: On the Tendency of Varieties to Depart Indefinitely from the Original Type

Context
Alfred Russel Wallace composed his essay "On the Tendency of Varieties to Depart Indefinitely from the Original Type" in 1858 while living in the Malay Archipelago. That essay reached Charles Darwin and prompted a rapid joint presentation of Wallace's paper and excerpts from Darwin's own unpublished writings to the Linnean Society of London. The episode crystallized the idea that species change over time through natural processes and marked a pivotal moment in the history of evolutionary thought.

Central thesis
Wallace proposed that populations of organisms tend to diverge progressively from their ancestral forms through the accumulation and preservation of advantageous variations. He asserted that variation is ubiquitous within populations and that environmental pressures favor those variants better adapted to local conditions. Over time, differential survival and reproduction cause descendant populations to depart further from the original type, eventually producing new species.

Mechanism of natural selection
The core mechanism Wallace described is natural selection: a process by which beneficial heritable differences increase in frequency because they confer an advantage in the struggle for existence. Individuals exhibiting traits that improve their chances of surviving and reproducing in particular circumstances are more likely to leave offspring carrying those traits. Wallace emphasized that selection acts continuously and cumulatively, shaping populations as environments change and as competition for resources imposes filters on variation.

Evidence and argumentation
Wallace marshaled observations from biogeography, comparative anatomy, and the distribution of species to support his claims. He highlighted how closely related forms often occupy adjacent but differing habitats and how island faunas display predictable patterns of divergence from mainland ancestors. Rather than exhaustive experimental proof, the essay relied on pattern recognition: consistent correlations between environmental differences and morphological divergence suggested a natural process driving the change, rather than occasional catastrophes or solely static creation.

Speciation and geographic separation
Wallace noted that isolation, geographic or ecological, facilitates the indefinite departure of varieties from their original type. When populations become separated or exploit different niches, distinct selective regimes act on each group, allowing unique combinations of attributes to accumulate. Over many generations these accumulating differences can become sufficiently pronounced that formerly interbreeding populations cease to interbreed, producing distinct species.

Philosophical and scientific implications
The essay advanced a naturalistic explanation for adaptive complexity and the origin of species, challenging prevailing views that invoked special creation as the primary explanation for biological diversity. Wallace's emphasis on selection as a sorting process diminished the need for inherent teleology or purposeful design in explaining organismal fit to environment. The argument reframed biological diversity as the result of ongoing interactions among variation, environment, and reproductive success.

Reception and legacy
The joint presentation of Wallace's essay and Darwin's material galvanized scientific attention and accelerated the dissemination of the selectionist view. Wallace emerged as a co-discoverer of natural selection and later carried the ideas into biogeography and field observations, influencing generations of evolutionary biologists. While some details and priorities differed between Wallace and Darwin in subsequent years, the 1858 essay remains a crucial statement of the principle that variation, filtered by environmental pressures, drives the progressive divergence of life.