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 themselves in a series beginning with an
animal like an elephant with the full number of five digits on each foot
and ending at the opposite extreme with the horse. A reasonable
interpretation of these facts was that the animals with fewer toes had
evolved from ancestors with five digits, of which the outer ones had
progressively disappeared during successive geological periods, while the
middle one enlarged correspondingly. The facts provided by palæontology
sustain this contention with absolutely independent testimony.
Disregarding some problematical five-toed forms like _Phenacodus_, the
first type of undoubted relationship to modern horses is _Hyracotherium_,
a little animal about three feet long that lived during the Eocene period
of the Cenozoic epoch. Its forefeet had four toes each, and its hinder