In the struggle for existence in the vegetable world, the tree line pushes as far up the mountain as conditions of climate and soil will permit. Then comes a season of fiercer storms, intenser cold and invading ice upon the peaks. Havoc is wrought, and the forest drops back across a zone of border warfare—for war belongs to borders—leaving behind it here and there a dwarfed pine or gnarled and twisted juniper which has survived the onslaught of the enemy, Now these are stragglers in the retreat, but are destined later in milder years to serve as outposts in the advance of the forest to recover its lost ground. Here we have a border scene which is typical in nature—the belt of unbroken forest, growing thinner and more stunted toward its upper edge, succeeded by a zone of scattered trees, which may form a cluster perhaps in some sheltered gulch where soil has collected and north winds are excluded, and higher still the whitened skeleton of a tree to show how far the forest once invaded the domain of the waste.
Oscillating boundaries
The habitable area of the earth everywhere shows its boundaries to be peripheral zones of varying width, now occupied and now deserted, protruding or receding according to external conditions of climate and soil, and subject to seasonal change. The distribution of human life becomes sparser from the temperate regions toward the Arctic Circle, foreshadowing the unpeopled wastes of the ice-fields beyond. The outward movement from the Tropics poleward halts where life conditions disappear, and there finds its boundary; but as life conditions advance or retreat with the seasons, so does that boundary. On the west coast of Greenland the Eskimo village of Etah, at about the seventy-eighth parallel, marks the northern limit of permanent or winter settlement; but in summer the Eskimo, in his kayak, follows the musk-ox and seal much farther north and there leaves his igloo to testify to the wide range of his poleward migration. Numerous relics of the Eskimo and their summer encampments have been found along Lady Franklin Bay in northern Grinnell Land (81° 50' N. L.), but in the interior, on the outlet streams of Lake Hazen, explorers have discovered remains of habitations which had evidently, in previous ages, been permanently occupied.[326] The Murman Coast of the Kola Peninsula has in summer a large population of Russian fishermen and forty or more fishing stations; but when the catch is over at the end of August, and the Arctic winter approaches, the stations are closed, and the three thousand fishermen return to their permanent homes on the shores of the White Sea.[327] Farther east along this polar fringe of Russia, the little village of Charbarova, located on the Jugor Strait, is inhabited in summer by a number of Samoyedes, who pasture their reindeer over on Vaygats Island, and by some Russians and Finns, who come from the White Sea towns to trade with the Samoyedes and incidentally to hunt and fish. But in the fall, when a new ice bridge across the Strait releases the reindeer from their enclosed pasture on the island, the Samoyedes withdraw southward, and the merchants with their wares to Archangel and other points. This has gone on for centuries.[328] On the Briochov Islands at the head of the Yenisei estuary Nordenskiold found a small group of houses which formed a summer fishing post in 1875, but which was deserted by the end of August.[329]
Altitude boundary zones.
An altitude of about five thousand feet marks the limit of village life in the Alps; but during the three warm months of the year, the summer pastures at eight thousand feet or more are alive with herds and their keepers. The boundary line of human life moves up the mountains in the wake of spring and later hurries down again before the advance of winter. The Himalayan and Karakorum ranges show whole villages of temporary occupation, like the summer trading town of Gartok at 15,000 feet on the caravan route from Leh to Lhassa, or Shahidula (3,285 meters or 10,925 feet) on the road between Leh and Yarkand;[330] but the boundary of permanent habitation lies several thousand feet below. Comparable to these are the big hotels that serve summer stage-coach travel over the Alps and Rockies, but which are deserted when the first snow closes the passes. Here a zone of altitude, as in the polar regions a zone of latitude, marks the limits of the habitable area.
"Wallace's Line" a typical boundary zone.
The distribution of animals and races shows the limit of their movements or expansion. Any boundary defining the limits of such movements can not from its nature be fixed, and hence can not be a line. It is always a zone. Yet "Wallace's Line," dividing the Oriental from the Australian zoological realm, and running through Macassar Strait southward between Bali and Lombok, is a generally accepted dictum. The details of Wallace's investigation, however, reveal the fact that this boundary is not a line, but a zone of considerable and variable width, enclosing the line on either side with a marginal belt of mixed character. Though Celebes, lying to the east of Macassar Strait, is included in the Australian realm, it has lost so large a proportion of Australian types of animals, and contains so many Oriental types from the west, that Wallace finds it almost impossible to decide on which side of the line it belongs.[331] The Oriental admixture extends yet farther east over the Moluccas and Timor. Birds of Javan or Oriental origin, to the extent of thirty genera, have spread eastward well across Wallace's Line; some of these stop short at Flores, and some reach even to Timor,[332] while Australian cockatoos, in turn, have been seen on the west coast of Bali but not in Java, Heilprin avoids the unscientific term line, because he finds his zoological realms divided by "transition regions," which are intermediate in animal types as they are in geographical location.[333] Wallace notes a similar "debatable land" in the Rajputana Desert east of the Indus, which is the border district between the Oriental and Ethiopian realms.[334]