Geographical Distribution—Continued explained

What happens

Themes in this chapter

• Geographical Distribution and Barriers

  The chapter extensively explores how barriers (land or water) separate and form zoological and botanical provinces, and how migration and subsequent modification explain the distribution of species.

• Natural Selection and the Struggle for Existence

  Darwin invokes natural selection as the mechanism accumulating modifications after migration, and discusses competition among immigrants and aborigines as shaping the inhabitants of different regions.

• Common Descent and the Tree of Life

  The chapter argues that all individuals of the same species, and species of the same genus, are descended from common parents, with migration and modification explaining their current distribution.

Characters to notice

• De Candolle

  Discussed in relation to the law that lower groups of organisms range more widely, as observed in plants.

• Edward Forbes

  Cited for insisting on the striking parallelism in the laws of life throughout time and space.

• Dr. Hooker

  Referenced for showing that in the Galapagos Islands the proportional numbers of plant orders differ from elsewhere.

• Mr. E. V. Harcourt

  Mentioned as informing Darwin about European and African birds blown to Madeira.

• Mr. J. M. Jones

  Cited for his admirable account of Bermuda, noting that many North American birds visit the island.

• Rev. W. B. Clarke

  Mentioned for his view that New Zealand and New Caledonia should be considered appurtenances of Australia based on geological character and mountain ranges.

Key passages

• “The relations just discussed—namely, lower organisms ranging more widely than the higher—some of the species of widely-ranging genera themselves ranging widely—such facts, as alpine, lacustrine, and marsh productions being generally related to those which live on the surrounding low lands and dry lands—the striking relationship between the inhabitants of islands and those of the nearest mainland—the still closer relationship of the distinct inhabitants of the islands of the same archipelago—are inexplicable on the ordinary view of the independent creation of each species, but are explicable if we admit colonisation from the nearest or readiest source, together with the subsequent adaptation of the colonists to their new homes.”

  Patterns like lower organisms having wider ranges, island species resembling those on the nearest mainland, and species within an archipelago being closely related, cannot be explained by the idea that each species was created independently. Instead, they make sense if we accept that species colonize from the nearest available source and then adapt to their new environments.

  Darwin summarizes key distribution patterns and contrasts them with the creationist view, advocating for colonization and adaptation.

• “As the late Edward Forbes often insisted, there is a striking parallelism in the laws of life throughout time and space; the laws governing the succession of forms in past times being nearly the same with those governing at the present time the differences in different areas.”

  Edward Forbes emphasized that the rules determining how species succeed each other over geological time are nearly identical to those that explain why different regions have different species today.

  Darwin invokes Forbes to highlight the consistency of evolutionary laws across time and space.

• “Although in oceanic islands the species are few in number, the proportion of endemic kinds (i.e. those found nowhere else in the world) is often extremely large.”

  Even though oceanic islands have relatively few species, a high percentage of those species are unique to the island and found nowhere else on Earth.

  Darwin notes the paradox of low species richness but high endemism on oceanic islands.