Of course the number of varieties steadily decreases as one ascends just as on approach ing the poles, because the means of supporting life diminish in proportion as a moderate cli mate and abundant vegetation are left behind. The hardy grizzly can stray to the utmost heights, as he is fond of doing, for his strength, power of locomotion and fasting abilities make it possible; but even he cannot remain there long, since food is very scarce beyond the tim ber. The eagle and greater hawks may soar above the naked icy crest of the loftiest sum mits, or perch upon the pinnacles, but they seek their prey and build their nests for the most part at far lower levels. A few small birds, like certain semi-arctic warblers and sparrows (especially the gay Leucosttcte finches) and one large one (the well-known ptarmigan), the little chief hare or "cony," which stores in its rocky tunnels a winter larder of roots and stems gathered at the edge of the snow during the brief summer, and a small variety of beetles and other insects more or less subterranean in hab its, alone brave the storms and famine involved in continuous residence upon the higher sum mits.
Next below lies the zone of hardy plants and of a longer list of animals, such as the Canada jay, dusky grouse, several hawks and owls, the kinglets, water-ouzel, snow-birds and Zono trichia finches, the bighorn sheep and a large variety of insects and snails. Below that, down among the pines and abundant shrubbery of the lower slopes and the foothills, one finds in sum mer the full measure of Rocky Mountain life.
An examination of these facts discloses that all the inhabitants of the lofty plateaus and the rocky peaks are arctic animals and some of them, like the ptarmigan, turn white in winter in true arctic fashion. These creatures, finding the same conditions at those great heights to which they are accustomed at lower levels in side the polar circle, can live and flourish on an arctic island, as it were, in the midst of the temperate zone; the long narrow snow peaks of the great range forming a tongue of polar cli mate stretching half way to the equator. It has thus been possible for the beautiful white goat, whose proper home is in British Columbia, to stray south along the crest of the Sierra Nevada and Rockies as far as these high moun tains run, but he must keep upon the very crest, whereas in Alaska he comes down to the shore. The insects and minute life of the peaks of Col orado belong to the same class with (are often identically the same species as) those collected by polar exploring expeditions. The same is true of plants. "Red snow" and arctic lichens may be gathered on Pike's Peak.
It is natural, then, that two different migra tory movements should be observable in the Rocky Mountains; one the regular seasonal movement southward in the fall and hack in the spring, affecting chiefly those birds that live at the base of the range, or near it; and another movement, which is regularly made by many animals, upward to the cooler and fresher. pas tures in the summer, and back to the less snowy and more sheltered dells near the base Of the mountains as winter approaches. This
vertical migration is very well understood in the case of the game, and the paths which the ani mals follow are often distinct.
The foregoing facts lead to an interesting generalization. While the southern peaks might be colonized by such stragglers from the north as the goat and the ptarmigan, whose legs or wings enable them to travel back and forth, there is a much longer list of small and prac tically stationary animals never found in the intervening valleys and totally unable to cross from one peak to another. Hence we must con clude that each lofty mountain top is a habitat by itself, entirely cut off from neighboring peaks where duplicates of its fauna and flora may be collected. On the tops of Mount Washington, in the White Mountains and Mount Marcy, in the Adirondacks, are insects and cryptogamous plants which do not occur anywhere between these isolated fragments of polar climate and the arctic circle, where the same butterflies, spiders and lichens are widespread and indigenous.
How shall this isolation of strictly circum scribed faunas upon mountain peaks be ex plained? Clearly it dates back to a time when communication between them existed. The ice cap which, during the last Glacial Epoch, grad batty overspread a large part of the north temperate zone and in the Rocky Mountain region covered the whole extent of their high lands with a thick mantle of snow and filled every canyon with local glaciers, of course crowded southward all the surviving life which had been wont, during the warm Tertiary time, preceding this cold period, to dwell far toward the north.
But when the epoch was on the wane, and the ice-front began to retreat, the relieved earth was again clothed with vegetation and re-ten anted by cold-loving animal life, which ad vanced northward closely in the rear of the re treating glacier. At a greater distance followed the more delicate animals and plants, gradually spreading northward as the moderating climate permitted, until they had established themselves m isothermal zones as we now find them. But as a subsiding flood will leave, stranded upon the top of the first points to appear above the surface, the driftwood and wreck of the del uge, so, as the warmth of southerly regions at a low elevation has increased, certain colonies of the advance guard of the army of animals and plants have found themselves stranded upon frigid mountain tops — islands of arctic climate —isolated from their fellows by warm valleys and plains in which their kindred speedily dis appeared, overcome in the battle of life by the greater increase of lowland species to whom the circumstances were more favorable; but here on the high cold peaks they have been able to keep a stronghold, each in its own in a limited area, though surrounded by utterly fatal condi tions.
Consult authorities cited under ZOOGEOGRA PHY, especially the writings of C. Hart Merriam.