The foregoing are the chief acquired characteristics of aquatic air-breathing animals and especially aquatic reptiles in adaptation to their new mode of life. The resemblances, sometimes striking, thus brought about in animals of very different origin and remote relationships have often been mistaken for evidences of kinship, that is, direct inheritance from common ancestors. Such acquired resemblances in unrelated animals are known as parallel or convergent evolution. It has often been difficult to distinguish between convergent evolution and direct evolution, and difficulties still perplex and trouble the student of natural history in every branch of life. Not till all such problems are solved can we hope to attain the true classification of animals and plants. The whales a century ago were considered merely breathing fishes; the ichthyosaurs until a quarter of a century ago were supposed to be the direct descendants of fishes; lizards and crocodiles were grouped together in a single order; and salamanders were called reptiles not very long ago. Perhaps the reader will be able from the foregoing to understand and appreciate better some of the difficulties that confront the paleontologist in his attempts to solve the problems of past life; to understand why he sometimes makes mistakes, for he has by no means yet learned all the permutations of the skeleton in any class of vertebrates, and is not sure that the laws he accepts are not subject to modifications and exceptions. If he is truly scientific he hesitates long in prophesying or conjecturing. CHAPTER VI SAUROPTERYGIA