In the lower part of the Tertiary rocks, we find at once a great variety of large beasts that gave suck to their young.  It is likely that these creatures had come into existence in a somewhat earlier time in other lands, where we have not been able to study the fossils; for to make their wonderful forms slowly, as we believe them to have been made, would require a very long time.  It is probable that during the Cretaceous time, in some land where we have not yet had a chance to study the rocks, these creatures grew to their varied forms, and that in the beginning of the Tertiary time, they spread into the regions where we find their bones.

Beginning with the Tertiary time, we find these lower kinsmen of man, through whom man came to be.  The mammals were marked by much greater simplicity and likeness to each other than they now have.  There were probably no monkeys, no horses, no bulls, no sheep, no goats, no seals, no whales, and no bats.  All these animals had many-fingered feet.  There were no cloven feet like those of our bulls, and no solid feet as our horses have.  Their brains, which by their size give us a general idea of the intelligence of the creature, are small; hence we conclude that these early mammals were less intelligent than those of our day.

It would require volumes to trace the history of the growth of these early mammals, and show how they, step by step, came to their present higher state.  We will take only one of the simplest of these changes, which happens to be also the one which we know best.  This is the change that led to the making of our common horses, which seem to have been brought into life on the continent of North America.  The most singular thing about our horses is that the feet have but one large toe or finger, the hoof, the hard covering of which is the nail of that extremity.  Now it seems hard to turn the weak, five-fingered feet of the animals of the lower Tertiary—­feet which seem to be better fitted for tree-climbing than anything else—­into feet such as we find in the horse.  Yet the change is brought about by easy stages that lead the successive creatures from the weak and loose-jointed foot of the ancient forms to the solid, single-fingered horse’s hoof, which is wonderfully well-fitted for carrying a large beast at a swift speed, and is so strong a weapon of defence that an active donkey can kill a lion with a well-delivered kick.

[Illustration:  FIG. 10.  FEET OF TERTIARY MAMMALS.]

The oldest of these creatures that lead to the horses is called Eohippus or beginning horse.  This fellow had on the forefeet four large toes, each with a small hoof and fifth imperfect one, which answered to the thumb.  The hind feet had gone further in the change, for they each had but three toes, each with hoofs, the middle-toed hoof larger and longer than the others.  A little later toward our day we find another advance in the Orohippus, when the little imperfect thumb has disappeared, and there are only four toes on the forefeet and three on the hind.