: Island Life

Introduction
Alfred Russel Wallace presents a sweeping analysis of island faunas and floras, comparing the living communities of oceanic and continental islands and exploring how they arise, change, and disperse. The book treats islands as natural experiments in biogeography, where isolation, area, age, and means of colonization interact to shape distinct assemblages of species. Wallace draws on a wide range of examples from around the world to build a coherent picture of how species get to islands, survive there, and often diverge into endemic forms.

Island types and colonization
Wallace distinguishes sharply between continental fragments and true oceanic islands, arguing that the former typically retain elements of nearby mainland faunas while the latter begin largely empty and are populated through dispersal. He examines the physical and biological filters that influence which organisms succeed in crossing seas: mobility, life-history traits, tolerance of salt and starvation, and the chance of surviving rafting or island-hopping. These factors explain recurring patterns, such as the relative scarcity of terrestrial mammals on remote volcanic islands and the abundance of birds, insects, and plants capable of long-distance transport.

Distribution, endemism, and adaptive change
A central theme is the high degree of endemism that islands often develop when colonists become isolated and subject to different selective pressures. Wallace emphasizes that once established, island populations may undergo rapid modification through natural selection, adapting to novel niches and sometimes radiating into multiple species. He links patterns of adaptive divergence to island size and habitat diversity, arguing that larger and older islands tend to support more species and more specialized forms, while very small or recent islands sustain fewer, more generalized populations.

Mechanisms and the role of natural selection
Natural selection is treated as the principal mechanism driving the modification and diversification of island species after colonization. Wallace uses island cases to illustrate how selection molds morphology, behavior, and ecology to local conditions, producing unique solutions that nevertheless fit within broader evolutionary principles. He also evaluates alternative explanations for island diversity, weighing the relative importance of isolation, chance dispersal events, and extinction, and treating selection as the key force that shapes surviving lineages.

Historical biogeography and land connections
To explain affinities between island faunas and adjacent continents, Wallace invokes a mix of overwater dispersal and former land connections. He reconstructs past geographic relationships to account for puzzling distributions, proposing land-bridge scenarios where necessary to explain the presence of less vagile groups. While limited by the geological knowledge of his time, this approach underscores his insistence on integrating earth history with biological evidence to make sense of present-day patterns.

Legacy and significance
The study casts islands as vital windows into evolutionary and biogeographic processes, demonstrating how isolation, colonization, selection, and extinction combine to produce strikingly distinct communities. Wallace's synthesis advanced the understanding of species distribution and provided compelling support for Darwinian evolution by natural selection, showing how the same mechanisms that act on continents operate in intensified and revealing ways on islands. The conclusions remain influential for biogeography, conservation, and evolutionary theory, highlighting why islands continue to be privileged arenas for studying the origin and fate of species.