A basic argument used by creationists is that evolution is "only a theory and cannot be proven." This creationist argument is based on the false assumption that the only way to "prove" evolution is for someone to actually observe large evolutionary changes, for example birds evolving from dinosaurs. Since this can't be accomplished in a few lifetimes, creationists claim that "proving" evolution is impossible. This argument totally ignores the enormous amount of factual data that supports the position that present life forms are descended from primitive ancestors. Whether evolution is true or not depends on a careful and rational examination of the facts. A clear distinction needs to be made between facts (things which can be observed and/or measured) and theories (explanations which correlate and interpret the facts. /P>
A fact is something that is supported by clear and incontrovertible evidence. For example, the Grand Canyon cuts through layers of different kinds of rock, such as the Coconino sandstone, Hermit Shale, and Redwall limestone. These rock layers often contain fossils, and when a layer contains fossils, they are characteristic of that layer. Facts may be interpreted in different ways by different individuals, but that doesn't change the facts themselves; they are incontrovertible.
Another fact is that fossil skulls have been found that are intermediate in appearance between humans and modern apes. They have brow ridges that are larger than humans, but smaller than apes; likewise their teeth and jaws are larger than humans, but smaller than modern apes.
Numerous fossils have been found that are clearly intermediate between dinosaurs and birds and run the gamut from non-flying reptiles with feathers to birds that retain some reptilian characteristics, like teeth and tail bones.
Persons may interpret the meaning of these fossils in different ways, but that doesn't alter the fact that they do exist.
Facts stand alone; they are independent of whether a person accepts evolution or rejects evolution. In the world of science there are easily a hundred thousand "facts" relating to biology, astronomy, geology, and so on. It is only natural to try to correlate all this information into explanations that make sense and that can predict other correlations. These explanations and correlations provide the "theories" of science.
In any language, usage determines the meaning of a word. In science, the meaning of the word "theory" has been clearly established by usage; its fundamental meaning is a logical explanation based on all the available evidence. In order for theories to be given credence by the scientific community, they must be capable of predicting further correlations.
Theories may be good, bad, or indifferent. They may be well established by the factual evidence, or they may lack credibility. But theories only explain the facts; they are not the facts themselves.
Before a theory is given any credence in the scientific community, it must be subjected to "peer review." This means that the proposed theory must be published in a legitimate scientific journal in order to provide the opportunity for other scientists to evaluate the relevant factual information and attempt to duplicate any experimental procedures involved. Creationists did not take the lack of peer review into account in their criticism of "cold fusion" and the National Geographic "bird/reptile" fossil, both of which were rejected upon peer review.
The statement that evolution is "a theory, not a fact" is based on the belief that a theory is something that can not be proven, and therefore is not a "fact." This view grossly distorts the meanings of the words "theory" and "fact," and diverts attention from the large amount of factual evidence supporting evolution. In addition, it fails to make a clear distinction between theory and fact.
Additional information is available in Lenny Flank's web site section: Is Evolution "Just a Theory"?