File:Atlas of zoogeography; a series of maps illustrating the distribution of over seven hundred families, genera, and species of existing animals (1911) (20351201781).jpg

Title: Atlas of zoogeography; a series of maps illustrating the distribution of over seven hundred families, genera, and species of existing animals
Identifier: AtlaszoogeograpVBart (find matches)
Year: 1911 (1910s)
Authors: Bartholomew, J. G. (John George), 1860-1920; Clarke, William Eagle, 1853-1938; Grimshaw, Percy Hall
Subjects: Zoology; Zoogeography
Publisher: (Edinburgh) Pub. at the Edinburg Geographical Institute, by J. Bartholomew & Co.
Contributing Library: Smithsonian Libraries
Digitizing Sponsor: Smithsonian Libraries

View Book Page: Book Viewer
About This Book: Catalog Entry
View All Images: All Images From Book
Click here to view book online to see this illustration in context in a browseable online version of this book.

Text Appearing Before Image:
GENERAL PRINCIPLES OF DISTRIBUTION nearest land. From the direction of the wind during the four days previous to the occurrence, it appeared to be beyond doubt that the insects had come from the islands named, and must therefore have crossed nearly a thousand miles of ocean! Large numbers of insects lay their eggs in timber, and hence the agency of floating logs in aiding dispersal must be taken into account, especially as such creatures in the egg-state may retain their vitality for a considerable time. Marine animals live under more uniform conditions than those on the land, but their means of dispersal may be briefly touched upon. Mammals, such as Seals and Walruses, sometimes undertake long sea journeys, -while the wanderings of the huge Cetaceans seem only to be limited by considera- tions of temperature. The same remark applies largely to fishes, but with them the depth of water may also have its influence on their dispersal. Fresh-water fishes may travel from one large river-system to another when the sources of the streams are near enough to each other, and through changes in land-level the direction of streams may be altered, and lead to considerable corresponding changes in the fauna. Lastly, the eggs of fishes may be, and no doubt often are, carried from one district to another by aquatic birds. CONDITIONS LIMITING DISPERSAL. Against the various phenomena favouring dispersal which have been noticed above, must be reckoned certain factors which act in the way of species to a varying degree of heat and cold, and on the whole it may be said that temperature does indeed play a not unimportant part in determining or limiting the range of animals. It may be added, that the closest depend- ence in this respect upon temperature is to be found ainong marine organisms. Indeed, this is often the only condition acting as a limit. The temperature of the water undoubtedly checks the distribution of sea-fishes, and in the case of shallow-water species a great depth acts as a perfect barrier. It has been mentioned above, that in the case of non-flying terrestrial animals, such as Mammals, it may be taken that an arm of sea exceeding 20 miles in width acts most effectively as a barrier to dispersal. The few cases in which animals swim across even this distance can only be regarded as possible methods of dispersal and not by any means the rule. When, therefore, we find a great similarity in the faunas of two regions separated by a wide tract of water, we must conclude that there has been in past ages a land connection between them. As will be seen later on, truly oceanic islands, i.e., islands not having been connected with a continental area within comparatively recent geological times, are practically without any Mammals, Reptiles or Amphibians, while on the other hand they possess both Birds and Insects. A wide river may also act in the same way as an arm of the sea, though to a less extent. Thus, in the plains of the Amazon, the river separates entirely distinct sets of species of Monkeys, Birds and even Insects. In the case of species inhabiting high altitudes, valleys may act in precisely the same way. SKETCH-MAP OF THE TRUNK LINES FOLLOWED BY BIRDS IN THEIR MIGRATIONS. After Prof. J. A. Palmen.
Text Appearing After Image:
limiting distribution. In many cases the range of a species is coincident with the area possessing the precise features suitable for its welfare. Thus, for example, strictly arboreal Mammals, such as Apes, Lemurs and Sloths, can only exist within the limits of the great forests to which they are adapted. Again, mountain-loving species do not thrive in the lowlands, and the result of this is that the colonies of such animals are sometimes widely separated and even altogether isolated. The Chamois, for example, is found in the Alps, the Pyrenees, and the Caucasus, but is entirely absent from the inter- vening districts, while the Mammals inhabiting the elevated regions of Tibet are singularly distinct and peculiar. Another important factor which has an influence on geographical distribution is that of temperature. The opinions of authorities vary greatly as to the amount of limitation due to the effects of temperature alone. Examples can readily be found which prove that temperature very consider- ably affects distribution, while on the other hand others are as easily met with which apparently support the opinion that this factor is not of very great importance. The fauna of the Arctic Regions is of such a character that some zoogeographers would constitute a separate circumpolar region which is inhabited by animals markedly distinct from those of more southerly areas. Again, the South American Mammals, known as Vicunas and Guanacos, have a distribution which decidedly proves that they are influenced by temperature. In Peru and Ecuador they inhabit the high regions of the Cordillera, while in the more temperate country of the Argentine Republic and Patagonia they roam over the low-lying plains. These animals, therefore, being addicted to a cool climate, cannot exist elsewhere except at a considerable elevation. The Dotterel, too, in Great Britain nests on our highest mountains, and on the fringe of the Arctic Ocean at sea-level. On the other hand, an animal may be spread over an area presenting a considerable range in temperature. The Tiger, as we have already seen, is found not only in the hottest jungle districts of India, but also at considerable heights in the Caucasus and the Altai Mountains,' and even in the cold Manchurian plains. Examples like the latter may of course be interpreted to prove the great adaptability of certain isolated That high mountain ranges act effectively as barriers to dispersal is shown by the fact that the species inhabiting one slope of such a range are often entirely distinct from those on the other; moreover, the fishes living in the rivers rising on different sides of the same range are likewise of different species. The influence of mountains is further borne out by the curious difference we find between the Old and New Worlds in this respect. In North and South America the chief ranges run from North to South, and consequently the animals are only limited in their range by conditions of temperature. In Europe and Asia, on the other hand, the principal ranges extend in a direction more or less parallel to the Equator, and hence we find in these continents a larger proportion of species with restricted distribution. Lastly, deserts play a not unimportant part in limiting the range of certain animals, especially those possessing weak powers of locomotion. Thus, animals like the Amphibia, which require moisture for the younger stages of development and which cannot wander much, find even a small tract of arid ground an insurmountable obstacle. The Sahara has for a very long period separated Northern Africa from the rest of the continent, and so distinct are these areas in regard to their faunas that they are by general agreement allocated to different zoological regions. Many other influences not yet mentioned may control the distribution of animals in various ways. Some groups are dependent for their welfare, or even their very existence, upon the presence or absence of other groups, and this is notably the case with carnivorous animals and their victims. Again, parasitic animals and their hosts have necessarily the same distribution, either wholly or in part, while insectivorous birds can of course only exist where there is an abundance of the particular group or groups upon which they feed. Vegetation is also an important factor in determining the limits of certain species. A butterfly, for example, would soon become extinct were the food-plant of its Caterpillar to fail, and the same applies to frugivorous birds and other groups indulging in a vegetarian diet. Thus the whole fauna and flora of a district is bound together by a complicated network of particular conditions, and the slightest altera-

Note About Images