AND ZOOGEOGRAPHY OF INDIAN OLIGOCH.ETA. 135 



says, " Since the present geographical distribution of earthworms 

 depends in the first place on the configuration of land and sea in 

 recent geological epochs, it is to be looked on as a valuable 

 document for the history of the earth." We are not bound, that 

 is, to accommodate our conceptions of the wanderings of the 

 ancestors of the present-day fauna to the views founded on 

 geological evidence only ; we also are in possession of important 

 documents, and their evidence may perhaps be of superior cogency 

 to that of geology. A zoologist is not likely to underrate the 

 value of the evidence furnished by zoology ; only we must be sure 

 what its value is. 



And firstly, in the present case, even if there were no geological 

 evidence, even if we were not told that " the geologic evidence 

 for the general permanency of the abyssal oceans is over- 

 whelmingly strong," it would be our duty not to introduce land 

 connections unnecessarily. It is an old philosophical rule that 

 " causa? non sunt multiplicands praater necessitatem " ; in the 

 present case we may substitute " bridges," and say " pontes non 

 sunt multiplicand! pra?ter necessitatem." We have a number of 

 agencies which are in existence before our eyes to-day : The slow 

 extension of distribution by the normal wanderings of earth- 

 worms, the extirpation of indigenous worms by younger forms 

 of later introduction, the existence of natural rafts on the sea, the 

 known ability of certain worms and their cocoons to endure salt 

 water, the polyphyletic origin of certain genera, and moderate 

 changes of land and sea ; and it may fairly be demanded that we 

 exhaust the possibilities of these before we have recourse to the 

 construction of bridges which we cannot see and which are at 

 any rate much more hypothetical in nature. 



Again, I speak only of those bridges which have been postulated 

 in order to explain the distribution of Oligochseta, and especially 

 of those Oligochaeta. which occur in the Indian and Australian 

 regions. My contention is that the greater part of these are 

 unnecessary in this connection ; whether they are a necessary 

 assumption or not for other reasons, I must leave to others. 



And first with regard to natural rafts. Matthew recalls the 

 fact that these have several times been recorded as occurring over 

 a hundred miles off the great tropical rivers such as the Ganges, 

 Congo, Amazon, and Orinoco ; and for one such observed, a 

 hundred may have drifted out unnoticed. Wallace, in his 

 'Island Life,' speaks of "those floating islands which are often 

 (italics mine) formed at the mouths of great rivers. Sir Charles 

 Lyell describes such floating islands which were encountered 

 among the Moluccas " (i. e. between Celebes and New Guinea, 

 where there is no large river) " on which trees and shrubs were 

 growing on a stratum of soil which even formed a white beach 

 round the margin of each raft. Among the Philippine Islands 

 similar rafts with trees growing on them have been seen after 

 hurricanes, and it is easy to understand how, if the sea were 



