Fig. 38.—Flying Foxes.
As is well known, there are many mammals which have the habit of hibernating, which pass the severe part of the year well protected in deep and carefully closed burrows, and are thus spared the necessity of leaving their haunts. Even among these, however, at least among those living in the temperate zones, there are some which migrate during their waking time, namely, the bats. Defective as the wing of a bat must appear when compared with that of a bird, it is nevertheless of such assistance in flight, that it makes journeys possible which seem out of all proportion to the size of the animal. Another fact makes travelling easier to the restless bat; it is not tied down by its offspring to any particular spot, for the young one attaches itself directly after birth to the breast of the mother, and is borne by her through the air till it is capable of independent life. The bat is thus one of the best-adapted of migratory mammals, and, under some circumstances, it makes full use of its advantages. As a general rule, the wanderings of the different species of bat are to be regarded simply as excursions made with a view to taking advantage of any district which is, for the time being, particularly rich in food; but they do sometimes become really long journeys, which lead some species to far distant lands, and they are then not without the regularity characteristic of all true migrations. The largest bats, the flying foxes, fly long distances every evening in search of the fruits on which they chiefly subsist; they do not hesitate to cross an arm of the sea fifty or sixty miles in breadth, and they must even have traversed the distance between Southern Asia and the East of Africa, as certain species occur in both these regions. The bats proper accomplish at least as much. Following the reappearance of the insects, which occurs at different times in regions of different altitude, they ascend from the plains to the mountain heights, and descend in autumn to the low grounds again; they pursue the numerous flies which congregate about the wandering cattle-herds of Central Africa, and they migrate also from the south towards the north and return southwards again, or in reverse order. The boreal bat appears at the beginning of the bright nights in the north of Scandinavia and Russia, and leaves these districts, which may be considered its head-quarters, towards the end of summer, to spend the winter among the mountains of Central Germany and the Alps. The pond-bat is regularly seen on the plains of North Germany during summer, but only exceptionally at that period among the mountains of Central Germany, in whose caverns it spends the winter. That other species of bat occurring in Germany change their place of abode in a similar manner can scarcely be doubted.
In the cases cited, which have been selected from a mass of available material, I have given examples of those migrations of mammals which we may call voluntary, because of their regularity; but in so doing I have by no means completed my task. Hunger and thirst, the poverty and temporary inhospitableness of a particular region, sometimes press so severely on certain mammals that they endeavour, as if despairing, to save themselves by flight. Abundant nourishment and good weather favour the increase of all animals, and affect that of a few plant-eating mammals to such an extraordinary degree, that, even under propitious conditions, their habitat must be extended. But if one or more rich years—in some cases a few favourable months—be followed by a sudden reverse, the famine soon passes all bounds, and robs the creatures not only of the possibility of subsistence, but also of all hope, or at least of all presence of mind.
It is under circumstances such as these that the field-voles of our own country, and the Siberian voles, assemble in enormous multitudes, leave their native haunts and migrate to other districts, turning back for no obstacles, avoiding the water as little as the forbidding mountains or the gloomy forest, fighting to the last against hunger and misery, but perishing hopelessly from diseases and epidemics which rage among them like plagues, reducing armies of millions to a few hundreds. Thus, too, the squirrels of Siberia, which, in ordinary years, undertake, at the most, only short excursions, assemble in vast armies, hurry in troops or companies from tree to tree, in compact masses from forest to forest, swim across rivers and streams, throng into towns and villages, lose their lives by thousands; but suffer no obstacle or hindrance—not even the most obvious dangers—to delay them or divert them from their path. The soles of their feet become worn and cracked, their nails ground down, the hairs of their usually smooth fur rough and matted. Through the forest lynxes and sables, in the open fields gluttons, foxes and wolves, eagles, falcons, owls and ravens follow them closely; pestilence claims more victims from their ranks than the teeth and claws of beast of prey or the guns and cudgels of men, yet they press on and on, apparently without hope of return. A Siberian sportsman of my acquaintance gave me a verbal account of the appearance of such an army of squirrels, in August 1869, in the town of Tapilsk, among the Ural Mountains. It was only one wing of a migrating army, of which the main body travelled through the forest about five miles farther north. Sometimes in single file, sometimes in companies of varying strength, but in unbroken succession, the animals pressed on, crowding as densely through the town as through the neighbouring forest; used the streets, as well as the hedges, and the roofs of buildings as paths; filled every court-yard, thronged through windows and doors into the houses, and created quite an uproar among the inhabitants—much more among the dogs, which killed thousands of them, evincing an unbridled bloodthirstiness till then unsuspected. The squirrels, however, did not seem to concern themselves in the least about the innumerable victims falling in their midst; in fact, they took no notice of anything, and allowed nothing to divert them from their route. The procession lasted for three whole days, from early morning till late in the evening, and only after nightfall each day was there a break in the continuity of the stream. All travelled in exactly the same direction, from south to north, and those that came last took the same paths as their predecessors. The rushing Tchussoveia proved no obstacle, for all that reached the bank of that rapid mountain-river plunged without hesitation into its whirling and seething waters, and swam, deeply sunk and with their tails laid across their backs, to the opposite bank. My informant, who had been watching the procession with growing attention and sympathy, rowed out into the midst of the throng. The tired swimmers, to whom he stretched out an oar, climbed up by it into the boat, where, apparently exhausted, they sat quietly and confidingly, until it came alongside a larger vessel, when they climbed into that, and remained on it for some time as indifferent as before. As soon as the boat touched the bank they sprang ashore, and proceeded on their journey as unconcernedly as if it had suffered no interruption.
