Darwin's idea of shared ancestry was supported by the evidence he gathered from fossils and biogeography.
He was a devoted student of nature and a collector of insects.
Darwin was sent to medical school to follow in his parents' footsteps.
His father encouraged him to enroll in the School of Divinity at Christ's College at Cambridge because he didn't want to study medicine.
Darwin attended many lectures on biology and geology at Christ's College to satisfy his interest in natural science.
He gained skills in the identification and collection of plants from his friend John Henslow.
Adam Sedgewick, one of the founding fathers of modern geology, was one of the people Darwin was working with in the summer of 1831.
The voyage was to take 2 years, but ended up taking 5 years, and the ship was to traverse the Southern Hemisphere.
Darwin encountered species that were very different from his native England.
As part of his duties as the ship's naturalist, Darwin began to gather evidence that organisms are related through descent with modification from a common ancestor and that adaptation to various environments results in diversity.
The origin of new species was one of the mysteries Darwin pondered.
Darwin was able to observe geological changes firsthand.
He saw raised beaches along the coast when he was in Argentina.
Many of the raised beaches had exposed layers of silt that contained fossils of extinct mammals.
It was suggested to Darwin that the Earth is very old by observing marine shells high in the cliffs of the Andes Mountains.
Darwin thought that there would be enough time for modification to occur if the Earth was very old.
Living forms could be descended from extinct forms.
It would appear that the species were not fixed.
During his exploration of South America, Darwin found fossils of extinct mammals.
The extinct animal must be related to the living ones.
The glyptodont weighed more than 2000 lbs.
The study of the geographical distribution of organisms is called "writing".
The distribution of species and the makeup of species groups in different regions give hints about past geological events, such as the movement of continents and the formation of volcanic islands.
Darwin compared the animals of South America to those he was familiar with in the Southern Hemisphere.
The cavy was found in the grassland of South America.
The cavy is native to South America and has long legs and ears, but not the face of a guinea pig.
Both animals ate grass and hid in bushes.
As he sailed southward along the eastern coast of South America, Darwin saw how similar species replaced one another.
The lesser rhea was replaced by the greater rhea in the north.
Darwin thought that related species could be changed according to the environment.
Further evidence of this phenomenon was found when he traveled to the Galapagos Islands.
The small group of volcanic islands off the western coast of South America are called the Galapagos Islands.
Life is present on these islands, which are too far from the mainland for most animals and plants to colonize.
There were different types of plants and animals found there, and they varied from island to island.
Darwin wondered if the variation in vegetation among the islands was related to the different types of tortoises on the islands.
Long-necked tortoises were only found in dry areas with tall cacti.
Short-necked tortoises were found in moist regions.
Darwin wondered if the tortoises were descended from a common family.
The tortoises with dome shells and short necks feed on grasses on the islands.
Those with shells that flare up in the front live on arid islands where they feed on cacti.
The finches have a nondescript nature compared to the other animals in the Galapagos.
Darwin didn't know that the birds were all finches, because they were different from the ones in England.
His thoughts about geographic barriers and their contribution to the origin of new species would eventually be formed by these birds.
John Gould identified the birds after Darwin returned to England.
Gould said the birds were a series of ground finches.
A finch is the most unusual of the finches.
The bird has a sharp beak that chisels through tree bark, but it doesn't have the long tongue of a true woodpecker.
The bird has a twig or cactus spine in its beak and uses it to poke into crevices.
For more information on the shape of finch beaks, see the Nature of Science feature, "Genetic Basis of Beak Shape in Darwin's Finches," in Section 17.2.