Charles Darwin's "The Origin of Species" provides the foundation for modern biology, but its influence extends far beyond biology to such outlying areas as ethics. In the introduction of the book Darwin begins with his argument of evolution by examining the different variation within domestic animal groups. His primary example of variation is the domestic pigeon. Darwin explains that all were developed from the rock pigeon. This was because of the selective traits that were very closely related to the physical structure of the rock pigeon. Species had fixed boundaries, many traditional scientists believed. If the selective pressure was abandoned, within a couple of generations the pigeons would have returned to "normal." Darwin was emphasizing only the ability of human selection to accentuate variations within the species. Particular traits within a species can be chosen, and man's ability to push selection in a particular direction can result in large variations within a species.
Darwin goes on to explain that he developed his notion of variation under nature. For Darwin, the difference between species and variations is vague and subjective. "Species are simply strongly marked and permanent varieties" (Darwin, p. 108). Variations occur within species, and some variations have a better chance of surviving than other variations. Natural selection, commonly termed 'survival of the fittest', is seen as a mechanism preserving the favorable variations and destroying the harmful variations. Therefore, varieties slowly develop into independent and permanent species. Nature makes no big leaps. More specialized species were better "adapted" than earlier species. ].
Similarly, Darwin sees that some variations have a better chance of surviving, and the species we see today are the result of natural selection operating over long periods of time. The belief serving as the foundation of "Uniformitarianism" is that nature is a continuum and the main processes we see at work can be analyzed over large time and space scales.