DISTRIBUTION OF THE TERRICOLOUS OLIGOCHAETES 



145 



Critical analyses of these phenomena have been carried out satisfactorily by 

 Wilcke (1955) but it must be remembered that some of the Oligochaetes 

 dwell in peat or moor soil, which was certainly the first environment that they 

 occupied; even now the Phreorictidae — presumed ancestral group of all the 

 living famihes of terricolous Oligochaetes — live in this environment; other, 

 more modern groups, e.g. the Ocnerodrilinae, the genera Alma and Drilocrius, 

 etc., have invaded this important ecological niche secondarily, but without 

 doubt this occurred in a very ancient time while other parallel groups were 

 speciahzing themselves in their present habitats. 



20 



ko 

 40 J 

 JO 



iOO 

 120 



1A0 

 ioo 



220 



I 

 1 



i i i 



I 



P i^'i 



i i i 



"I 



i 





i mm 

 i I 





■ j 



i 



Fig. 1 1 . Chronology of Mammals, of Terrestrial Oligochaeta, and of three types 



of soil in millions of years. The data for Mammals after Rensch (1954), the data 



of soil types after Wilcke (1955). 



The diagram in Fig. 1 1 establishes the parallel between the chronology of 

 the Mammal evolution as schematized by Rensch (1954), and the chronology 

 of the Oligochaete evolution as it can be reconstructed from their geographical 



