Visiting Oxford, the Harvard professor/sleuth gets a crash course in Darwin's survival of the fittest in a high-spirited whodunit that's "vintage Langton" (Booklist).   William Dubchick is too keen a student of the writings of Charles Darwin to not see that the world of biology has evolved past him. Decades ago, he was the foremost mind in Oxford University's department of natural sciences, but as the field's focus narrowed to the microscopic level he became nothing more than a gray-haired, cantankerous relic. He has a small fiefdom, manned by Helen Farfrae, a committed disciple who, Dubchick is annoyed to learn, someone is trying to kill.   It is into this world that Homer Kelly, Emersonian scholar and part-time sleuth, comes to spend a semester lecturing. Though expecting a vacation, he finds Oxford to be a swamp of theft, fraud, and murder. Besides the attempts on Farfrae's life, he must reckon with a murdered priest, the theft of a dodo's portrait, and suspicious claims that long-lost Darwinian artifacts have been found. With an academic climate like this, it's amazing that any of the Oxford dons live to see tenure.