Appeal to authority is a fallacy of defective induction, where it is argued that a statement is correct because the statement is made by a person or source that is commonly regarded as authoritative.Scientists don't believe the theory of evolution is correct because it was originally formulated by Charles Darwin, or because it was presented in his book "The Origin of Species by Means of Natural Selection".
Scientific presentations of new theories consist of two parts: the theory itself, and the gathered data which this theory explains. The availability of the gathered data means other scientists can check its validity and see whether it actually supports the proposed theory, or matches some other theory better. For example, the finches Darwin gathered in the Galapagos islands are actually still stored in the bird collection of the British Natural History Museum. Additionally, future scientists can gather additional data and perform new experiments to confirm, expand on, or refute Darwin's original theories. So far new gathered data has confirmed Darwin's basic theories, and vastly expanded on them (understandable, since Darwin was not even aware of Mendelian genetics, let alone DNA). But this new research is not being done to defend Darwin's legacy. Young scientists would be overjoyed to discover a radical new experimental finding or theory which overturns Darwin, and are feverishly looking for this. This very possibility of being wrong is what makes evolution a scientific theory.
An earlier post quoted Dawkins on how children are natural essentialists. I argue children are also, both by nature and nurture, believers in the argument from authority. Children want all statements to have invariant mappings to the categories "true" or "false". The vague category "as best we can tell right now, pending future information" is confusing and upsetting. Teachers and parents are also natural authority figures, and don't have the time (and often background) to explain the rationale behind all their instructions. Elementary memorization is useful, and a child which questioned everything would be impossible to teach. But for many students this attitude continues to their study of science, so Newton, and Einstein, and Darwin become unquestioned authority figures. They don't go back to see that these scientist's work consisted of the same type of presentation discussed above: theories with supporting evidence.
Critics of "Darwinism" are often actually modeling their critique on their early childhood religious upbringing. Early authority figures in their life (their parents) presented additional authority figures (church leaders) whose authority is ultimately derived from an unquestionable final authority figure (the religion's founder(s)). The founders words are contained in unquestionable texts (the revealed sacred documents), and all necessary truths can be obtained through detailed study of those texts. This technique of finding truths through interpretation of texts written by founders (and later texts written about texts written by founders) can arguably be useful in some fields such as law. But it is of only historical interest in science. The correctness of modern evolutionary theory is not bound up in the life of Darwin or the text of "The Origin of Species."
An amusing collision between intelligent design "science" and argument from religious authority is here. Dembski is at risk of being "Expelled"...