The Scientific Method/Empiricism and Inductivism
Take the example of the Megalodon in the field of Paleontology. From only a handful of teeth and vertebrae, paleontologists "tell" us that the Megalodon was, basically, a 20m long Great White with similar structure and behavioral patterns. The teeth of a Megalodon are similar in shape to that of a Great White so it has been assumed that it's morphology and behavior are similar (although because the teeth are larger, it's prey would be larger). While this may be accurate, it may also be completely wrong (there is strong support for the theory that Megalodon and great whites are not related, the latter being a descendant of the broad-tooth Mako shark). The only real clue that we have towards the size and behavior is that many bones of large whales have been found with tooth marks almost identical to that of the Megalodon, as sharks have full cartilaginous skeletons. However, there is no evidence, other than its similarity to the great white's, that the teeth and vertebrae even came from a shark and not some other animal which happened to have similar dentition and spinal structure.