Popular Science Monthly/Volume 10/March 1877/The Journeyings and Dispersal of Animals
|THE JOURNEYINGS AND DISPERSAL OF ANIMALS.|
IN his recent elaborate work on the "Geographical Distribution of Animals," Mr. Alfred Russel Wallace gives much attention to the subject of migrations as among the means of dispersion, and from his copious treatment of the subject, and various other sources, we glean the following brief particulars upon this subject:
Winged insects are perhaps, of all, most admirably adapted for the special conditions found in one locality, and the barriers against their permanent displacement are numerous. Thus many insects require for their subsistence succulent vegetable food during the entire year, which, of course, confines them to tropical regions; some are dependent on mountain-vegetation; some subsist on water-plants; and yet others, as the Lepidoptera, in the larva state, are limited to a single species of plant. Insects have enemies in every stage of their existence; foes are at hand ready to destroy not only the perfect form, but the pupa, the larva, and the egg; and any one of these enemies may prove so formidable, in a country otherwise well adapted to them, as to render their survival impossible. But, on the other hand, most varied means of dispersal carry insects from their natural habitats to distant regions. They are often met far from land, carried thence by storm or hurricane. Hawk-moths are sometimes captured hundreds of miles from shore, having taken passage on ships which neared tropical countries, and Mr. Darwin narrates that he caught in the open sea, seventeen miles from the coast of South America, beetles, some aquatic and some terrestrial, belonging to seven genera, and they seemed uninjured by the salt-water. Insects, in their undeveloped states, make their abodes in solid timber, which, transported by winds and waves, may carry its undeveloped, winged freight great distances. Tropical insects are not unfrequently captured in the London docks, where they have been carried in furniture or foreign timber. Insects are very tenacious of life, and nearly all can exist for a long time without food. Some beetles bear immersion in strong spirit for hours, and are not destroyed by water almost at the boiling-point. These facts enable us to understand how not only by means of its delicate wings, but by winds, waves, volcanic dust, and a thousand other agencies, insects may be carried to remote regions.
Mollusca, which are less highly organized than insects, have, of course, limited appliances for journeying, and their dispersal and distribution may involve long periods of time. Fresh-water mollusks are very widely distributed, and this is accounted for by Mr. Darwin by the fact that ponds and marshes are frequented by wading and swimming birds. These carry away with them the seeds of plants and the eggs of mollusks. True land-shells are exceedingly sensitive to saltwater, and yet they are found all over the globe. Experiments on their power to resist sea-water show that a membranous diaphragm, which they sometimes form over the mouth of the shell, enables them to survive many days' immersion in it. They may lie dormant for a long time, some having lived between two and three years shut up in boxes; and one snail, from the Egyptian Desert, was found to be alive after having been glued for four years to a tablet in the British Museum. These facts render it quite possible that they may cross the sea in the chinks of drift-wood, and this is probably the means of their dispersal.
The inhabitants of the sea seem to have unlimited facilities for journeying, but when we remember that cold water is essential to many fishes, tropical warmth to others, and the deep sea an effectual barrier to a large number of species, it is apparent that the Atlantic may be as impassable a gulf to fishes as to land-animals. Distinct river systems are sometimes inhabited by the same species of freshwater fish, which indicates that they have some means of dispersal over land. This may be accomplished by changes of level giving rise to altered river-courses and new water-basins, to transportation of the eggs by ducks, geese, aquatic birds, and even water-beetles, and to the agency of whirlwinds and hurricanes, which carry up considerable quantities of water, and with it small fishes.
Reptiles have very limited means of dispersal. Snakes are dependent on climate, being comparatively scarce in temperate and cold regions. They entirely cease in 62° north latitude, and are not found above 6,000 feet on the Alps. They swim rivers easily, but, since they are rarely met on oceanic islands, it is inferred that they have no means of crossing the sea. Lizards are also tropical animals, though they are found higher on the mountains and farther north than are snakes. They possess some means of crossing oceans, and frequently inhabit islands where there are neither snakes nor mammalia. The amphibia extend farther north than true reptiles, frogs being found, sometimes, beyond the arctic circle. Salt-water is fatal to them, and they are probably effectually limited by deserts and oceans.
