152 MAN AS A DISTRIBUTING AGENT. [CH. Ill 



due to the interference of man. Such islands as St 

 Helena have had their fauna and flora radically changed 

 by these means. 



But there are other cases in which man has probably 

 had an influence which are not quite so obvious as that 

 just cited. Many small land Mollusca are readily earned 

 by ships laden with timber or other material upon which 

 such animals are found ; and the same kind of argument 

 applies to numerous other cases. When therefore we find 

 an identical species upon the two sides of a wide ocean 

 or other barrier hardly to be crossed by the animal in 

 question, we must not at once assume that this is evidence 

 of past land connection, or of unsuspected facilities for 

 crossing the barrier; it may be merely a question of 

 transport by ships. A consideration of the distribution of 

 certain genera of earthworms will serve to illustrate the 

 kind of argument and tests which may be applied to sift 

 these cases. 



The continent of Europe, and, so far as it is known, 

 northern Asia and the whole of North America is chiefly 

 tenanted by members of the genera Allwrus, Allolobophora 

 and Lumbricus. There are but few other genera than 

 these met with in the regions named. But these same 

 genera also occur in every other part of the world. They 

 are abundant for example in South America and in New 

 Zealand. A gathering of earthworms from tropical regions 

 is in fact, so far as my own experience goes, rarely without 

 Lumbricidse. This might be urged as an argument for 

 the antiquity of this particular family ; but as a matter of 

 fact the structural relations of this family to others seems 



