• Layout of the Beagle. The ship was 90ft long by 24 feet wide. Darwin’s

cabin was only 10ft by 11ft.

Darwin’s Voyage on the Beagle 1831-1836

– Darwin was Fitzroy's fourth choice– Purposes of voyage: Provide maps of South America useful to the British Navy. Britain viewed South America as a market for its manufactured products and a source of raw materials.

Darwin brought: microscope, telescope, compass, barometer, rain gauge, guns, preserving fluids for specimens.... and Lyell's Principles of Geology (Vol. 1).

St. Jago in Cape Verde Islands

Due to quarantine restrictions the Beagle cannot stop at the Canary Islands.

First stop is the Cape Verde islands

St. Jago is a volcanic island with both vegetation and bare rocks. Darwin sees a horizontal white band on some rocks 30 feet above sea level. On close examination Darwin discovers this band is made up of compressed sea shells and corals.

How had the sea shells got there?

His geological speculations begin (based on Lyell's work).

Frontispiece of Lyell’s “Principles of Geology

– The temple is that of Jupiter Serapsis at Pozzouli. The three columns show evidence of having been underwater at some time in the past. The bottom parts are smooth, above that is a layer of shells embedded in stone.

– Conclusion: Temple initially built by Romans above the water line had been partly submerged for a time.

– Lyell was a gradualist. He had adopted Hutton's ideas of slow never ending change. Geological processes still operating were shaping the earth, including the slow rise and fall of land.

South America

Bahia – Darwin entranced by plants and animals in jungle.

Fossils – Finds his first fossils in the southern pampas, at Punta Alta, including a jawbone and tooth characteristic of a megatherium, a huge extinct animal with similarities to the modern-day sloth.

Megatherium

Sloth

Darwin reads Lyell's second volume, which focused on how animals and plants respond to a changing landscape.

Lyell is anti-Lamarck. Lyell believes each species was adapted to the place in which it came into existence. If the environment changes too much, the species died out.

Darwin, having seen and touched Megatherium bones accepts this. Lyell has explanation for extinction of species but not for their coming into being.

Darwin continues to search out fossils as the Beagle tracks up and down the coast. Finds megatherium bones together with shells of creatures that still existed.

Envisions megatheriums living in the not-too distant past as the coast of South America slowly rose.

Darwin sees a Lyellian world, but has no answer for species creation.

Geographical Distribution of Species

Rhea a native pampas bird that Darwin was familiar with.

Darwin hears from locals about a smaller darker rhea that lives only in the southern part of the pampas.

Darwin finds this smaller rhea in the south, and is told it is the only kind of rhea in that area.

Two very similar species with separate but overlapping ranges. Why?

Concepcion Earthquake, Chile, 1835

Severe earthquake caused massive destruction and loss of life. The earthquake also raised surrounding land by several feet.

Darwin finds mussel beds now lying above high tide, with the mussels all dead.

Another confirmation of Lyell's theory.

An example of destructive power of nature rather than a benevolent nature designed for man's use.

Galapagos Islands/ Mainland Comparisons

Galapagos are dry volcanic islands filled with reptiles, including giant land tortoises.

Birdlife – Darwin notices three varieties of mockingbirds, each found on only one island. Similarities to mockingbirds found on mainland.

Finches were the most common bird – he collects six types of finches from three islands, but assumes each type could be found on all the islands. Similarities to mainland birds.

Tortoises – Local prisoners believed each island had its own particular kind of tortoise, characterized by slightly different shell shape. Darwin uninterested as he believes tortoises originally brought to islands by sailors as food source.

Galapagos finches

Due to very different beak size Darwin assumed that some of the finches were wrens.

Charles Darwin's Journal of Researches

Coral Atolls

Lyell believed coral atolls grew as a result of slowly rising underwater volcanoes.

Darwin speculates that land rising in South America must mean land falling somewhere else. Perhaps coral reefs grew as a result of sinking volcanoes?

Fitzroy's soundings around an atoll showed that coral grew on a underwater mountain surrounded by very deep water. Showed coral reef had grown upward as volcano submerged.

Darwin's geological speculations bearing fruit.

From The Voyage of the Beagle

Voyage home

Darwin draws up catalog of his collections and reflects on what he has found.

His mockingbirds got him thinking:

“When I see these Islands in sight of each other, & possessed of but a scanty stock of animals, tenanted by these birds, but slightly differing in structure & filling the same place in Nature, I must suspect they are only varieties…If there is the slightest foundation for these remarks the zoology of the Archipelagoes – will be well worth examining; for such facts would undermine the stability of species.”

Did islands have a unique role in making species less stable? How pliable was nature? How had colonization by one species from the mainland turned into different varieties?

Darwin - Back in England

Lyell had used Darwin's fossils of giant extinct sloths, armadillos and llamas to show that living species were closely connected to the species that they replace. More food for Darwin's beginning speculations on evolution.

John Gould examines Darwin's birds – his wrens and finches. Gould realizes that they are all finches, differing in beak shape, and that they are all separate species, not just varieties. And they have close relatives on the mainland. Somehow the original finch immigrants on the Galapagos had changed enough to produce new species. Darwin realizes the importance of isolation in creating new species.

Darwin finds his niche in the world, examining what Lyell couldn't, the origin and stability of species.