If the division of the terrestrial world into continents is partially but misleadingly rooted in geology, as recent GeoCurrents posts have argued, we must also ask whether it reflects the distribution of animal and plant life. Are continents, in other words, entities of biological significance? To answer this question, it is useful to begin with the work of Alfred Russel Wallace (1823–1913), the “father of zoogeography.” Wallace was one of the world’s greatest naturalists, deserving far greater recognition than he receives. Most impressively, he derived the theory of evolution through natural selection independently from Charles Darwin. Indeed, Darwin only decided to publish The Origin of Species after he received a letter from Wallace that outlined the same key ideas.
Wallace is probably best remembered for his pioneering work in biogeography. The line that he drew separating the fauna of Southeastern Asia from that of Australia – Wallace’s Line – still appears on many maps, although it was later repositioned by other authorities (see the map below). Wallace’s interest in zoogeography was of global scope, as reflected by this two-volume study The Geographical Distribution of Animals: With a Study of the Relations of Living and Extinct Faunas as Elucidating the Past Changes of the Earth’s Surface, first published 1876.
In this work, Wallace distinguished six great zoogeographical regions, or faunal realms, which he subdivided into smaller units. Slightly modified maps of Wallace’s scheme are still commonly encountered, remaining a cornerstone of zoogeography. To be sure, other authors have devised their own regionalization systems, one of which will be examined in a forthcoming post. But it still behooves us to compare Wallace’s pioneering map with the familiar depiction of continents to see the extent to which they coincide.
The simple answer is that Wallace’s scheme accords reasonably well with the standard six-continent model – which combines Europe and Asia to form Eurasia – but with a few important off-sets and one significant addition. Wallace’s Nearctic region focused on North America, for example, is distinct from his Neotropical region focused on South America. The division is not at the continental boundary along the Isthmus of Panama, however, but is rather located well within North America, at the highland/lowland transition zone in southern and central Mexico. Similarly, Wallace’s “Ethiopian” region includes southern Arabia but excludes the part of Africa located north of the Sahara, which is instead appended to the Eurasian Palearctic region. The barrier separating these two faunal regions is thus one of aridity rather that one of water. Eurasia is itself divided, with the Indian subcontinent, along with most of Southeast Asia and southern China, forming its own “Oriental” region. Here the division is both topographic, marked by the Himalayas and other imposing highlands, and climatic, with the tropics separated from the temperate zone. Wallace’s scheme also extends the Australian zoogeographical region well beyond the continent of Australia. Not surprisingly, it includes New Guinea, which sits on the same chunk of continental crust. But it also encompasses New Zealand, which is part of the mostly submerged continent of Zealandia, as well as eastern Indonesia and the larger archipelagos of Melanesia. To sum it up, one might say that Wallacian zoogeography is partially but imperfectly lodged in the architecture of continents.
For most of the Cenozoic Era (the “age of mammals,” 66 million years ago to the present), the linkage between continental landmasses and distinct faunal assemblages was closer than it is today. South America, most notably, had been an isolated “island” with a unique set of animals for millions of years. It started to rift away from Africa some 130 to 110 million years ago, during the Cretaceous period, and did not gain a connection to North America until roughly 2.7 million years ago. The subsequent linkage of the two American landmasses resulted in the “Great American Biotic Interchange” (or GABI). In this exchange, many South American species moved north just as many North American species moved south. The result was a degree of homogenization of the faunas of the two continents. The main zoological division also moved north into southern Mexico, as it came to be based more on climate than on the legacy of physical separation.
As is mentioned above, several studies have devised their own schemes of zoological regions, most of which remain relatively close to that of Wallace. One of the most detailed of these revisions is found in a 2013 article modestly entitled “An Update of Wallace’s Zoogeographic Regions of the World.” As we shall see in the next GeoCurrents post, the maps in this impressive article depict a much more intricate situation, as well as one that strays further from the continental model.