Scientists at the beginning of the 1800s know of some kinds of fossils, and they were very aware of homologous and vestigial structures. Many scientists suspected that some kind of evolution had given rise to living things around them. However, they had no unifying theory to explain how evolution might have occurred. Two scientists led the way in the search for a mechanism of evolution. The first was Jean Lamarck. The second was one of the greatest figures in biology, Charles Darwin.
Assume that there were salamanders living in some grasslands. Suppose, Lamarck argued, that these salamanders had a hard time walking because their short legs couldn't trample the tall grasses or reach the ground. Suppose that these salamanders began to slither on their bellies to move from place to place. Because they didn't use their legs, the leg muscles wasted away from disuse and the legs thus became small. Lamarck's theory said that the salamanders passed this acquired trait to their offspring. In time the salamander's legs were used so rarely that they disappeared. Thus, Lamarck argued, legless salamanders evolved from salamanders by inheriting the acquired characteristic of having no legs. Lamarck presented no experimental evidence or observation and his theory fell out of scientific favor. The next significant idea came from the British scientist Charles Darwin.
Charles Darwin (1809-1882), like many people of genius, did not at first appear to have extraordinary talents. From a young age Darwin disliked school and preferred observing birds and collecting insects to study. He was sent to medical school in Scotland when he was 16. Young Darwin found medicine "intolerably dull." He was much more interested in attending natural history lectures. Seeing that Darwin lacked enthusiasm for becoming a doctor, his father suggested he study for the clergy. Darwin was agreeable to the idea and enrolled in the university at Cambridge, England, in 1827. Here again, Darwin admitted, "My time was wasted, as far as the academic studies were concerned." However, Darwin found that his friendship with John S. Henslow, professor of botany, made life in Cambridge extremely worthwhile. Through long talks with Henslow, Darwin's knowledge of the natural world increased. Henslow encouraged Darwin in his studies of natural history. In 1831 Henslow recommended that Darwin be chosen for the position of naturalist on the ship the HMS Beagle.
The Beagle was chartered for a five-year mapping and collecting expedition to South America and the South Pacific. Darwin's job as ship naturalist was to collect specimens, make observations, and keep careful records of anything he observed that he thought significant. At the beginning of the voyage Darwin read a geology book given to him by Henslow. This book, Principles of Geology by Charles Lyell, spurred his interest in the study of land forms. In Chile Darwin observed the results of an earthquake: the land had been lifted by several feet. In the Andes he observed fossil shells of marine organisms in rock beds at about 4,300 m. He came to agree with Lyell that over millions of years earthquakes and other geologic processes could change the geology of the land. Because the land changed, new habitats would form. Darwin realized that animals would have to adapt to these changes. During the Beagle's five-year trip the captain often dropped Darwin off at one port and picked him up months later at another. One reason that Darwin was so eager to study life on land was that he suffered from terrible seasickness and couldn't wait to get off the Beagle. During his time on land Darwin trekked hundreds of miles through unmapped region. He observed thousands of species of organisms and collected many different types of fossils. On the long sea voyages he used his time to catalog his specimens and write his notes.
In 1837 Darwin began his first notebook on evolution. For several years Darwin filled his notebooks with facts that could be used to support the theory of evolution. He found evidence from his study of the fossil record: he observed that fossils of similar relative ages are more closely related than those of widely different relative ages. Comparing homologous structures, vestigial organs, and embryological development of living species gave him additional evidence of evolution. He consulted animal and plant breeders about changes in domestic species. He ran his own breeding experiments and also did experiments on seed dispersal.