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 palaeontology 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 limbs ended with three toes armed with small hoofs, but one of its relatives of the same time has a vestige of another digit on the hind foot.  By the geological time mentioned, therefore, the earliest true horses had already lost some of the toes that their progenitors possessed.  In the Miocene the extinct species, obviously descended from the Eocene forms, had lost more of their toes; still higher, that is, in the rocks formed during succeeding periods of time, the animals of this division are much larger and each of their feet has only three toes, of which the middle one is the largest while the ones on the sides are small and withdrawn from the ground so as to appear as useless vestiges.  To produce modern horses and zebras from these nearer ancestors, few additional changes in the structure of the feet are necessary, for the lateral toes need only to become a little more reduced and the middle one to enlarge slightly to give the one-toed limb of modern types, with its splint-like vestiges still in evidence to show that the ancestor’s foot comprised more of these terminal elements.  Comparing the animals of successive periods, these and other skeletal structures demonstrate that the ancestry of each group of species is to be found in the animals of the preceding epoch, and that the whole history of horses is one of natural transformation,—­in a word, of evolution.