 Though the Beagle departed from the Galápagos with over 30 adult tortoises on deck , these were not for scientific study , but a source of fresh meat for the Pacific crossing . Their shells and bones were thrown overboard , leaving no remains with which to test any hypotheses . It has been suggested that this oversight was made because Darwin only reported seeing tortoises on San Cristóbal ( C. n. chathamensis ) and Santiago ( C. n. <unk> ) , both of which have an intermediate type of shell shape and are not particularly morphologically distinct from each other . Though he did visit Floreana , the C. n. nigra subspecies found there was already nearly extinct and he was unlikely to have seen any mature animals . 
