The Science Book

146 CHARLES DARWIN

This epic voyage opened the eyes
of the young Darwin, still only in his
twenties, to the incredible variety
of life. Wherever the Beagle docked,
Darwin keenly observed all aspects
of nature. In 1835, he described
and collected a group of small,
insignificant birds on the
Galápagos Islands, a Pacific Ocean
archipelago 560 miles (900 km)
west of Ecuador. He thought there
were nine species, six being finches.
After his return to England,
Darwin organized his mass of
data and oversaw a multivolume,
multiauthor report, The Zoology of
the Voyage of HMS Beagle. In the
volume on birds, the renowned
ornithologist John Gould declared
that there were in fact 13 species
in Darwin’s specimens, all of them
finches. Within the group, however,
were birds with differently shaped
beaks, adapted to different diets.
In his own, bestselling account
of his adventure, The Voyage of the
Beagle, Darwin wrote, “Seeing this
gradation and diversity of structure
in one small, intimately related
group of birds, one might really
fancy that from an original paucity
of birds in this archipelago, one
species had been taken and

modified for different ends.” This

was one of the first clear, public

formulations of where his thoughts

on evolution were heading.

Comparing species

Darwin’s finches, as the Galápagos

specimens became known, were

not the only trigger for his work on

evolution. In fact, his thoughts had

been mounting throughout the

Beagle’s voyage, and especially

during his visit to the Galápagos.

He was fascinated by the giant

tortoises he saw, and by the way

the shapes of their shells differed

subtly from island to island.

He was also impressed by the

species of mockingbirds. They,

too, varied between the islands,

yet they also had similarities not

only among themselves, but with

species that lived on the South

American mainland.

Darwin suggested that the

various mockingbirds might have

evolved from a common ancestor

that had somehow crossed the

Pacific from the mainland; then

each group of birds evolved by

adapting to the particular

environment on each island and

its available food. Observing giant

tortoises, Falkland Island foxes,

and other species supported these

early conclusions. But Darwin

was sensitive about where such

blasphemous ideas would lead:

“Such facts would undermine the

stability of species.”

Other parts of the jigsaw

On his way to South America in

1831, Darwin had read the first

volume of Charles Lyell’s Principles

of Geology. Lyell argued against

Cuvier’s catastrophism history

and his theory of fossil formation.

Instead, he adapted the ideas of

geological renewal put forward by

James Hutton into a theory known

as “uniformitarianism.” Earth was

continually being formed, altered,

and reformed over immense time

periods by processes such as wave

erosion and volcanic upheaval that

were the same as those happening

today. There was no need to invoke

disastrous interventions by God.

Lyell’s ideas transformed

the way Darwin interpreted the

landscape formations, rocks, and

Natural selection is the...

principle by which slight

variation (of a trait), if useful,

is preserved.

Charles Darwin

This giant tortoise is only found on

the Galápagos Islands, where unique

subspecies have developed on each

island. Darwin gathered evidence

here for his theory of evolution.