It would seem, at first, that birds are limited by no barriers, and that a study of their habits could scarcely throw any light upon the causes of animal distribution; but remarkable contrasts in the extent of their range are presented by different groups of birds. Thus, the gulls (Laridæ) and petrels (Procellaridæ) are great wanderers, a few being found, with scarcely any variation, over almost the entire globe; other species being restricted to one of the great oceans; while parrots, pigeons, and many small perching birds, are confined to islands of limited extent, or to single valleys or mountains. Some birds, such as the apteryx, ostrich, and cassowary, have no power of flight, and, of course, limited means of dispersal. The short-winged birds, such as wrens and toucans, are able to fly but a short distance, and only species endowed with great powers of flight can cross extended widths of sea. Violent gales sometimes carry small birds accidentally to foreign countries, as is shown by the large numbers of North American stragglers which reach the Bermudas. Inadequate supply of food, afforded by the vegetation of a country, oceans, and even large rivers, may serve as effectual barriers to the dispersal of birds. The presence of enemies, of either the young, the eggs, or the parent-birds, may limit the range of a species. In the Malay Archipelago pigeons are said to "abound most where monkeys do not occur, and in South America the same birds are comparatively scarce, in the forest-plains, where monkeys are very abundant, while they are plentiful on the open plains and campos, and on the mountain plateaux, where these nest-hunting quadrupeds are rarely found." When we consider that the pigeon is the most careless and awkward of birds in the construction of its nest, it is not difficult to understand how formidable an enemy it must have in the artful and wary monkey.
The term migration is strictly applied to the annual movements of birds and fishes, which take place in large bodies, and are immediately connected with the process of reproduction. In all temperate regions a large number of birds reside temporarily. Some arrive in spring, and leave in autumn; others arrive in autumn, remain during the winter, but depart in spring; and yet others, birds of passage, pass through the country twice a year, without long delay. The species which winter here are those which build their nests and rear their young farther north, and in returning, on the appearance of spring, they simply act as do those whose homes are nearer the equator. The birds of passage, like our winter-birds, have their breeding-quarters nearer the poles, but, like our summer-birds, seek a warmer climate for the winter. The arrival of migratory birds from warmer regions seldom varies more than a week or two, though their departure is more dependent on the weather, and consequently less constant.
The migratory birds of Europe seem to have a definite route: they "go southward to the Mediterranean, move along its coasts east or west, and cross over in three places only—either from the south of Spain, in the neighborhood of Gibraltar, from Sicily, over Malta, or to the east by Greece and Cyprus." The passage is mostly accomplished by night, and is undertaken only when there is a steady wind from the east or west, and when there is moonlight. It is an interesting fact that the males often leave before the females, and both parents before their offspring; the latter, however, rarely go so far as do the old ones. On returning they vary their course, some following the old, others adopting a new line of travel. In connection with the routes taken by European birds, it is suggestive and interesting to note that fossil remains of huge animals, and the shallow waters in this part of the Mediterranean, indicate that Gibraltar, Sicily, and Malta, must have been at no very distant date united with Africa; the submersion of this land involved considerable time, and the change could hardly have been perceptible from one year to another. It is natural to conclude that the migration, which was at first a land passage, would be kept up over marshes, then over a channel, and finally over the sea. The sea-passage is a dangerous one for birds, and, from the immense flocks of quails which annually undertake it, large numbers are drowned when the weather proves unfavorable for the passage.
The migratory movement of the North American birds is almost wholly limited to the Atlantic coast, a smaller number being permanent residents than on the Pacific coast, or in corresponding European localities. In Massachusetts the regular number of summer visitors is 106, while there are only 30 species which remain all the year. The number of permanent residents increases as we go southward, but during the breeding-season in any single locality it increases as we go northward until we reach Canada, where more species rear their young than in the Southern States. The extent of the migration of certain birds has greatly altered within a limited period of observation. A Mexican swallow (Hirundo lunifrons) first appeared in Ohio in 1815; its yearly range increased, until in 1845 it had reached Maine and Canada, and now its annual migrations extend to Hudson's Bay. The rice-bird, or "bobolink," enters the Southern States in April, passes northward until in June it reaches Canada, and stops in its westerly course at the Saskatchewan River, in 54° north latitude, having widened its range continually as wheat and rice were cultivated over more extensive areas.
