igoS.] W. MiCHAEivSEN : Oligochœta of the Indian Empire and Ceylon. 125 



both its recent dominions. But we may state that there was once a land bridge 

 between New Zealand and India. In other cases we may even learn from certain 

 facts the direction of a certain migration. 



We have to state another premise. If the former track of migration passed through 

 a certain district, we are inclined to look in this district for traces of the migrating genus. 

 The want of traces intermediate between far-separated recent localities does not^ how- 

 ever, always permit us to draw the conclusion that the land bridge used by the migrating 

 genus has vanished totally by diving into the sea. In certain cases the genus in ques- 

 tion, once occupying a district intermediate between those it now occupies, may have 

 been extirpated in the intermediate district by the appearance of a younger and more 

 vigorous tribe. In the regions here in question one such a younger and vigorous genus 

 occurs, viz. Pheretima, phyletically the youngest genus of the M egascolecince. The 

 species of this genus prove to be of such a vigour of distribution that they have 

 suppressed all terrestrial rivals in their proper dominion and totally extirpated most 

 of them. Indeed, only some scarce Moniligastridœ' and some scarce species of the 

 nearly allied genus Plionogaster have survived in their proper dominion. Besides 

 these we find only a single Woodwardia and a single Perionyx endemic in the mar- 

 ginal part of their dominion, in Burma; not to speak of the lyUmbricid of Japan, 

 Helodrilus japonicus (MICHI,SN.), which itself belongs to such a vigorous family that 

 it is not surprising that it was able to endure the rivalry of Pheretima: In the coun- 

 tries and islands around the dominion of Pheretima, m.vnélym.^e'w Zealand, Australia, 

 Ceylon and India, we find a great number of phyletically old forms. ' If a certain 

 genus or sub-family is wanting in a larger district of these countries, we may suppose 

 that it never has occurred here, for none of the genera of these coutitries has been 

 able to get supremacy over other forms. In these countries we' find the most different 

 genera and sub-families living peacefully side by side. ' ' 



If we now look for the tracks along which foreign terrestrial OligOchaetes may have 

 immigrated into the Indian region, or along which Indian Oligochaetes may have emi- 

 grated to other regions, we have, as stated above, to examine the geographical relations. 

 The principal relations point to a south-eastern direction, towards Australia and New 

 Zealand. But these relations are not uniform. Only some of the phyletically oldest 

 Megascolecinœ (the genera Megascolides, Diporochœta and Spenceriella) found their 

 way equally to New Zealand {Megascolides and Spenceriella only to the North Island 

 of New Zealand) and India, emigrating from their oldest dominion, Australia. Other 

 Oligochaetes found a path only between New Zealand and India, avoiding Australia 

 (the Odochcetinœ), or between Australia and India {Plutellus, Woodwardia, Periony- 

 chella, Notoscolex and Megascolex). The path used by the latter group was not uniform 

 throughout its extent, some of these genera invading only the northern parts of 

 India, e.g., Burma {Woodwardia, Perionychella); others only the southern part 

 including Ceylon {Notoscolex, Megascolex). We may draw the conclusion from these 

 facts that the track used by these migrants was no uniform broad land bridge extending 

 between India and Australia -f New Zealand in full breadth. The connection between 

 these different regions was of a more complicated figure. There must have been smaller 



