species go into the existing divisions, in such a manner as to fill them up precisely where they are most deficient, thus occupying what would otherwise be gaps in the existing system of nature. The principal difficulty which they occasion to the zoologist and botanist is that, by filling the intervals between genera previously widely separated, they give to the whole a degree of continuity which renders it more difficult to decide where the boundaries separating the groups should be placed. We also find that the animals and plants of the earlier periods often combined in one form powers and properties afterward separated in distinct groups; thus in the earlier formations the sauroid fishes unite peculiarities afterward divided between the fish and reptiles, constituting what Agassiz has called a synthetic type. Again, the series of creatures in time accords with the ranks which a study of their types of structure induces the naturalist to assign them in his system; and also within each of the great sub-kingdoms presents many points of accordance with the progress of the embryonic development of the individual animal. Nor is this contradictory to the statement that the earlier representatives of types are often of high and perfect organization, for the progress both in geological time and in the life of the individual is so much one of specialization that an immature animal often presents points of affinity to higher forms that disappear in the adult. In connection with this, earlier organic forms often appear to foreshadow and predict others that are to succeed them in time, as the winged and marine reptiles of the Mesozoic foreshadow the birds and cetaceans. Agassiz has admirably illustrated these links of connection between the past and the present in the essay on classification prefixed to his "Contributions to the Natural History of America." In reference to "prophetic" types, he says: "They appear now like a prophecy in those earlier times of an order of things not possible with the earlier combinations then prevailing in the animal kingdom, but exhibiting in a later period in a striking manner the antecedent consideration of every step in the gradation of animals."