PART I.
GENERAL PRINCIPLES OF DISTRIBUTION.
CONDITIONS FAVOURING DISPERSAL.
Many factors are always at work either aiding or hindering dispersal, and it is necessary at the outset to enumerate the more important. Obviously those animals which possess a more rapid or perfect means of locomotion succeed best in establishing themselves over a ־wide area. The typical Bats of the family Vespertilionidse (Plate 3, Map vi) have great powers of flight, and hence are well-nigh cosmopolitan. Two species at least which are natives of North America regularly visit the Bermudas, a distance of 600 miles from the mainland. Many birds again annually perform marvellous journeys in proceeding to and from their seasonal haunts— sometimes several thousands of miles. Insects, too, have a world-wide distribution, and some particular species, such as the Painted Lady Butterfly (Pyrameis cardui), have an enormous geographical range.
Many Mammals are accustomed to roam over wide areas, and some are even known to climb mountains, cross rivers, or swim over considerable tracts of sea. In the Himalayas, for instance, certain Monkeys ascend to a great height in the hot season, returning to lower levels in the winter, while Wolves in other regions and Lemmings in Scandinavia indulge in similar habits. Tigers and Pigs are examples of Mammals which can cross broad rivers and even narrow arms of the sea, the limit to such powers being apparently a width of about 20 miles. But the dispersal of Mammals, Reptiles, and many other purely terrestrial forms of life is materially aided in other ways. The masses of tangled vegetation, uprooted trees and driftwood which are often washed down the larger rivers are frequently tenanted by quite a miscellaneous assortment of living creatures such as Monkeys, Cats, Crocodiles, Snakes, and Molluscs. By means of strong winds and tidal currents, such animals may be transported to regions some hundreds of miles from their old home, and in this manner the range of many species has without doubt been considerably extended in the past. In Arctic Regions masses of floating ice may act in the same way, and indeed it is recorded that during a single winter no fewer than twelve Polar Bears were thus stranded upon the coast of Iceland.
Although, as we have seen, the wide distribution of Birds is chiefly due to their great powers of flight, yet to this factor we must add also that of the migratory instinct. But “ for the purposes of the study of geographical distribution,” as Wallace says, “ we must, except in special cases, consider the true range of a species to comprise all the area which it occupies regularly for any part of the year, while all those districts which it only visits at more or less distant intervals, apparently driven by storms or by hunger, and where it never regularly or permanently settles, should not be included as forming part of its area of distribution.” As an example of the tremendous area included within the migratory flights of a species, we may take the Curlew Sandpiper. This species breeds on the tundras of West Siberia, bordering the Arctic Ocean, and yet in winter travels to the Cape of Good Hope, Tasmania, and Patagonia. Such cosmopolitan distribution as is shown on Plate 17, Map i, where the Tringinee (to which the Curlew Sandpiper belongs) are indicated by a blue line, may thus be accounted for. One of the main causes for the development of the migratory habit is the question of food-supply. Since many birds live on insects and their larva;, it is absolutely necessary for them to leave their temperate haunts as winter approaches, and seek warmer climes. The intense love implanted in birds for their native land, and the fact that the tropical and sub-tropical regions do not afford a suitable nursery for the young of the hardy northern races, sufficiently account for the return of the migrants to summer haunts in the spring.
Considering now the lower Vertebrates, we find that they undoubtedly possess some means of crossing the sea. Whether they actually swim across, or are only transported in the egg-state or involuntarily on driftwood, as in the case of Mammals, is a disputed point. But that Reptiles do become accidentally transported to new homes is proved by the case, often quoted, of a Boa-constrictor which was found to have reached the island of St Vincent, fully a couple of hundred miles from its native country, by means of a floating cedar-tree. Amphibians may owe their dispersal, not only to this means, but also to the agency of birds, chiefly Waders and aquatic species, which carry the eggs attached to their feet from one pond or river to another.
Birds act in the same way as carriers of certain molluscs. A Mallard was once shot in the Sahara to whose feet adhered the eggs of some species of Snail, which, falling to the ground perhaps miles away, might hatch and thus extend its range. Rivers and torrents may carry Molluscan shells down to the sea, and since some forms are able to secrete a diaphragm which closes up the shell, these may float away some hundreds of miles and yet survive.
