THE discovery of general principles in the study of fossils is much hampered by the imperfection of the geological record. As every geologist is aware, we are dependent for our knowledge of the life of past ages on a few isolated episodes which have been locally preserved. There is no continuous history of the life of long periods in the rocks of any region that has hitherto been well explored. Cessations in the deposit of sediment, the recurrence of unfavourable conditions, and extensive migrations, among other causes, have all contributed to this result. An increasing acquaintance with scattered episodes in the secular development of life, however, tends to reveal its main outlines; and if we are unable to discover the actual facts we can at least arrive at an approximation to them which serves all immediate purposes. If we can determine the “fashion,” so to speak, which prevailed during each successive period in the geological history of a race of animals, we are able to distinguish between those changes in anatomical structure which led to stagnation or extinction, and those which were necessary for evolution to a higher plane. An acquaintance with the precise links between one grade and the next is not of supreme importance.