120 EARTHWORMS AND THEIR ALLIES [ch. 



with an archaic race ; and for this reason also we 

 should place the genus Dichogaster in the position of 

 being the most ancient of these Oligochaeta. For the 

 genus occurs in Central America and in certain parts 

 of the East as well as in Africa. So that we can 

 fairly dismiss the view that the Lumbricids by virtue 

 of their greater range over a given area are the most 

 ancient type and that their range is associated merely 

 with their antiquity. Nor does it appear that geo- 

 graphical or meteorological consideration can have 

 had effect in the present instances. For conditions 

 favourable to earthworms prevail in tropical Africa, 

 as in Europe and much of North Asia. 



Climate as Affecting Migration. 



That excessively rigorous climatic conditions affect 

 the range of earthworms as well as fresh- water forms 

 is quite clear from the conditions which obtain in the 

 most northern climes. At any rate in those regions 

 where physical conditions render it impossible for 

 these Annelids to have their being. A perpetual 

 mantle of snow and a temperature far below freezing 

 point are absolute barriers to the extension of range. 

 And yet there are some few Oligochaeta which do 

 not in the least suffer from a somewhat milder taste 

 of such conditions. Thus species of Enchytraeidae 

 have been met with on glaciers and even found in 



