124 



Memoirs of the Indian Museum. 



[VoiL. I, 



whilst entering and occupying this north-western district. If Eudichogaster really 

 is the ancestor of Dichogaster, the north-western district of India on the other hand 

 must be regarded as the focus of distribution of the large genus Dichogaster, the 

 headquarters of which is tropical Africa from ocean to ocean, from which region 



• Eudichogaster 



it Dichogaster .^ 

 endem. ? 



it finally extends to the West Indies and Central America. Also in India (in Travan- 

 core) the gentis Dichogaster seems to be represented by an endemic species. But 

 as the smaller species of this genus are very apt to be exported by man, we 

 cannot be sure about the endemic nature of this species. An endemic Dichogaster in 

 the western part of India indeed would accord very well with the view of an, Indian 

 origin of this genus, without being a necessary condition thereof. 



The Trigastrince of India, then, represent probably a relation of the western 

 part of this country to tropical East Africa, India in this case being the older 

 dominion of this phyletic branch and tropical Africa being taken possession of bv 

 Indian emigrants. 



GKOIvOGICAIv HISTORY. 



The endemic terrestrial Oligochsetes give us one of the best documents for the 

 geological history of a country. The sea, as well as all deserts, is an insurmountable 

 obstacle to their migration. As these obstacles have changed in different geological 

 periods, the paths of their migration were different in accordance with the periods 

 during which different groups of Oligochsetes were in the climax of their migratory 

 capability. Consequently the recent geographical distribution and the relations be- 

 tween the different groups enable us to determine the different paths of the former 

 migrations, and thence the configuration of land and sea in former periods. 



It is true that we are not always able to state in what direction a certain migration 

 must have gone. For example Octochœtus occurs in New Zealand as well as in 

 India, but we cannot say whether it went from New Zealand to India or in the opposite 

 direction, or, indeed, whether it immigrated from a third district, now abandoned, into 



