Page:Popular Science Monthly Volume 10.djvu/609

Rh north. But year after year it got warmer, and the local glaciers shortened, extending less and less into the valleys. The trapped insects could not then rejoin their mates, but instead climbed the mountain, dwelling farther and farther up as time progressed and the climate changed. At length they reached the summit of Mount Washington, where we still find some of them, and whence there is no escape. The plants formed patches in congenial spots on the sides of the mountain, driven upward by the new flora filling the valley from the southward. Now, in examining these colonists, we find among them the herb-like willow (Salix herbacea), its short stems hardly rising above an inch from the ground, and other species of plants which we have to go far north to meet again. There, too, the White Mountain butterfly (Œneis semidea) appears year by year, swaying in feeble flight over its narrow range, while its congeners are found one thousand miles to the northward in Labrador. These examples could be easily multiplied.

But this sort of mountain-trap was not large enough to hold such game as men and reindeer. These both went northward, and are not to be found alive with us; but the one left his implements, and the other its bones, to tell of their presence at that time, and in these latitudes.

When we come to the question as to the descendants of this early North American man, we cannot avoid studying for a moment the movements or migrations of man over the surface of the earth generally. I think we may divide his migrations into two main classes from their motives:

A primitive migration—one influenced solely by physical causes affecting his existence, and which must have been in more extended operation in early times when he was unprovided with means of his own invention against a change in his surroundings. Such migrations are operative now among certain of our Indians, who move from place to place with the game upon which they subsist and with the season.

A culture migration—one arising from a certain stage of intellectual advancement when the movements of man are determined by ultimate and not immediate considerations. The movements of the Indo-European races fall within this category. Besides these, there are to be distinguished accidental migrations, which man submitted to against his will. We know that insects and plants are so transported. Birds and ocean-currents carry seeds from land to land. Insects on a blade of grass or a fallen bough are carried down a river by the current to found colonies of their race far from their place of origin. And such circumstances give rise to races and varieties among species modifiable by the peculiarities of their new localities. The accidental migrations of man may be considered as belonging to the epoch of culture-migration, since they must more usually have