PART I. 

 GENERAL PRINCIPLES OF DISTRIBUTION. 



GENERAL PRINCIPLES OF 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 ex- 

 ception 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 2, Map iv), 

 Swallows (Plate 12, Map ii), and Plovers, (Plate 17, Map i) among Birds, 

 and many families of Insects, are examples of almost universal dis- 

 tribution. 



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 IV. (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. 



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 V espevtilionidee, (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 per- 

 form marvellous journeys in proceeding to and from their seasonal haunts — 

 sometimes several thousands of miles. Insects, too, have a world-wide dis- 

 tribution, and some particular species, such as the Painted Lady Butterfly 

 {Pyrantels 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 drift- 

 wood 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 Tringinse (to which the Curlew Sand- 

 piper 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 larvae, 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 wdiich 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 



