For example, he saw fossilized bones of long-extinct animals, and on different Galapagos islands he observed differences among various species of birds that were obviously closely related, yet distinctly adapted to their environments.Darwin also read with interest the work of the geologist Charles Lyell (Principles of Geology), who claimed, based on his scientific studies, that the earth was much older than the 6,000 years calculated by Bishop Ussher based on biblical genealogies and a literal reading of scripture. Most scientists, including devout Christians, had long been convinced from a study of the increasingly large fossil record that evolution was a fact, but they had been unable to explain it satisfactorily.
However, his book On the Revolution of the Celestial Spheres was added to the Index of Forbidden Books maintained by the Roman Catholic Church, and Christians were forbidden to read it.
Galileo Galilei built telescopes and began looking through them at the heavens.
In 1996 Pope John Paul II, in an address to the Pontifical Academy of Sciences, said, Today, almost half a century after the publication of the encyclical [Humani Generis], new knowledge has led to the recognition of the theory of evolution as more than a hypothesis. A contemporary French scientist, Comte du Buffon, estimated the age of the earth at 75,000 years, based on the rate of cooling that he had measured.
It is indeed remarkable that this theory has been progressively accepted by researchers, following a series of discoveries in various fields of knowledge. British naturalists William Smith and John Phillips observed the deposition of fossils in regular strata throughout Britain and estimated the age of the earth at 96 million years.
The convergence, neither sought nor fabricated, of the results of work that was conducted independently is in itself a significant argument in favor of this theory. Charles Lyell, the father of modern geology, introduced the principle of uniformitarianism, that is, that the processes that are currently going on in the earth have been going on more or less unabated since the earth's creation.
Most scientists who identify themselves as Protestants also accept Darwin's theory as foundational to the modern, scientific study of biology. This principle led to even older estimates of the earth's age, with many scientists in Darwin's day accepting an age of as much as 400 million years.
Darwin's proposal of descent with modification as a result of natural selection seemed to fit the evidence better than any other previous theory.
Many scientists agreed with the concept of evolution, but they doubted that natural selection was a sufficiently powerful factor to drive species to change over time.
Such large-scale changes, they argued, were driven primarily by factors internal to living beings that led to advancement up the evolutionary ladder; natural selection worked only on the margins, leading primarily to evolutionary dead ends.
Still other scientists opposed Darwin's theory because it seemed to conflict with two biblical principles: immutability of species and the relatively young age of the earth.
He was familiar with the work of Copernicus, and his own studies confirmed the heliocentric (sun-centered) view of the solar system.