(7) Across space (pages 82-93)
By the 15th century, European explorers were accumulating more and more knowledge of the earth’s flora and fauna. As their discoveries increased, naturalists speculated that there might be important ‘centers of special creation’ around the globe. But in Origin Darwin argues that natural selection gives a more plausible explanation of the distribution of life.
Looking at the distribution of animals and plants around the globe, Darwin observes major patterns. These patterns cry out for explanation:
• Environmental conditions do not account for the placement of species. The species found in similar habitats are different on separate continents.
• Barriers of any kind have a significant impact on the distribution of species.
• Species on the same continent or in the same sea have an affinity with one another.
According to Darwin’s theory, each group of organisms can be traced back to a single species in a single place; the species then migrated and diversified in new places. There is no pre-programmed outcome; the varieties that have advantages in particular situations will survive.
Darwin now brings up several examples that challenge his theory: How can closely related species exist on the summits of different mountain ranges? How can the same or similar species exist on islands and on the closest mainland, though separated by hundreds of miles of open sea? If Darwin could show that the same species in different locations came from a single origin, and subsequently dispersed, his theory would hold.
Origin starts by laying out global factors that enhance or impede migration: for instance, climate change; or change in the levels of land and sea. But what really engages Darwin’s attention are what he calls ‘occasional’ means of distribution.
He tests whether and to what extent seeds can travel. Seeds can float…be carried by birds…eaten by freshwater fish… ride in patches of earth attached to icebergs… Through his tests, Darwin learned that there is a limit to how far seeds can travel while retaining their vitality. This is why the flora of distant continents remain distinct. But given that all these means of transport have been in action year after year, for centuries and millennia, it makes sense that many plants are widespread.
Consider that in the circumpolar regions, the flora and fauna are remarkably similar around the world; or that species flow up and down the flanks of mountains throughout the planet, showing the migration of arctic, sub-arctic, and temperate forms in tune with change climate. This explains the existence of different but related species in alpine regions.
Acutely aware that this explanation does not remove all difficulties, Darwin is confident that it goes far enough to explain in a reasonable way the fact that the same alpine species can be found on widely separated mountain summits. And what about when the same species is found in separated bodies of water? Organisms become fitted for short and frequent migrations from pond to pond, or from stream to stream, until they encounter a major barrier and their spread stops.
Coming to one last difficulty – the same or similar species found on continents as on islands far offshore – Darwin insists that individuals of the same species have descended from the same parent, and therefore have proceeded from a common birthplace.
The number of species on oceanic islands is scanty. No terrestrial mammals are found on islands further than 300 miles from the continent, but aerial mammals such as bats appear on almost every island. Believers in ‘special creation’ cannot explain why a large number of the best adapted plants and animals have not been created on oceanic islands, or why the island creatures are so unusual. But the theory of natural selection explains why: the low number of species is due to their need to migrate in from somewhere else, and their peculiar characteristics are due to their modification in one locale.
Evolution does not progress in any particular direction, but is rather a response to present conditions at the moment. Of all the characteristics of oceanic islands that Darwin’s theory explains, the most striking is the affinity of island species to those of the nearest mainland. Here the grand example is the Galapagos Archipelago, below the equator about 600 miles off the west coast of South America.
While admitting ignorance about the full effects of climate change, oscillating land and sea levels, glacier action, and means of transport, Darwin still finds his theory convincing: If we allow for enormous periods of time for migration and modification, we can understand how all individuals of the same species, wherever located, have descended from the same parents.
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