[Illustration: Charles Lyell, 1797-1875, revolutionized the study of geology partly by publicizing the earlier work of James Hutton, who died the year Lyell was born in Scotland, and partly by infusing the science with his own highly disciplined point of view. His greatest contribution was the firm establishment of Hutton’s principle of uniformitarianism, or uniformism, which became the foundation for all modern geological work. Put simply, this is the principle that the processes we see operating to form and shape the Earth today have always operated in the past. Once this is admitted, it becomes clear that past geological time is vast, not short, a truly stunning notion for Lyell’s time but a commonplace fact today. The first volume of his _Principles of Geology_ was published in 1830; in his later works he championed Darwin’s own revolutionary point of view, adding his own powerful arguments in support of the idea of natural selection.]
[Illustration: Alfred Wallace, 1823-1913, was the co-originator, with Darwin, of the principle of natural selection, or “survival of the fittest.” The main difference between the two was that Wallace did not believe that natural selection explained things as well as Darwin thought it did, which has been borne out to a large extent by modern studies of genetic variation. Wallace worked in South America, along the Amazon and Rio Negro rivers, and in East Asia. He showed that the animals on either side of a line between Borneo and the Celebes Islands are radically different in their makeup and origin. Now known as “Wallace’s Line,” his work has been vindicated by additional modern studies. Although Wallace did not become as well known as Darwin, his brilliant, independent studies lent a great deal of weight to the Darwinian view of evolution.]
[Illustration: _The largest divisions of geologic time are eras, shown above in chronological order from the oldest on the bottom to the most recent on top. The scale at left shows the relative duration of each era. As the chart shows, geologists further divide time into periods and, in the Cenozoic Era, into epochs. The fossilization of animals in the Agate Springs area of Nebraska took place in the Miocene Epoch. Adjustments to this time chart are made as new data becomes available, so it should not be thought of as an unchanging reference. This diagram is adapted from one in The Emergence of Man series published by Time-Life Books._]