 It was said that tortoises differed in the shape of the shells from island to island, and Darwin noted Lawson's statement that on seeing a tortoise he could "pronounce with certainty from which island it has been brought".Though Darwin remembered this later, he did not pay much attention at the time.However, he found a mockingbird and "fortunately happened to observe" that it differed from the Chatham Island specimen, so from then on carefully noted where mockingbirds had been caught.