Geographical Distribution (Continued)

“On the theory of descent with modification, the great law of the long enduring, but not immutable, succession of the same types within the same areas, is at once explained.” — Charles Darwin

Darwin continues his exploration of biogeography, now examining freshwater species, oceanic islands, and the remarkable effects of glacial periods. Each pattern he explores reinforces the same conclusion: species distributions are explained by history—by descent, dispersal, and modification—not by independent creation suited to each environment.

Freshwater Species

Freshwater habitats present an interesting puzzle: lakes and rivers are isolated from each other, yet freshwater species are often widely distributed.

The Freshwater Paradox

• Lakes and rivers are separated by land—barriers for aquatic organisms
• Yet many freshwater fish, plants, and invertebrates have wide distributions
• The same species may occur in disconnected watersheds

How do freshwater species disperse across land?

Mechanisms of Freshwater Dispersal

• Flooding: Rivers occasionally connect during floods
• Birds: Wading birds carry eggs, small organisms, and seeds on feet and feathers
• Wind: Dried ponds leave eggs and seeds that can be blown to new waters
• Past connections: Watersheds that are now separate were once connected

Darwin conducted experiments showing that snail eggs, fish eggs, and aquatic plant seeds could survive being dried and later transported.

Oceanic Islands: Further Evidence

Darwin elaborates on oceanic islands, which provide crucial evidence for evolution:

What Oceanic Islands Lack

• No native land mammals (except bats, which can fly)
• No amphibians (cannot tolerate salt water)
• Often no indigenous freshwater fish

These are exactly the groups that cannot disperse across oceans. Their absence confirms that island species arrived by dispersal, not by creation in place.

What Oceanic Islands Have

• Birds: Can fly to islands
• Insects: Can be blown by storms
• Plants: Seeds can float or be carried by birds
• Reptiles: Some can survive salt water or rafting

Endemic Species on Islands

Islands are famous for unique species found nowhere else:

Endemic Species

Species found only in one location. Oceanic islands often have high proportions of endemic species because their isolation allows colonizers to evolve in new directions without competition from continental relatives.

Why Islands Have Endemics

• Colonizers arrive and find empty niches
• Isolation prevents gene flow with mainland populations
• New selective pressures favor novel adaptations
• Over time, island populations diverge into distinct species

This explains both the uniqueness of island species and their similarity to mainland forms—they’re modified descendants of mainland colonizers.

The Effects of Glacial Periods

Darwin discusses how ice ages dramatically affected species distributions:

Glacial Migration

During ice ages, arctic species spread southward as temperatures dropped. When glaciers retreated, some populations became isolated on mountaintops—“islands” of cold habitat in a warming world. This explains why similar alpine species occur on disconnected mountain ranges.

Alpine Species Distribution

• Same or similar species occur on Alps, Pyrenees, and Scandinavian mountains
• Similar arctic and alpine floras across northern continents
• Mountain peaks harbor relict populations from glacial times

This once-puzzling pattern makes perfect sense when viewed historically: species that were once continuous became fragmented as climates changed.

Analogies Between Isolated Regions

Darwin notes that isolated regions often show similar patterns of unique forms:

Parallel Patterns

• Australia has many endemic species and genera
• Madagascar has unique fauna resembling African ancestors but distinct
• New Zealand has ancient lineages found nowhere else
• Each isolated landmass is a natural experiment in independent evolution

Summary: Geography Supports Evolution

Darwin synthesizes the biogeographic evidence:

The Case from Geography

• Distributions are explained by barriers and dispersal, not by suitable environments
• Species in each region are related regardless of habitat
• Island species resemble nearby mainland forms
• Islands lack groups that cannot disperse across oceans
• Endemic species result from isolation and divergence
• Past climate changes explain current distributions

Every one of these patterns follows from evolution and common descent. None is explained by—or even expected from—independent creation.

Key Takeaways

• Freshwater species disperse via floods, birds, and past connections
• Oceanic islands lack mammals and amphibians—groups that can’t disperse across oceans
• Endemic species result from isolation allowing divergence from mainland ancestors
• Ice ages explain the current distribution of alpine and arctic species
• Each isolated landmass shows independent evolution from ancestral colonizers
• Biogeographic patterns are powerful evidence for descent with modification