origin and
development of many of the different branches that grew out of the
mammalian limb from different places and at different times during the
Mesozoic and the following age, called the Cenozoic, or age of recent
animals. It is unnecessary, however, for us to review more of the details:
the main result is obvious; namely, that the appearance of the great
classes of vertebrates is in the order of comparative anatomy and
embryology. Not only, then, is the fact of evolution rendered trebly sure,
but the general order of events is thrice and independently demonstrated
to be one and the same. Surely we must see that no reasonable explanation
other than evolution can be given for these basic facts and principles.

Turning now to the second division of palaeontological evidence, we come to
those groups where abundant materials make it possible to arrange the
animals of successive epochs in series that may be remarkably complete.
For the reasons specified, the backboned animals provide the richest
arrays of these series, and such histories as those of horses and
elephants have taken their places in zooelogical science as classics. But
even among the invertebrates significant cases may be found. For example,
in one restricted locality in Germany the shells of snails belonging to
the genus _Paludina_ have been found in superimposed strata in the order
of their geological sequence. The ample material shows how the several
species altered from age to age by the addition of knobs and ridges to the
surface of the shell, until the fossils in the latest rocks are far
different from their ancestors in the lowermost levels. Yet the
intervening shells fill in the gaps in such a way as to show almost
perfectly how the animals worked out their evolutionary history. This
example illustrates the nature of many other known series of mollusks and
of brachiopods, extending over longer intervals and connecting more widely
separated ages like the Secondary and the present period.

Since the doctrine of evolution and its evidences began to occupy the
thoughts of the intellectual world at large, no fossil forms have received
more attention than the ancient members of the horse tribe. As we have
learned, a modern horse is described by comparative anatomy as a one-toed
descendant of remote five-toed ancestors. When the hoofed animals of
modern times were reviewed as subjects for comparative anatomical study,
the odd-toed forms arranged themselv