Summary: Chapter XI
Darwin examines the role of geographical isolation and
migration in the development of species in particular environments.
He begins by observing that the similarity and dissimilarity of
organisms in particular locales has little to do with the climate
of the region. Similar climates exist all over the world, but these
climates support vastly different species. Barriers to species migration
affect their development in important ways. Separated land masses
(large, continuous areas of land, such as the continents of Australia,
Africa, and South America) support extremely different species,
as do separated bodies of water. In a location in which migration
is impossible, species develop according to the conditions of that
particular environment and become highly distinct from species in
other locations. Within a continent or sea, species show relationships
(or affinities) to one another. In places where migration is possible,
the introduction of new species shapes natural selection just as
much as the environment does.
The divergent development of species based on geographical
separation leads to an important question: Were species created
at one point on the earth’s surface, or did they appear simultaneously
at many different points? Darwin believes that each individual species formed
in one particular location and then spread to other parts of the
world through migration. For some species, such as land-living animals,
migration is difficult. Those species may not spread as far as a
result. However, many ways exist for plants to migrate from location
to location, even given geographical barriers such as bodies of
water. Some plant seeds can survive in seawater and float from one
continent to another. Birds may also carry seeds in their beak, feet,
or stomach (to be expelled later) as they fly from continent to continent.
Even icebergs may transport seeds to different locations.
Geographical changes have played a role in both limiting
and allowing for species migration. Darwin does not believe, as
some geologists do, that islands were land masses that broke off
of the continents. Even if this were true, the large geographic
changes could not have occurred within a short enough time to affect
living species. However, as the level of the land changed due to
the oscillation of sea levels, water barriers gradually appeared
and disappeared. These changes created periods of geographic isolation, during
which new species were able to develop, and periods of geographical
connection, during which migration allowed the new species to spread
throughout other regions.
To illustrate the role of geological change in species
development, Darwin points out two large geological changes that
explain some patterns of species dispersal. First, the Glacial period
(commonly known as the Ice Age) created a uniform climate in which
only certain species could survive. At the end of that period, the
remaining species had to migrate to mountaintops to find a climate
cold enough for them to survive. This migration accounts for the
similarity of species on mountain peaks throughout the world. Second,
a connection between lands stretching from Western Europe through Siberia
to eastern America—a supposed land bridge now covered by the Bering
Strait—would have allowed species to migrate by land from Europe
and Asia to North and South America, before rising waters cut off
the connection between these lands. Darwin hypothesizes that ice
melting after the Ice Age increased the ocean’s water level and
disconnected these lands from one another, creating a geographical
separation of species on these continents that allowed the species
to modify separately and diverge. This hypothesis explains how analogous
species appear in many different parts of the world, without any
apparent means of migration. These geological changes also explain
how natural selection occurs in isolated environments, creating
the divergent development of species in different locales.
Summary: Chapter XII
Darwin continues his discussion of the geographical distribution
of species by considering how similar species can exist in different freshwater
environments that are geographically isolated from one another.
Obviously, water-bound species cannot migrate from pond to pond
with ease. As a result, a species of fish cannot exist in the freshwater
environs of two separate continents. However, oftentimes the same
fish species may be found in freshwater ponds of the same continent.
Darwin hypothesizes that changes in land levels allowed rivers to
flow into one another at certain points, and the migration of fish
ceased when the bodies of water were fully separated. Freshwater
species also may have been able to migrate when ducks carried plants
and shells from one pond to another. Birds, Darwin observes, may
also eat seed-eating freshwater fish and expel the seeds in their
excrement at a different location.
Darwin addresses the issue of species formation on oceanic islands,
which are geographically isolated. Darwin argues that the ability
to migrate has shaped which species are found on islands, noting
that islands hold fewer species than larger continents. However,
islands hold more “endemic species”—species that are found only
on that island and nowhere else in the world. Similarly, species that
are better able to migrate, such as birds, show less adaptation than
species that are unable to migrate. This observation suggests the
capacity for migration is just as important as environmental conditions
in the formation of adapted species. For example, plants that bear
hooked seeds are more widespread across the world than are plants
with non-hooked seeds. The hook of the seeds allows them to attach
to a bird better, making the seeds more likely to be carried to
another area by migration.
