AND ZOOGEOGItArHY OF INDIAN OLIC40CH^TA. 133 



raising the level of the land of the globe by 100 fathoms; an 

 elevation of five times this amount would alter the boundaries 

 very little more. W. D. Matthew sums up the evidence strongly 

 in favour of general permanency (5) : — "The geologic evidence 

 for the general permanency of the abyssal oceans is over- 

 whelmingly strong. The continental and oceanic areas are now 

 maintained at their different levels chiefly through isostatic 

 balance, and it is difficult to believe that they could formerly 

 have been reversed to any extensive degree." 



(c) The Objection to the Inolo- Australian Bridges. 



I propose later to enumerate the several land-bridges which 

 have been invoked to explain the distribution of the genera of 

 earthworms common to India and other parts of the world. But 

 there is none of them the former existence of which seems to be 

 better attested than that between Australia and India,; this has 

 almost become axiomatic in the minds of students of the Oligo- 

 chasta. The reason is, as has been said, the large number of 

 genera that are common to India and the Australian region. 



There can be nd reasonable doubt that the western part of the 

 Malay Archipelago has been joined on to the Asiatic mainland 

 at no distant time; according to Wallace, "all the wide expanse 

 of sea which divides the islands of Java, Sumatra., and Borneo 

 from each other, and from Malacca and Siam, is so shallow that 

 ships can anchor in any part of it, since it rarely exceeds forty 

 fathoms in depth"; while the eastern part of the Archipelago 

 has, with equal probability, formed a part of Australia. Michaelsen 

 assumes not only the passage of numerous genera of Megascolecidaa 

 from the Australian side, but (or perhaps as an alternative) 

 suggests that some may have passed back into Australia, from 

 outside (16). 



But how does the hypothesis of land-bridges square with the 

 other known facts of distribution ? I have given some reason 

 for thinking that the whole of the earthworm fauna of the world, 

 and in particular that part of it with which we are dealing at 

 present, is of recent origin. Megascolex, for example, is one of 

 the youngest genera; its immediate ancestor Notoscolex is one 

 ■stage further back ; both are separated by a long line of ancestors 

 from the earliest earthworms, which alone seems sufficient to bring 

 their oi-igin down to late Tertiary times ; Megascolex appears to be 

 ■evolving still, and has not as yet settled down to the comparative 

 fixity of an old-established genus. 



Now it is well known that, broadly speaking, Australia has no 

 indigenous Eutherian population. The great groups of terrestrial 

 Eutherians originated in the Eocene — some in the very early 

 Eocene — and spread rapidly thereafter. How, on the supposition 

 •of a land-bridge, are we to let the Australian earthworms out to 

 India without letting the Asian mammals into Australia? If the 

 door is open for the particularly slow-moving worms, it is open 



