II.

Voyage of the Beagle

Darwin’s job as naturalist aboard the Beagle gave him the opportunity to observe the various geological formations found on different continents and islands along the way, as well as a huge variety of fossils and living organisms. In his geological observations, Darwin was most impressed with the effect that natural forces had on shaping Earth’s surface.

At the time, most geologists adhered to the so-called catastrophist theory that Earth had experienced a succession of creations of animal and plant life, and that each creation had been destroyed by a sudden catastrophe, such as an upheaval or convulsion of Earth’s surface (see Geology: History of Geology: Geology in the 18th and 19th Centuries). According to this theory, the most recent catastrophe, Noah’s flood, wiped away all life except those forms taken into the ark. The rest were visible only in the form of fossils. In the view of the catastrophists, species were individually created and immutable, that is, unchangeable for all time.

The catastrophist viewpoint (but not the immutability of species) was challenged by the English geologist Sir Charles Lyell in his three-volume work Principles of Geology (1830-1833). Lyell maintained that Earth’s surface is undergoing constant change, the result of natural forces operating uniformly over long periods.

Aboard the Beagle, Darwin found himself fitting many of his observations into Lyell’s general uniformitarian view. Beyond that, however, he realized that some of his own observations of fossils and living plants and animals cast doubt on the Lyell-supported view that species were specially created. He noted, for example, that certain fossils of supposedly extinct species closely resembled living species in the same geographical area. In the Galápagos Islands, off the coast of Ecuador, he also observed that each island supported its own form of tortoise, mockingbird, and finch; the various forms were closely related but differed in structure and eating habits from island to island. Both observations raised the question, for Darwin, of possible links between distinct but similar species.