106 EARTHWORMS AND THEIR ALLIES [CH. 



But while we can lay down no general explanation 

 of the phenomenon, it is possible to furnish some 

 explanation of particular cases. Thus the genus 

 Microscolex is the only exotic genus which appears 

 to have established itself in Europe, from which 

 country indeed it was early known as an apparently 

 indigenous inhabitant. We must put this and some 

 similar cases down to ability to do without great 

 heat. It is probable in fact that the original home 

 of Microscolex is the antarctic half of the globe ; and 

 this of itself would allow of its establishing a new 

 home in the northern hemisphere, did other circum- 

 stances allow of it. 



It might be urged that this genus has been able 

 to establish itself in Europe because it has in fact 

 had the chance denied to other species. There are 

 a good many, however, which would in that case be 

 in the same category. Some years ago I received 

 from time to time a very large number of earthworms 

 from the Royal Gardens at Kew which had been 

 accidentally imported thither from many quarters 

 of the globe, among which I described some eighteen 

 or twenty new species including, for instance, the 

 African genus Gordiodrilus. There are plenty of 

 facts of a similar nature and Dr Michaelsen has 

 pointed out that botanical gardens act as centres 

 of dispersion for accidentally introduced Oligochaeta. 

 We must therefore come to the conclusion that 



