_The Wild Horse._ The steps in the evolution of the horse are stated so fully in the text that it is not necessary to repeat them here. Almost any good text in geology gives the same facts. It should be remembered that horses with more than one toe on each foot did not live when the Tree-dwellers did, but during earlier periods. The teacher who wishes to read further regarding the wild horse will find materials in the _Century Dictionary_ under _horse_, in _Chambers' Encyclopedia_, in H. N. Hutchinson's _Creatures of Other Days_, in N. S. Shaler's _Domesticated Animals_, and in _McClure's Magazine_, Vol. 15, p. 512. _The Musk Sheep._ The appearance of the musk sheep in western Europe during the mid-Pleistocene period marked the change that was beginning to take place in the climate. As the climate increased in severity all the arctic species came down from the north and occupied the land during the late Pleistocene period. The musk sheep is the most arctic in its habits of any of the herbivores, and at the present time is restricted to the high latitudes of North America. It thrives in desolate, treeless, barren grounds, not even being driven from its haunts by the extremest cold. It is closely allied to the species which is the parent of our domestic sheep, although that species did not appear in western Europe until prehistoric times. The musk sheep goes in herds of from twenty to thirty individuals, and when alarmed the animals huddle together like frightened sheep. Its food is grass, lichens, moss, and tender shoots of the willow and pine. It is much sought after for its skin, which makes a fine robe. It is sometimes known as the musk ox and occasionally as the musk bison. _Plant Life._ The characteristic trees of the mid-Pleistocene period were evergreen. Of these the most abundant forms were the spruce, the fir, and the yew tree. The trees which shed their foliage were represented by the oak and the birch. The banks of rivers were shaded by thickets of laurel and by the sloe, the original form of the wild plum tree. The marshes afforded rich pastures for grass-eating animals as well as hiding-places, for they were partly covered by a heavy growth of alders. Wild peas, beans, stringy-rooted carrots, ruta-bagas, and turnips grew on the hillsides. The cabbage with its thick leaves, which had not yet developed into a head, was present. Seeds of grasses were available, but not used, for man had not yet learned to gather them and convert them into nourishing food. The teacher may be interested in referring to Candolle's _Origin of Cultivated Plants_ and Darwin's _Plants and Animals Under Domestication_. SPECIAL SUGGESTIONS If possible read the entire book, including the preface, carefully before beginning the work. If in addition to this you can read parts of the following books and articles, do so; for in this way it will be easier to grasp the full significance of the work.