WALLACE'S CURRENT. 315 



stances, easily cross the boundary originally set to their passage 

 by the current on the longer voyage to Celebes ; very easily, 

 indeed, when, by slight variations in the strength or direction of 

 the monsoons and in the surface currents caused by them, a 

 temporary change was produced in the direction of the normal 

 current flowing between Celebes and Borneo known as Wallace's 

 current which is merged in the return current of the Pacific 

 Ocean. All those animals, on the other hand, which might 

 have other means of transport at their command, would be ren- 

 dered independent of the agency of this current, whether in 

 separating or in mingling the faunas ; but of course only so 

 far as they were not monophagous, and thus absolutely depen- 

 dent for food on certain plants of which, again, the extension of 

 range was subject to the action of the said current. In pursu- 

 ance of this mode of viewing the matter we should then have 

 to inquire whether those insects and birds which appear to have 

 migrated from the Indian region to the Australian, and vice 

 v?rsd, may not be polyphagous and easily satisfied with various 

 kinds of food ; and, on the other hand, whether, as an inevitable 

 corollary, those forms which are confined to particular islands 

 or districts may not be monophagous or dependent on certain 

 forms of food whose extension of range from one island or 

 region to another is prevented by the agencies under considera- 

 tion. 



This, however, is not the place for pursuing this inquiry in 

 detail, nor do we as yet possess sufficient materials for it in the 

 form of well-confirmed observations. But so long as the 

 general observations we do possess allow of no positive conclu- 

 sions, we are, on the other hand, not justified in rejecting any 

 possibility as erroneous, and consequently Wallace's hypo- 

 thesis must for the present remain open to discussion ; the 

 arguments here laid down in opposition to it are so too, in the 

 same degree and for the same reason, and it must be left to the 

 future to decide between them. Still, I am of opinion that the 

 hypothesis I have put forward may claim the advantage of ap- 

 pealing for proof only to such elements as can be brought 

 under direct observation, while Wallace's is intrinsically in- 

 capable of demonstration by observation. 



