Fig. 35.—A Wild Duck defending her Brood from a Brown Rat.
Till the end of the first half of last century our forefathers shared their dwellings with the black rat, and knew the brown rat only by hearsay, if at all. The first was a rat with many, but not all the vices of its race. It lived in our houses, ate grain, fat, and all kinds of provisions, gnawed doors, boards, and furniture, racketed at night like a noisy ghost through old castles and other spook-favouring buildings, caused much annoyance, many a fright, strengthened superstition and the fear of ghosts in many a mind; but it was possible to live with it, one could manage to get along. A capable cat held it in check; a skilful rat-catcher was more than a match for it. Then its most terrible enemy appeared, and its star began to wane. In 1727, swarms of brown rats, which seem to have come from India, either directly or by way of Persia, were seen to swim the Volga, and we soon learned what awaited Europe. Following canals and rivers, the brown rats reached villages and towns, entered, in spite of men and cats, the lower stories of our dwellings, filled vaults and cellars, ascended gradually to the garrets, ousted its relative after long and inexorable warfare, made itself master in our own houses, and showed us in a thousand ways what a rat could do. It possessed and exercised all the vices of its family, mocked at all our attempts to drive it away, and remained in possession of the field, which, up till now, we have tried to wrest from it with dogs and cats, by traps and snares, poison and shooting. Almost at the same time as it swam over the Volga, it reached Europe by another route, coming from the East Indies to England on board ship. Then began its world-wanderings. In East Prussia it appeared as early as 1750, in Paris three years later, Central Germany was conquered in 1780, and here, as everywhere else, the towns were first colonized, and the flat country round taken in by degrees. Villages not easily reached, that is to say, not lying on river-banks, were only invaded in the last decade of this century: in my boyhood it was still unknown in my native village, and the black rat, now being crowded out even there, held undisputed possession of many places where its rival now reigns supreme. Many isolated farms were only reached later, about the middle of the present century, but the victorious march still goes on. Not content with having discovered and conquered Europe, towards the end of last century the brown rat set out on new journeys. In the sea-ports already colonized, the rats swam out to the ships, climbed on board by the anchor chains, cables, or any other available ladders, took possession of the dark, protecting hold, crossed all seas, landed on all coasts, and peopled every country and island, where its chosen protector and compulsory host—civilized man—has founded homesteads. Against our will we have helped it, or at any rate made it possible for it, to carry out a greater extension of range than has been attained by any other mammal not in subjection to man.[56]
Another remarkable illustration of wandering is afforded by the souslik, a destructive rodent about the size of a hamster, belonging to the family of squirrels and sub-family of marmots. Eastern Europe and Western Siberia are its head-quarters. Albertus Magnus observed it in the neighbourhood of Ratisbon, where it is now no longer found, though it has recently appeared in Silesia. Forty or fifty years ago it was unknown here, but, at the end of the forties or beginning of the fifties, it appeared no one could tell whence, and from that time it has pressed slowly westward. Its migrations, too, have been helped by man, for, though it is not confined to cultivated fields, these afford the habitat most suited to its taste.
The same holds true of many species of mice, which extend their territories as the soil is cultivated. On the other hand, man narrows the possible range of many mammals by deforesting, by draining marshes, and by otherwise changing the character of whole tracts of country. In this way, far more than by direct persecution, he does much to influence the migration of the mammals which have established themselves in these areas. For the fundamental law holds good for mammals as for other creatures, that suitable districts, and these only, will be colonized sooner or later, notwithstanding the arbitrary and usually rough and cruel interference of man.
Quite different from such wanderings are the expeditions made by mammals to secure a temporary betterment. These are probably undertaken, if not by all species, at least by representatives of every family in the class; they vary in duration and distance, and may even have the character of true migrations, but they always come to an end after a certain time, and the wanderer ultimately returns to his original place of abode. The intention or hope of reaching better grazing or hunting grounds, the desire to profit by some casual opportunity for making life more comfortable, may be said to be the chief motive of such expeditions. They take place all the year round, in every latitude and longitude, even in districts where the conditions of life do not vary materially at different times. The mammal begins and ends them either alone or in bands, companies or herds, according as it is wont to live with its fellows; it follows the same routes with more or less regularity, and appears at certain places at approximately the same time, yet it is always guided by chance circumstances.