It must be similar circumstances which compel the lemmings to the migrations which have been known for centuries. For many successive years the heights in the tundras of Scandinavia, North Russia, and the North of Siberia afford them comfortable quarters and abundant nourishment; for the broad ridges of the fjelds and the extensive plains between them, the highlands and the low grounds, offer room and maintenance for millions of them. But not every year do they enjoy the accustomed abundance for the whole summer. If a winter in which much snow falls, and which is therefore favourable to them, as they live safely below the snow, be followed by an early, warm, and agreeable spring, their extraordinary fertility and power of increase seem to have almost no bounds, and the tundra literally teems with lemmings. A fine warm summer increases their numbers past computing, but it also accelerates the life-course of all the plants on which they feed, and before it is over these are partly withered, partly devoured by the greedy teeth of the insatiable rodents. Scarcity of food begins to be felt, and their comfortable life comes to an end in panic. Their fearless, bold demeanour gives place to a general uneasiness, and soon a mad anxiety for the future takes possession of them. Then they assemble together and begin to migrate. The same impulse animates many simultaneously, and from them it spreads to others; the swarms become armies; they arrange themselves in ranks, and a living stream flows like running water from the heights to the low grounds. All hurry onwards in a definite direction, but this often changes according to locality and circumstances. Gradually long trains are formed in which lemming follows lemming so closely that the head of one seems to rest on the back of the one in front of it; and the continuous tread of the light, little creatures hollows out paths deep enough to be visible from a long distance in the mossy carpet of the tundra. The longer the march lasts, the greater becomes the haste of the wandering lemmings. Eagerly they fall upon the plants on and about their path and devour whatever is edible; but their numbers impoverish even a fresh district within a few hours, and though a few in front may pick up a little food, nothing is left for those behind; the hunger increases every minute, and the speed of the march quickens in proportion; every obstacle seems surmountable, every danger trifling, and thousands rush on to death. If men come in their way they run between their legs; they face ravens and other powerful birds of prey defiantly; they gnaw through hay-stacks, climb over mountains and rocks, swim across rivers, and even across broad lakes, arms of the sea, and fjords. A hostile company, like that behind the migrating squirrels, follows in their wake: wolves and foxes, gluttons, martens and weasels, the ravenous dogs of the Lapps and Samoyedes, eagles, buzzards, and snowy owls, ravens and hooded crows fatten on the innumerable victims which they seize without trouble from the moving army; gulls and fishes feast on those which cross the water. Diseases and epidemics, too, are not awanting, and probably destroy more than all their enemies together. Thousands of carcasses lie rotting on the wayside, thousands are carried away by the waves; whether indeed any are left, and whether these return later to their native Alpine heights, or whether all, without exception, perish in the course of their journey, no one can say with certainty; but so much I know, that I have traversed great tracts of the tundra of Lapland where the paths and other traces of a great migrating army were to be seen almost everywhere, while not a single lemming could be discovered. Such tracts, I have been told, remain thus for several successive years, and only after long periods become gradually repeopled with the busy little rodents.[60]
What hunger causes in the North is brought about by the tortures of thirst in the richer South. As the brackish pools which have afforded water to the zebras, quaggas, antelopes, buffaloes, ostriches, and other animals of the steppes, dry up more and more under the burning heat of a South African winter, all the animals whose necessities have hitherto been supplied by the steppes assemble about the pools which still contain a little water, and these become scenes of stirring, active life. But when these, too, evaporate, the animals which have congregated around them are compelled to migrate, and it may happen that despair takes possession of them, as it did of the little rodents already described, and that, collecting in herds like the wild horses and Chinese antelopes (dzieren) of the steppes of Central Asia, or the bisons of the North American prairies, they rush straight on for hundreds of miles, to escape the hardships of winter.
In the South, too, the wild horses are the first to turn their backs on the inhospitable country. Till the drought sets in, these beautifully-marked, strong, swift, self-confident children of the Karroo, the zebra, quagga, and dauw, wander careless and free through their vast domain, each herd going its own way under the guidance of an old, experienced, and battle-tried stallion. Then the cares of the winter season begin to make themselves felt. One water-pool after another disappears, and the herds which gather about those which remain become more and more numerous. The general distress makes even the combative stallions forget to quarrel and fight. Instead of small companies, herds of more than a hundred head are formed, and these move and act collectively, and finally forsake the wintry region altogether before want has enfeebled their powers or broken their stubborn wills. Travellers describe with enthusiasm the spectacle presented by such a herd of wild horses on the march. Far into the distance stretches the sandy plain, its shimmering red ground-colour interrupted here and there by patches of sunburnt grass, its scanty shade supplied by a few feathery-leaved mimosas, and, as far away as the eye can reach, the horizon is bounded by the sharp lines of mountains quivering in a bluish haze. In the midst of this landscape appears a cloud of dust which, disturbed by no breath of air, ascends to the blue heavens like a pillar of smoke. Nearer and nearer the cloud approaches, until at length the eye can distinguish living creatures moving within it. Soon the brightly-coloured and strangely-marked animals present themselves clearly to the spectator’s gaze; in densely thronged ranks, with heads and tails raised, neck and neck with the quaintly-shaped gnus and ostriches which have joined their company, they rush by on their way to a new, and possibly far-distant feeding-ground, and ere the onlooker has recovered himself, the wild army has passed by and is lost from view in the immeasurable steppe.
Fig. 39.—Springbok Antelopes.