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."

4. The periods into which geology divides the history of the earth are
different from those of Scripture, yet when properly understood there
is a marked correspondence. Geology refers only to the fifth and sixth
days of creation, or, at most, to these with parts of the fourth and
seventh, a