DISTRIBUTION OF THE TERRICOLOUS OLIGOCHAETES 141 



aid of birds appears decidedly impossible, and even more so since no case 

 of bird migration across the middle of the Atlantic is known. 



It is well known that the Oligochaetes are exclusively terricolous or fresh- 

 water taxa and everything that is known about their phylogenesis leads us to 

 assume that they have evolved exclusively in such habitats. Only a few forms 

 of hmicolous Oligochaetes and two genera of Megascolecidae have acquired 

 secondarily a certain tolerance of brackish and even salt water. The two 

 genera of Megascolecidae which tolerate sea water are Pontodrilus (the name 

 is significant) and Plutellus, often considered as one. Only for these forms is 

 the passage from one continent to another thinkable, but mainly by means of 

 slow displacement along the shores : the presence of five endemic species of 

 Plutellus in the Cahfornian subregion can be explained by postulating the 

 arrival of an ancestor from the eastern coasts of Asia. This is the only case 

 where I consider it permissible to suppose than an earthworm was able to 

 move, if not across the sea, then at least along its shores, to reach America. 



I want to add one last objection before moving on to other points : if we 

 admit that the relationship between the earthworms of the two Atlantic 

 coasts is due to passive transport of some ancient forms across this ocean, 

 how do we justify the great differences existing between the earthworm 

 faunas of Madagascar and the African mainland, which are separated by a 

 body of water so much narrower than the Atlantic? 



The hypothesis that the principal zoological groups of the two coasts of 

 the Atlantic came from the north — a hypothesis which has found its most 

 authoritative defender in Matthew (1915) and has recently been supported 

 by Darhngton (1957) — does not offer as serious theoretical difficulties as the 

 preceding idea. Nevertheless, it raises myriads of doubts and problems calUng 

 for very complicated, additional hypotheses. 



To explain the presence of the Acanthodrihnae in South Africa and in the 

 Chilean-Patagonian region, intentionally ignoring their presence in New 

 Zealand and other lesser southern islands and admitting that they started 

 from the northern continents, we must explain also why they have disappeared 

 from immense continental areas where once they must have been common. 

 If we assume that the environmental conditions became hostile to them in 

 these zones and that the appearance of new zoological groups has completely 

 replaced them, we must ask ourselves why it is that two species of Acantho- 

 drihnae, Microscolex phosphoreus and M. dubius, recently have been able to 

 invade Europe, North America, and many tropical countries where they now 

 flourish and compete successfully with indigenous forms. 



Certain facts regarding the biology of earthworms, which it would be well 

 to make clear immediately, make the application of Matthew's (1915) theory 

 to our case difficult. Earthworm species have often a latitude of adaptation 

 unparalleled by any other invertebrate: Dendrobaena rubida lives on the 

 island of Disko off Greenland, on the Himalayas and throughout the tropical 



