108 EARTHWORMS AND THEIR ALLIES [ch. 



There is no doubt that wherever land has been 

 at all long under cultivation in any part of the world 

 that land will be found to produce species of the 

 European genera Lumbriciis,Helodrilas, Eisenia, etc. 

 More than this the recently imported European forms 

 will be found to have largely or almost entirely driven 

 out the native species, which have retired more into 

 the interior of the country. There is thus here no 

 barrier placed by temperature. It should be remarked, 

 however, that while these earthworms are most abun- 

 dant in the less tropical regions, they occur in such 

 tropical districts as Peru, though in less striking 

 numbers. Whether those of North America are really 

 indigenes or not remains perhaps a matter for dis- 

 cussion ; but it is at least noteworthy that the vast 

 preponderance of species occurring there are also 

 European and even British. In this particular case, 

 which is on the whole the most emphatic of all the 

 cases of peregrine earthworms, some explanations 

 are possible, or at least have been offered. In the 

 first place it would appear that earthworms are more 

 abundant as individuals in northern countries where 

 the soil is rarely dry for prolonged periods. And as 

 has been already pointed out there is a close relation 

 between earthworms and agriculture. Dunghills are 

 fertile gathering grounds for some species, and 

 ploughed fields and gardens always swarm with 

 several species. In the tropics these animals are 



