: The Geographical Distribution of Animals

Introduction
Alfred Russel Wallace's The Geographical Distribution of Animals (1876) presents a systematic survey of how animal life is spread over the surface of the Earth and why those patterns exist. Drawing on an enormous body of published records, museum collections, and his own field experience, Wallace organized global faunas into coherent regions, traced historical connections between lands, and sought natural explanations for barriers and affinities among animal groups. The book sets biogeography on a scientific footing by combining comparative description with hypotheses about past geological events and dispersal processes.

Main Themes
Wallace emphasized that present distributions are the result of historical events as much as of current climate and habitat. He argued that geological changes, such as the rise and fall of land bridges, mountain-building, and glaciations, have repeatedly altered connections between regions and thereby shaped which lineages could spread and which became isolated. Barriers like oceans, deserts, and mountain ranges, together with the physiological and ecological limits of species, explain the striking regional differences among faunas.

Zoogeographical Regions and Wallace's Line
A central achievement of the book is the delineation of major zoogeographical regions. Wallace recognized six broad regions, Palaearctic, Nearctic, Neotropical, Ethiopian, Oriental, and Australian, each with characteristic families and higher taxa. He also drew critical boundaries within complex areas, most famously the demarcation running through the Malay Archipelago that separates the Oriental and Australian faunas. That boundary, now known as "Wallace's Line," highlights how narrow marine channels and geological history can produce profound faunal contrasts over short distances.

Mechanisms and Causes
Wallace combined ideas of dispersal, extinction, and vicariance to explain patterns. He treated centers of origin and routes of migration as central concepts, proposing that many groups originated in particular areas from which they spread when physical connections allowed. He paid close attention to the differences between continental islands, which often preserve fragments of continental faunas, and oceanic islands, which must be colonized by long-distance dispersal and therefore possess depauperate and highly specialized biotas. He also considered the modifying roles of climate and competition in shaping ranges.

Examples and Evidence
The book marshals diverse examples to illustrate general principles: the predominance of marsupials in Australia contrasted with placental dominance elsewhere; the unique lemur fauna of Madagascar; the distinct bird and mammal assemblages of South America versus Africa; and the affinities of northern Holarctic faunas across Eurasia and North America reflecting past land connections. Wallace used fossil records where available and compared distributions of different groups to infer historical connections, often pointing to congruent patterns across taxa as evidence of shared history.

Legacy
The Geographical Distribution of Animals established biogeography as a rigorous comparative science and provided powerful support for evolutionary explanations of biodiversity. Wallace's regional scheme and his emphasis on historical cause-and-effect remain foundational. His ideas about barriers, dispersal, and the role of geological history continue to inform modern biogeography, evolutionary biology, and conservation planning, while his clear articulation of phenomena such as Wallace's Line remains a touchstone for studies of faunal turnover and endemism.