BARBARA KLEIN: Charles Darwin had poor health much of his life. He suffered headaches and problems with his skin and stomach. No one was able to find out what disease he may have had during his lifetime. Recently, some experts have suggested that he might have become infected with a tropical disease. Others suggest Darwin’s health problems were caused by conflict in his mind over his theory. Poor health would later force him to leave London and settled at Down House near Kent, England.

A copy of Charles Darwin's notebook containing his idea of an evolutionary tree. The notebook is in the American Museum of Natural History in New York City.

Darwin began work on a series of secret notebooks containing his thoughts about the evolutionary process. He began to think that animals developed from earlier, simpler organisms. As early as eighteen thirty-seven, he imagined this process as a tree with branches representing new species. Unsuccessful branches ended. But successful evolutionary changes continued to form new branches.

STEVE EMBER: Charles Darwin’s personal life was also expanding. In eighteen thirty-nine, he married Emma Wedgwood, his cousin. He told her his ideas about how species evolve over time -- what he called the transmutation of species.

Emma did not agree with her husband. But the two had a strong and happy marriage. They had ten children together. Seven of them survived.

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