When the fruits of the sacred fig and other trees surrounding the temples of the Hindoos are beginning to ripen, the Brahmins who tend temple and trees await with unctuous devotion the arrival of their four-footed gods. And not in vain, for the two divinities, Hulman and Bunder, two species of monkey, unfailingly appear to strip the luscious fruits from the trees piously planted and tended for their benefit, and also to rob and plunder in the neighbouring fields and gardens as long as it is worth while. Then they disappear again, to the sorrow of their worshippers and the joy of the other inhabitants of India, whose possessions they have ravaged, as they gathered in their spoils in their usual ruthless fashion. In Central Africa, when the chief cereal of that country, the dhurra or Kaffir-millet, comes to maturity, a dignified and inventive baboon, tried and experienced in all the critical situations of life, leads down the flock of which, as leader, he is justifiably proud, to see whether Cousin Man has been good enough to sow the nutritive grain for him this year also. Or, about the same time, a band of long-tailed monkeys, under not less excellent leadership, approaches the edge of the forest in order not to miss the right moment for a profitable, and, as far as possible, undisturbed ravaging of the fields. When the golden orange glows among the dark foliage in South American plantations, the capuchin monkeys make their appearance, often from a great distance, to share the fruit with the owner. Other plant-eaters too are led by the hope of gaining an easier livelihood into regions and districts which they usually avoid; insectivores periodically follow the insects when they are for the time abundant at this place or that, and large beasts of prey keep in the wake of herbivorous mammals, especially of the herds belonging to man. The lion journeys from place to place, following the wandering herdsmen on the steppes of Africa; Russian wolves followed close on the retreat of Napoleon’s defeated army, pursuing the unfortunate fugitives as far as the middle of Germany. Otters undertake land journeys to get from one river-basin to another; lynxes and wolves in winter often traverse very wide stretches of country. Such journeys bring about a change of residence, but they do not constitute a migration in the true sense of the word. It is only exceptionally, too, that they are undertaken from real necessity, which we must look upon as the cause of all true migrations; in most cases they are undertaken simply to gratify a passing desire.
Quite otherwise is it with those mammals which, every year about the same time, leave their habitat for some other region often far distant, from which at a definite time they will return to their former abode. These migrate; for they do not seize a chance opportunity, but obey, consciously or unconsciously, a compelling necessity.
The fundamental cause of all true migration among mammals, is some very distinct and decided seasonal change. In countries of everlasting spring true migrations do not take place, for want is never imminent. Summer must contrast with winter, whether the latter bring frost and snow, or heat and drought; scarcity must alternate with superfluity before the sluggish mammal makes up its mind to migrate.
To a slight extent migrations take place among all mountain animals. The chamois, the steinbock, the Alpine hare, the marmot all migrate when the snow begins to melt, or a little later; they clamber over hillsides and glaciers to the heights above, where the pasturage, now laid bare, promises rich and abundant nourishment, and they return to the lower slopes of the mountain before winter sets in. The bear, by nature omnivorous, by habit a thief, undertakes a similar migration at the same season, and completes it before winter sets in,—at least so it is in the mountains of Siberia; the various wild cats and dogs which live among the mountains do the same. Such changes of residence occur also on the mountains of southern countries, even of those lying within the tropics. In India and Africa certain species of monkey ascend and descend the mountains at regular intervals; elephants seek the high grounds on the approach of summer, the low grounds in winter; on the Andes in South America the guanacos flee before the snow into the valleys, and before the summer-heat to the shoulders of the mountains. All these migrations are confined by the mountains within comparatively narrow limits. They only involve a change of altitude of from three to nine thousand feet, or a journey which may be accomplished in a few hours, or, at most, in a few days. They have, however, the regularity characteristic of true migrations, especially in the precise periodicity of their occurrence, and not less in the constant choice of the same routes.