Insects owe their wide distribution, like birds, to their great powers of flight, but high winds also contribute not a little in this respect. The examples of insects having been met. with far out of sight of land are very numerous, and one only must suffice to illustrate this method of dispersal. The ship Pleione, returning home some years ago from New Zealand, upon reaching a point some 960 miles south-west of the Cape Verde Islands, encountered some hundreds of Moths belonging to a species which is common in the Eastern Tropics, but not found in South America, which was the
GENERAL PRINCIPLES OP DISTRIBUTION.
Every portion of our globe is the home of numerous forms of animal life. Each particular tract of land or water possesses a fauna of its own, and even the most arid desert, the densest forest, the highest and bleakest mountain summit, the shallowest lake, the swiftest stream or the deepest ocean abyss, has its own share of struggling inhabitants, all living in greater or less harmony with their surroundings. In past ages the fauna was almost equally prolific and diverse, as is shown by the innumerable remains which crowd the sedimentary rocks of all the great geological epochs.
But the distribution of animal forms upon the earth is by no means uniform or regular, and a casual reference to our Plates will at once reveal the fact. The Anthropoid Apes (Plate 1, Map i), for example, occur in isolated areas in West Africa and South-eastern Asia ; the Cavies (Plate 6, Map iii) are exclusively South American; the Tenrecs (Plate 4, Map ii) are confined to Madagascar; the whole Order of Marsupials (with the exception of the Opossums) is practically restricted to Australia and New Guinea; while the Plantain-Eaters (Plate 13, Map iii) are exclusively African. On the other hand, the Dogs (Plate 5, Map v) and Mice (Plate 6, Map vi) among Mammals, the Thrushes (Plate// S, Map Swallows (Plate 12, Map ii), and Plovers, (Plate 17, Map i) among Birds, and many families of Insects, are examples of almost universal distribution.
Again, when we study the subject from a geographical point of view, we find a corresponding lack of uniformity. All countries have a certain proportion of species peculiar to themselves, in addition to those which they harbour in common with other regions. In the early days of zoological science, the character of the fauna of any particular area was supposed to be entirely due to its climatic and physical peculiarities. This explanation, however, does not account for the numerous and interesting differences which present themselves when two countries of similar climate and physical conditions are compared. Brazil and West Africa, for instance, are both largely covered with dense tropical forest, while their climatic conditions are very similar. Yet the animal life of one is of a totally different character from that of the other. In West Africa we find Elephants, Antelopes, and Gorillas, while in Brazil these are all absent and replaced by Tapirs, Sloths, and Monkeys with long, prehensile tails. On the other hand, countries with a vastly different climate may possess the same actual species of animal. The Tiger, for example, ranges from the tropical jungles of India to the Caucasus, Altai Mountains and the frigid plains of Manchuria. Lastly, the features of distribution are not necessarily dependent upon a matter of distance. There is a much greater similarity between the faunas of Great Britain and Japan, countries situated at the extremes of the great Eurasian continent and many thousand of miles apart, than there is between those of the small islands of Bali and Lombock in the Malay Archipelago, which are separated by a strait only about 15 miles wide !
The study of the distribution of animals over the earth’s surface is, therefore, not so simple as it may seem. For the full comprehension of the subject the accumulation of a large mass of facts and the formation of theories to account for the various phenomena which call for explanation become necessary. The zoologist must trace out in detail the exact area or areas inhabited by the several species, genera, and larger groups of animals, and this process to be reliable must be based upon a true and natural classification of the animals themselves. The latter can only be attained by a due consideration of the theory of evolution (or descent with modification) as generally understood at the present day. With this must be intimately associated a knowledge of extinct forms and their distribution in time and space, and this again depends upon an acquaintance with the extent and relative position of the various fossil-bearing strata which build up the huge series of sedimentary rocks.
In a work like the present it is obviously impossible to place before the reader anything like a complete account of this important subject. But an attempt may at least be made to indicate briefly the main features in the problem, such as, for example, the factors which make for or against the dispersal of animals, the actual methods of dispersal, the influence of temperature, vegetation, and so on. For a complete account of these and other phenomena, and for all subjects connected with the great branch of science known to the modern naturalist as “ Zoogeography,” reference must be made to one or more of the general works whose titles will be found in Section IY. (Bibliographical).
NECESSITY FOR DISPERSAL.
Owing to the rapid rate of multiplication of many animals, the struggle to procure sufficient food for all individuals in any particular area must always be a keen one. This competition affects not only individuals of the same species, but also allied species of the same genus, or even animals of different genera and families. Hence it may be assumed that an extension of geographical range is an advantage, if not an absolute necessity, in the case of the majority of animal forms.
A