|A visit to the Galapagos Islands in 1835 helped Darwin
formulate his ideas on natural selection. He found several species of finch adapted to different environmental niches.
The finches also differed in beak shape, food source, and how food was captured.
The genius of Darwin (left), the way in which he suddenly turned all of biology
upside down in 1859 with the publication of the Origin of Species,
can sometimes give the misleading impression that the theory of evolution
sprang from his forehead fully formed without any precedent in scientific
history. But as earlier chapters in this history
have shown, the raw material for Darwin's theory had been known for
decades. Geologists and paleontologists had made a compelling case that
life had been on Earth for a long time, that it had changed over that
time, and that many species
had become extinct. At the same time, embryologists and other naturalists
studying living animals in the early 1800s had discovered, sometimes unwittingly,
much of the best evidence for Darwin's theory.
Pre-Darwinian ideas about evolution
It was Darwin's genius both to show how all this evidence favored the evolution of species from a
common ancestor and to offer a plausible mechanism by which life might evolve. Lamarck and others had
promoted evolutionary theories, but in order to explain just how life changed, they depended on speculation.
Typically, they claimed that evolution was guided by some long-term trend. Lamarck, for example, thought
that life strove over time to rise from simple single-celled forms to complex ones. Many German biologists
conceived of life evolving according to predetermined rules, in the same way an embryo develops in the womb.
But in the mid-1800s, Darwin and the British biologist Alfred Russel Wallace independently conceived of a
natural, even observable, way for life to change: a process Darwin called
The pressure of population growth
Interestingly, Darwin and Wallace found their inspiration in economics. An English parson named Thomas Malthus
published a book in 1797 called Essay on the Principle of Population in which he warned his fellow
Englishmen that most policies designed to help the poor were doomed because of the relentless pressure of
population growth. A nation could easily double its population in a few decades, leading to famine and misery
When Darwin and Wallace read Malthus, it occurred to both of them that animals and plants should also be
experiencing the same population pressure. It should take very little time for the world to be knee-deep in
beetles or earthworms. But the world is not overrun with them, or any other species, because they cannot reproduce
to their full potential. Many die before they become adults. They are vulnerable to droughts and cold winters and
other environmental assaults. And their food supply, like that of a nation, is not infinite. Individuals must
compete, albeit unconsciously, for what little food there is.