23.i Merrill: Distribution of Dipterocarpaceae 17 



transition one, in which the Indian and Australian faunas min- 

 gle, where from east to west Australian types diminish rapidly, 

 and where from west to east the Asiatic types decrease. The 

 decreases are most startling in some groups, for instance, the 

 true fresh-water fishes. In this group Sumatra presents two 

 hundred twelve species, Borneo two hundred ninety-two, Java 

 one hundred thirty-one, and Celebes only four. Such distribu- 

 tion is significant in itself. A very few fresh-water fishes 

 entered Celebes by some fortuitous circumstances, and it is not 

 at all surprising that one entered Lombok. Weber 20 thus lays 

 entirely undue stress on the finding of a single cyprinoid in 

 Lombok, overlooking the real significance of the very numerous 

 cyprinoids in "Sunda Land" and their entire absence east of 

 Lombok and Sumbawa. Here is a most excellent illustration 

 of an efficient and long-continued barrier to the migration of 

 the fresh-water fishes eastward in the form of narrow arms of 

 the sea which they could not cross. That a very few, by for- 

 tuitous circumstances, did succeed in crossing this line in several 

 hundred thousand years, is utterly insignificant in view of the 

 very large number that occur west of this line. 



Barbour 21 states the case thus : 



Neither Wallace's nor any other line can be held to form a real 

 zoological boundary. A transition zone with a fairly definite western 

 frontier [italics mine] and with an eastern frontier incapable of equally 

 clear definition seems really to be the condition which serves to separate 

 the Malayan from the Papuasian subregions. This zone may be about 

 equally well defined for any of the groups of land animals, and the 

 boundaries for the distribution of the several groups coincide with reason- 

 able accuracy. 



It is realized fully that the region under discussion is a transi- 

 tion one; that no sharp line can be drawn anywhere that will 

 separate the Australian and Asiatic floras and faunas, or those 

 of eastern and western Malaysia, when all groups are taken 

 into consideration. The real significance of Wallace's Line is 

 that it delimits and separates two regions that fundamentally 

 have had a different geological history; one at times a continent 

 over which plants and animals could march unimpeded except 

 by such barriers that continents usually present, the other a 

 constant archipelago where intermigrations have been inter- 



50 Weber, M., Siboga-Expedetie. Introduction et description de l'expe- 

 dition (1902) 16. 



"Barbour, T., A contribution to the zoogeography of the East Indian 

 Islands, Mem. Mus. Comp. Zool. Harvard Univ. 44 (1912) 1-203, t 1-8 



