The Science Book

A CENTURY OF PROGRESS 147

fossils he found on his explorations,
which he now saw “through Lyell’s
eyes.” However, while he was in
South America, volume two of
Principles of Geology arrived. In
it, Lyell rejected ideas of gradual
evolution of plants and animals,
including Lamarck’s theories.
Instead, he invoked the concept
of “centres of Creation” to explain
species’ diversity and distribution.
Although Darwin admired Lyell as
a geologist, he had to discount this
latest concept as the evidence for
evolution mounted.
Another piece of the jigsaw
was revealed in 1838 when Darwin
read An Essay on the Principle
of Population by the English
demographer Thomas Malthus,
which had been published 40 years
earlier. Malthus described how
human populations can increase
in an exponential way, with the

potential to double after one

generation of 25 years, then double

again in the next generation, and

so on. However, food supplies

could not expand in the same way,

and the result was a struggle for

existence. Malthus’s ideas were

one of the main inspirations for

Darwin’s theory of evolution.

The quiet years

Even before the Beagle had

returned to England, the interest

generated by the specimens

Darwin had sent back had made

him a celebrity. After his return,

his scientific and popular accounts

of the voyage increased his fame.

However, his health deteriorated

and gradually he withdrew from

the public eye.

In 1842, Darwin moved to the

peace and quiet of Down House in

Kent, where he continued to amass

evidence to support his theory of

evolution. Scientists around the

world sent him specimens and

data. He studied the domestication

of animals and plants, and the

role of selective breeding, or artificial

selection, especially in pigeons. In

1855, he started breeding varieties

of Columbia livia, or rock doves, and

they would feature prominently in

the first two chapters in On the

Origin of Species.

Through his work on pigeons,

Darwin began to understand

the extent and relevance of

variation among individuals.

He rejected the accepted wisdom

that environmental factors were

responsible for such differences,

insisting that reproduction was

the cause, with variation somehow

inherited from parents. He added

this to the ideas of Malthus and

applied them to the natural world. ❯❯

The finches of the Galápagos have evolved
differently shaped beaks adapted to specific diets.

Geospiza

magnirostris

Camarhynchus

parvulus

Geospiza fortis

Certhidea

olivacea

Medium-sized

ground finch has

a smaller beak for

crushing smaller,

softer seeds.

Warbler finch

has a thin beak for

probing for small

insects and

spearing them.

Large ground

finch has a large,

strong beak for

crushing large,

woody seeds.

Small tree finch

has a short, sharp

beak for grasping

insects.