viii] MOVEMENT AND MIGRATION 123 



warm extra-European countries, but yet that it has 

 evidently not spread to those countries in the course 

 of nature but by man's interference, it seems possible 

 that time alone has prevented this ; and that there- 

 fore this family Lumbricidae is one of the most 

 recently evolved families of Oligochaeta. Certain 

 structural features support this way of looking at the 

 matter. The same arguments precisely apply to the 

 genus Pherethna, which is also regarded by most 

 systematists as a recently developed race of earth- 

 worms. Anyhow the conclusion which the facts seem 

 to warrant is that the effects of climate in influencing 

 distribution are seen to have an unequal effect upon 

 earthworms, some genera being debarred by climatic 

 conditions while others are indifferent to the same. 



Mountain Ranges and the Migration 

 OF Earthworms. 



In many groups of animals the interposition of a 

 lofty chain of mountains presents an insuperable 

 barrier to migration. The barrier is effective for 

 more than one reason. Lack of vegetation and a 

 differing climate are among the more obvious causes 

 which render Alpine chains important as affecting 

 distribution. There is plenty of evidence in the way 

 of positive fact that mountains are not necessarily 

 barriers to the spread of earthworms. The recent 



