23. i Merrill: Distribution of Dipterocarpaceae 29 



at times during the preceding geologic periods; similarly, the 

 area delimited by the Australian continental shelf, carrying upon 

 it New Guinea, was also a continental area. Interposed between 

 these two stable continental regions, that is, from the Lombok 

 Passage and the Macassar Strait extending to the eastward as 

 far as the west end of New Guinea and northward through most 

 of the Philippine Archipelago, we find a region in sharp contrast 

 to the two above-mentioned stable continental areas. The entire 

 intermediate region has been unstable, subject to elevations and 

 depressions, from at least the early Pleistocene to the Recent and 

 is still orogenetically active and, as a result, still unstable. In 

 other words, archipelagic rather than continental conditions 

 have persisted in this vast region since the early Pleistocene, 

 and probably earlier. 



2. Wallace's Line, so named by Huxley, separating the fauna 

 of eastern and western Malaysia, was located by Wallace through 

 the Macassar Strait and extended southward through the Lom- 

 bok Passage between Bali and Lombok. It is essentially the 

 western boundary of the insular unstable area and, as a corollary, 

 the eastern boundary of the ancient stable continental area. 

 Wallace's Line is a striking faunal and floral boundary (although 

 not an absolutely separating one) when the distribution of all 

 groups of animals and plants is taken into consideration. It is 

 essentially based on fundamental geologic differences between 

 Sunda Land and the region to the east. West of Wallace's Line 

 animals and plants have been able to migrate from one part of 

 the previously existing continental area to other parts, subject 

 only to those limitations that are found in continental areas. 

 East of this line all intermigrations of Australian and Asiatic 

 types of animals and plants have been interrupted by the con- 

 stant archipelagic conditions existing in the region under dis- 

 cussion and, at times, all or most intermigrations have been 

 inhibited by impassable barriers in the form of separating arms 

 of the sea. As between the various islands in this vast region 

 and the previously existing continental areas to the west and 

 •southeast, land connections have never been more than narrow 

 isthmuses. The evidence is that this fundamental dividing line 

 between eastern and western Malaysia did not extend to the ea3t 

 between Celebes and Mindanao, as originally placed by Wallace, 

 but extended northward through the Sibutu Passage and the 

 Sulu Sea to the Mindoro Strait and thence northward and then 

 eastward into the Pacific Ocean between Formosa and the Batan 

 Islands. There have been no direct land connections between 



