The recent epoch.  The deposits laid since glacial times graduate into those now forming along the ocean shores, on lake beds, and in river valleys.  Slow and comparatively slight changes, such as the warpings of the region of the Great Lakes, have brought about the geographical conditions of the present.  The physical history of the Recent epoch needs here no special mention.

THE LIFE OF THE QUATERNARY

During the entire Quaternary, invertebrates and plants suffered little change in species,—­so slowly are these ancient and comparatively simple organisms modified.  The Mammalia, on the other hand, have changed much since the beginning of Quaternary time:  the various species of the present have been evolved, and some lines have become extinct.  These highly organized vertebrates are evidently less stable than are lower types of animals, and respond more rapidly to changes in the environment.

Pleistocene mammals.  In the Pleistocene the Mammalia reached their culmination both in size and in variety of forms, and were superior in both these respects to the mammals of to-day.  In Pleistocene times in North America there were several species of bison,—­one whose widespreading horns were ten feet from tip to tip,—­a gigantic moose elk, a giant rodent (Castoroides) five feet long, several species of musk oxen, several species of horses,—­ more akin, however, to zebras than to the modern horse,—­a huge lion, several saber-tooth tigers, immense edentates of several genera, and largest of all the mastodon and mammoth.

The largest of the edentates was the Megatherium, a. clumsy ground sloth bigger than a rhinoceros.  The bones of the Megatherium are extraordinarily massive,—­the thigh bone being thrice as thick as that of an elephant,—­and the animal seems to have been well able to get its living by overthrowing trees and stripping off their leaves.  The Glyptodon was a mailed edentate, eight feet long, resembling the little armadillo.  These edentates survived from Tertiary times, and in the warmer stages of the Pleistocene ranged north as far as Ohio and Oregon.

The great proboscidians of the Glacial epoch were about the size of modern elephants, and somewhat smaller than their ancestral species in the Pliocene.  The mastodon ranged over all North America south of Hudson Bay, but had become extinct in the Old World at the end of the Tertiary.  The elephants were represented by the mammoth, which roamed in immense herds from our middle states to Alaska, and from Arctic Asia to the Mediterranean and Atlantic.

It is an oft-told story how about a century ago, near the Lena River in Siberia, there was found the body of a mammoth which had been safely preserved in ice for thousands of years, how the flesh was eaten by dogs and bears, and how the eyes and hoofs and portions of the hide were taken with the skeleton to St. Petersburg.  Since then several other carcasses of the mammoth, similarly preserved in ice, have been found in the same region,—­ one as recently as 1901.  We know from these remains that the animal was clothed in a coat of long, coarse hair, with thick brown fur beneath.