Land masses of the Oriental and Australian Regions are separated by several strings of islands and therefore it is difficult to draw a line separating the two regions distinctly. Wallace (1860) suggested a line running between Bali and Lumbok; Borneo and Celebes and between Philippines and Sangi and Talaud. Huxley named it Wallace’s Line. The differences between the fauna of Bali and Lumbok, separated by 15 miles sea was especially emphasized while marking the line.
Some years after Wallace’s observations, another line called Weber’s Line was suggested, which was thought to divide the fauna of the two regions better. The observation was based mainly on mollusks and mammals. It runs between Moluccas and Celebes and between Kei Islands and Timor.
Geologically, Wallace’s line marks the eastern limit of what was once a land mass joined to Malay mainland and Weber’s Line more or less marks off the western limit of Australian continent.
Dickerson et al. (1928) considered the area between Wallace’s and Weber’s lines as transitional area, which is not part of any region and therefore considered it a separate subtraction and named it Wallacea. Its islands were probably under the sea for most part and thus lost all their original flora and flora when they re-emerged. Later, they were recolonised irregularly from both the regions. That is why they contain unique fauna. For example, Celebes has few mammals, very few Amphibians (only frogs), no freshwater fish and some peculiar birds from both the regions.
Some zoogeographers suggest keeping both Wallace’s and Weber’s lines and the intervening transitional area as Wallacea. The following table gives Oriental and Australian characters of the reptilian and avian fauna of the islands of Wallacea.
FAUNA OF WALLACEA
Fishes. Out of many freshwater fishes on Borneo, none reaches Celebes, except 2-3 species carried by man. On Java there is a decrease in fish fauna from west to the east and very few Cyprinids reach Lesser Sundas and they are also carried by man. Bali does not have any significant fish fauna from Java.
Amphibia. Oriental frogs (Megaphrys) and Microhyla extend up to Bali only, Bufo to Celebes, Bali and Lumbok and Rhacophorus to Celebes and Timor.
From the Australian side, Leptodactylids reach New Guinea and Aru Islands and Hyla extends to Moluccas and Timor.
Reptiles. Majority of the Oriental reptiles do not cross Wallace’s Line and none extend beyond Weber’s line. From the Australian side only few elapid snakes reach up to Moluccas.
Aves. Distribution of birds is complex. Some of the Oriental birds extend up to New Guinea and some Australian species reach beyond the Wallace’s Line. Seventy five percent of the birds on Lumbok are Oriental but on Bali only 14.5% Australian birds exist.
Mammals. Most of the Oriental mammals stop on Borneo and Java, very few reaching Celebes and Bali and none cross over to Moluccas and Lesser Sundas. The Australian mammals stop in New Guinea and a only a small fraction reach Aru and Kei Islands. Only one species of flying phalanger reaches Celebes and Timor. There is very little overlapping of the Australian and Oriental fauna in Wallacea.