h, and which we may call the Series of
Time. Take as an illustration the class of Echinoderms. The first
representatives of this class were a sort of Star-Fishes on stems; then
were introduced animals of the same order without stems; in later
periods come in the true Star-Fishes and Sea-Urchins; and the highest
order of the class, the Holothurians, are introduced only in the present
geological epoch. Compare now with this the ordinal division of the
class as it exists today. The present representative of those earliest
Echinoderms on stems is an animal that upon structural evidence stands
lowest in the class; next above it are the Comatulae, corresponding to
the early Echinoderms without stems; next in our classification are the
Star-Fishes and Sea-Urchins; and the Holothurians stand highest, on
account of certain structural features that place them at the head of
their class. The Series of Time and the Series of Rank, then, accord
perfectly, and investigations of the embryological development of these
animals have shown that the higher Echinoderms pass through changes in
the egg that indicate the same kind of gradation, for the young in some
of them have a stem which is gradually dropped, and their successive
phases of development recall the adult forms of the lower orders. Take
as another illustration the class of Polyps. First in time we find a
kind of Polyp Coral, one among the early Reef-Builders, who built their
myriad lives into the solid crust of our globe then as their successors
do now. These old Corals have their representatives among the present
Polyps, and from their structure they are placed lowest in their class,
while the embryological development of the higher ones recalls in the
younger condition of the germ the same peculiar character. I might
multiply examples, and draw equally striking illustrations from the
other classes; and though these correspondences cannot be fully
established while our knowledge of the embryological growth of animals
is so scanty, and information about their geological succession, yet
wherever we have been able to trace the connected history of any group
of animals in time, and to compare it with the history of their
embryological development and their structural relations as they exist
to-day, the correspondence is found to be so complete that we are
justified in believing that it will not fail in other instances. I may
add that a gradation of exactly the same character contro