, travelling about in the water and 

 able to penetrate the human skin. In reality the animal 

 is quite harmless, and is, in fact, unable to maintain 

 itself ; it lives only a short time, and uses this for laying 

 its eggs. From these develop tiny larvae with a pointed, 

 zigzag boring apparatus, which pass through the skin 

 of May-flies and gnat-larvae, live in them for some time. 