A nocturnal concourse of birds sometimes occurs in the neighborhood of large towns near the end of summer, in still, cloudy weather. The notes of well-known birds may be recognized by the skillful ornithologist, at one time faint in the distance, at another near by, while occasionally the stroke of a wing gives a sense of nearness to these remarkable visitors. It is supposed that these noises proceed from migratory birds which, having lost their way, are attracted by the light from street-lamps.
It is thus obvious that the migration of birds is no mere arbitrary matter, but is governed by laws susceptible of intelligent interpretation. Want of food is the most evident cause of their journeyings. As it becomes scarce near the end of summer in the extreme northern limits, those individuals which feel the pressure of want seek it elsewhere, and, in doing so, they press upon the haunts of other birds, until the movement which began in the north has extended to the southern limit. The power of flight in birds makes it possible for them to cross a moderate breadth of sea and unlimited extent of country, and, traveling as they do, mostly at night and high in the air, their movements seem mysterious, simply because they are difficult to observe. But, let us map their comings and goings faithfully as we may, there yet remains the unanswered question, How do these little visitants find their way so unerringly from one place to another, over great distances and apparently unexplored routes?
Some of the largest Mammalia are not stopped by any physical obstacle in their journeys over whole continents. The rhinoceros, the lion, and the tiger, have great powers of dispersal, and their possible range is unlimited wherever there are land and sufficient food. The elephant climbs to mountain-tops, difficult of ascent for man, crosses rivers, and finds its way through the densest jungles. Other groups are much more limited in their wanderings, as witness the monkeys, lemurs, and apes, animals so strictly adapted to an arboreal life that they cannot roam far beyond forest limits. Equally essential to the existence of others is the desert or open country. The range of many mammals appears to be limited by climate, or by its resulting vegetation. Thus the Quadrumana are chiefly found within an equatorial belt of 30° wide, but these animals live almost exclusively on fruit, which is the abundant product of the tropics. The polar bear and walrus, which, in a natural state, are limited by the frozen ocean, in confinement may live in temperate regions; the tiger, once regarded as a purely tropical animal, now has his permanent home in Mantchooria, a country of almost arctic climate; and, in post-Tertiary times, the elephant and rhinoceros roamed over the northern continents, even to regions beyond the arctic circle. Hence it does not follow that animals, which we now see inhabiting extremely warm or extremely cold climates, may not, under changed conditions, thrive equally well elsewhere.
Valleys and rivers often prove effectual barriers to mammals. Thus, in the plains along the Amazon, many species of insects, birds, and monkeys, are found extending to the river-bank on one side, which do not cross to the other. And on the northern bank of the Rio Negro there are found two monkeys, the Brachiurus couxion and the Jacchus bicolor, which are never seen on its southern bank. Many mammals can swim well for short distances, but none over any great extent of sea. It is not unusual for the bear and bison to swim across the Mississippi, and from Lyell's "Principles of Geology" we learn that in 1829, during the floods in Scotland, pigs six months old, which were carried to sea, swam five miles back to shore; and it seems entirely probable that wild-pigs, from their greater activity and power of endurance, might cross arms of the sea twenty or thirty miles wide, and facts in the distribution of these animals lead us to infer that they have sometimes done so. Lemmings, rats, and squirrels, often migrate in enormous bands, but they generally perish in the sea-water. And, admitting that many mammals have power to swim considerable distances, it remains true that a channel ten or twenty miles wide would, in most cases, prove an effectual barrier to them. The bats, provided as they are with wings, and the Cetacea, which swim, have exceptional powers of dispersal. In the arctic regions glaciers give rise to icebergs; these descend to the sea, often carrying with them masses of earth and some vegetation. Such arctic quadrupeds as frequent the ice, as well as occasionally true land-animals, might often be carried from place to place in this way. But the uprooted trees and rafts of drift-wood which float down large rivers and out to sea, are more effectual agents in the dispersal of animals. Such islands or rafts are sometimes seen drifting hundreds of miles from the mouth of the Ganges, bearing upon their surfaces erect, living-trees. And the Amazon, Orinoco, and in fact most large rivers, present at times similar spectacles. Here, then, is most ample opportunity for carrying all small arboreal animals out to sea, and, although they are liable to perish, unusual tidal currents may bear them great distances safely from their native country.