Darwin never completely accepted the term "evolution" for his theory. He called it "descent with modification." In fact, the first edition of The Origin of Species by Means of Natural Selection never uses the word "evolution."
The problem was that this term was already used for something else. It had been applied to the series of progressive changes which occur in the development of an embryo.
Darwin's difficulty with this was that, while the development of an embryo is definitely a directed, progressive process, his proposed theory was not about a progressive process, or a process directed by anything other than the vagaries of environment, competition and reproductive success.
While this might seem to be a trivial quibble, it is actually a deep and fundamental problem. Ever since the day Darwin's theory was published, it has been burdened by the implication that it was all about progress from "lower" to "higher," or from "simple" to "complex," or from "primitive" to "advanced." This expectation permeates almost all of our casual consideration of evolution, and has become something of a cultural icon. Aristotle's "Great Chain of Being" has infiltrated our cultural consciousness, and our view of the history of life is colored by the metaphor of the ladder of progress. You see this expressed everywhere in the cartoons showing the "evolution" of man, inserting the football player in his crouch between the chimp (not, incidentally, a human ancestor) and the Neandertal; at the museum, with its linear display of animal models depicting the "progress" from Crossopterygian fish to mammal. If you start looking for it, it's everywhere. Next time you are at a party with a lot of people present, start a discussion of which is "higher" on the evolutionary scale, cats or dogs. You'll get lots of argument, but chances are nobody will point out that the question is nonsense. There is no evolutionary scale, and cats and dogs are simply different adapted to different, though rather similar, life styles.
Finally, it bears remembering the implications of the concept of common descent. This idea is that all organisms on this planet ultimately stem from the same common ancestral species and the evidence for that is very strong. What that means is that, though you may be uncomfortable with the idea, those dogs and cats, and earthworms, and bacteria, and humans have evolutionary histories of the same length. You are not "more evolved" than an earthworm (whatever that means); your evolutionary pathway simply meandered in different directions.
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Updated 25 September 2004