Wallace's Line in the Light of Recent Zoogeographic Studies

BY ERNST MAYR American Museum of Natural History, New York Z OOGEOGRAPHY has had a fate very much like taxonomy. It was flourishing during the descriptive period of biological sciences. Its prestige, however, declined rapidly when experimental biology began to come to the foreground. Again as with taxonomy, a new interest in zoogeography has been noticeable in recent years. It seems to me that this revival has had two causes. One is the interest of the student of geographic speciation in the findings of the zoogeographer. A study of past and present distributions yields much information on isolation of populations and on the dispersal of species. Itis in this connection that I became interested in zoogeography. The other reason is the introduction of new methods. The intensive xploration of all corners of the globe during the past fifty years has led to an accumulation o; sufficient faunistic data to permit the application of statistical methods. Furthermore, the science of ecology has reached a level of maturity at which it is beginning to affect profoundly zoogeographic methods and principles. It seemed worth while to me to study the controversial and still wide open subject of the borderline between the Australian and Oriental Regions with the help of such modern methods. A. R. Wallace, who is generally considered the foremost representative of classical zoogeography, states in his famous essay On the zoological geography of the Malay Archipelago (1860): "The western and eastern islands of the archipelago belong to regions more distinct and contrasted than any other of the great zoological divisions of the globe. South America and Africa, separated by the Atlantic, do not differ so widely as Asia and Australia. " There is much truth, in this statement. Except for bats and a few rodents, the only native mammals of Australia are marsupials and monotremes. These same two groups are entirely lacking in Asia and are replaced by a wide variety of placental mammals, such as monkeys, shrews, squirrels, ungulates, and so forth. An equally pronounced faunal difference xists among birds, insects, and other groups of animals of the two regions. Australia and Asia are connected by a belt of islands, the Malay Archipelago, and the question naturally comes up as to where in this island region the borderline is to be drawn between these two fundamentally different faunas. After reviewing the zoological evidence known to him, Wallace (l.c.) comes to the following conclusion: "We may consider it established that the Strait of Lombok [between Bali and Lombok] (only 15 miles wide) marks the limit and abruptly separates two of the great zoological regions of the globe. " With these words he drew a zoogeographic boundary which was destined to gain fame under the name of its author: "Wallace's Line," a term first used by Huxley (1868) (Fig.' 1). It runs between Bali and Lombok in the south, then through Makassar Strait between Borneo and Celebes, and finally turns into the open Pacific between Mindanao (Philippines) and the Sanghir Islands. This convenient borderline found quick acceptance in the zoological literature and was without hesitation adopted by nearly all 'the zoogeographers publishing between 1860 and 1890. Sarasin (1901) and Pelseneer (1904) should be