Darwin turned wholeheartedly to the problem of evolution.
Ever since his Beagle trip he had been convinced
that the difference between what naturalists called 'varieties'
and what they called 'species' was much less significant than previously
thought. If pigeon breeders could create varieties as different
as pouters, runts, and fantails, what would prevent nature from
doing the same? And, given millions of years, wasn't it possible
that a pigeon could be turned into something so radically different
we would no longer be willing to call it a pigeon–or even a bird?
Darwin was not the first to have these kinds of thoughts.
Seventy years before, his grandfather, Erasmus, had devoted a whole
section of his book Zoonomia to the issue of evolution.
In 1844, Robert Chambers anonymously published his controversial
book, The Vestiges of the Natural History of Creation, a
sweeping history of the cosmos that came down strongly on the side
of evolution, largely on the evidence of fossils (the 'vestiges'
of creation). The book was flawed, but popular, and it brought the
idea of evolution into the public eye. The opposition to evolution
was still strong, but it included among its number a wide range
of opinions, from those who thought that all species had been created
at the beginning of the world in the same form as they now had,
to those who thought that new species were being continuously created
to fill new environmental niches, to those who thought that variation
within species was within Nature's power but the creation of new
species remained in God's hands.
Darwin had two things to contribute to this debate: a
wealth of observations on adaptation, and, more importantly, a
theory that could explain how new adaptations arose without the
guiding hand of a divine Creator. His observations were gained
by his own experience on the Beagle, his eight
painstaking years of work on barnacles, and the advice and expertise
of friends like Hooker. His theory was his own creation. Darwin
solved the problem of evolution by pointing to a mechanism that
depended on nothing but variation and chance: natural
selection. Many more individuals were born than could
be supported by the environment, which meant that some had to die.
Which ones died? Obviously, those that were least well adapted
to the environment. Given that there is variation in the population,
and that that variation is heritable (i.e. can be passed from one
generation to the next), it was clear that the most useful adaptations
would be preserved. If enough of those adaptations were accumulated,
a new species could arise.
Although this sketch of the theory was already in place
in Darwin's notes, in 1854 he was still struggling with a few pieces
that he had yet to make sense of. One had to do with the population
of islands like the Galapagos. According to his theory, animals
and plants had arrived on the islands millions of years before
and had slowly adapted to fit the unique environments on each island.
But how had they gotten there in the first place? He was unwilling
to accept the possibility that these volcanic islands had once
been closer to the mainland of South America. Instead, he tried
to prove that seeds and even eggs might have been transported on
ocean currents from the mainland. He conducted experiments: soaking
seeds in salt water for weeks to see if they would still germinate
(most did) and figuring out which seeds would float (most didn't).
The second puzzle piece was why there was such a great diversity
of life in the world. If every species was continuously adapting
to fit the environment as best it could, why didn't all species
converge to the same form? Shouldn't there be some 'best species'
that would dominate all others? Darwin solved this problem by drawing
an analogy with modern industry. It was not true that there was
a single 'best job' in any task. In fact, production got more
efficient the more specialized each worker became. The same held
true in the natural world: species specialized so that they could
capitalize on particular aspects of the environment. In fact, a
species that was failing in the competition in one particular environmental
niche could become startlingly successful if it simply shifted
niches so that it was no longer in head-to-head competition similar
species.
To shore up his understanding of variation under artificial
selection–the kind of selection that had produced pets and domestic
farm animals–Darwin started to learn all he could about pigeons.
Breeding pigeons was a pastime that few aristocrats threw themselves into,
but Darwin eagerly built a shed in the yard behind Down House.
He started making trips into London to speak to the professionals,
downing beers with them while they boasted about how they could
see differences of 1/16th of an inch between two pigeon's beaks.
Darwin studied not only living, breeding pigeons, but also dead
ones; for a while, his workshop became a shop of horrors as he killed
and 'skeletonized' pigeons of all varieties and ages, not to mention
the occasional rabbit or chicken, studying the sometimes striking
differences in structure between different varieties.
Meanwhile, Darwin was testing out his theory of evolution
on friends like Hooker and zoologist Thomas Henry Huxley. No one was
entirely convinced yet, but some progress was being made. When
Lyell, the geologist whose Principles of Geology had
so inspired Darwin while on the Beagle, found out
about Darwin's theory, he urged him to publish it as soon as possible,
if only to ensure that he would not be scooped. Darwin demurred:
he was much more interested in producing a watertight case for
evolution by natural selection than in gaining credit for a theory
that could be easily dismissed. Nonetheless he started working
on a manuscript in May of 1856. It started as a sketch, but it
quickly became obvious that, when completed, it would rival Lyell's Principles in
length, and would probably have to be published in multiple volumes.
