Peto's paradox

Peto's Paradox is the observation, due to Richard Peto that at the species level the incidence of cancer does not appear to correlate with the number of cells in an organism. For example the incidence of cancer in humans is much higher than the incidence of cancer in whale. This is despite the fact that a whale has many more cells than a human. If the probability of carcinogenesis were constant across cells one would expect whales to have a higher incidence of cancer than humans.