The Encyclopedia Americana (1920)/Zoogeography
ZOOGEOGRAPHY, the science of mapping out the surface of the earth with reference to its faunæ; the study of the distribution of animal life. It has always been obvious to travelers, and even the most superficial students of nature, that the various regions of the earth's surface were characterized, among other differences, by local peculiarities in animals and plants. It is a commonplace of knowledge that the animal life of the tropics is different from that of the arctic regions; that the hirds and mammals and small creatures of Africa differ almost altogether from those of South America or Australia. Closer examination shows that such differences exist in a greater or less degree between lesser regions, as the east and west sides of a continent, groups of islands separated by a sea-space, and so forth. On the other hand, there may be found striking resemblances in the faunæ of certain separate regions, or a sameness over an extensive area, as Europe and Asia.
The importance and significance of these facts impressed themselves upon scientific men only within recent times. As long as it was held that each species must have been created, as a general rule within the geographical area which it now occupies, the most curious facts of distribution could be regarded only with “sterile wonder.” But when the idea came to be entertained that allied species have had a common origin, it was obviously implied that they or their ancestors must have had a common birthplace, and consequently, when we find members of a group severed from their nearest kindred, we feel bound to inquire how this came about. Thus, we have to explain how the tapirs are confined to the Malayan region and South America; the camels to the deserts of Asia and the Andes; marsupials to the Australian region and America; how the birds, mammals and reptiles of North America resemble those of Europe more than those of South America and so on.
Means of Dispersal.—Accepting as a starting point the proposition that the various forms of life originated in some parent-stock or stocks at a particular place or places, the present distribution of their descendants as we know them, must depend mainly upon the powers of dispersion which each possessed, and the character of the physical influences and obstructions which acted as controlling factors or barriers in guiding their dispersal along certain lines and altogether prevented it elsewhere. Theoretically a new stock would spread equally in all directions from its point of origin; actually a very irregular and complicated kind of distribution has ensued in most or all cases. Hence an inquiry as to the means and limitations of dispersion possessed by animals and plants is of first importance.
It is scarcely necessary to draw attention to the facilities for diffusion possessed by animals endowed with great locomotive powers, and especially among land animals, by those having the power of flight (q.v.); but it is important to note that some animals, which in the adult state have only feeble powers of locomotion, are better endowed in this respect when young. Such, for example, is the case with echinoderms, marine mollusks and similar lowly aquatic forms, all of which develop from free-swimming and often far-drifting larvæ. (See Deep-Sea Life). But accidental modes of dispersal must also be taken into account. The carrying power of winds is known to be sufficient to bear along in the air fine dust across seas many hundreds of miles in width, and we have in that agency alone an adequate means of accounting for the dispersion of all plants propagated by minute spores. For that reason the distribution of most cryptogamic plants hardly enters into the problem, since these mosses, fungi, seaweeds and the like are almost universal. What part winds may have played in carrying the seeds of flowering plants is more doubtful; but observations show that even for such seeds, especially when provided with some kind of feathery appendage, winds may occasionally serve as a means of transport for very long distances. See Plant Geography; Plants, Migration of.
But in the case of animals also, winds are a more important means of transport than one might at first suppose. Birds and insects are often blown immense distances out of their course; and to this cause, for instance, is due the arrival every year of American birds on the coasts of Great Britain and France, while European birds almost never reach America — a fact plainly due to the prevalent easterly direction of the winds, and especially the gales in spring and autumn, when birds are migrating. Insects have been caught on ships more than 300 miles from land. Further, there are well-authenticated cases of even crabs, frogs and fishes being carried long distances by storms, and in this way it is possible to account for the transference of fish, etc., from one river system to another. Still more frequently, in all probability, are the eggs of such creatures transported by this means.
Next, marine currents also form, beyond doubt, a highly important means of dispersal both for plants and animals, and that in various ways. First, seeds may float on the surface of the ocean, and be carried by currents hundreds of miles, and become stranded on a distant shore still in a condition fit for germination. The experiments of Darwin to determine the vitality of seeds in sea-water first enabled us to appreciate the importance of this factor. Further, marine currents often carry on their surface various kinds of natural rafts, which may transport both plants and animals. In the polar regions icebergs and icefloes may serve this purpose; and elsewhere trunks of trees, and even fragments torn from the land. Such fragments, forming small islands with erect trees upon them, have been seen at a dastance of 100 miles from the mouth of the Ganges and other rivers. Wallace points out that ocean waifs of one kind or another are almost the only means we can imagine by which land-shells can have acquired the wide distribution for which they are remarkable. Again, locomotive animals are very frequently the means of dispersing both plants and other animals. Seeds may be attached to the fleece or fur of mammals or the plumage of birds, or may be enclosed in clumps of earth clingling to the feet or some other part of bird or beast, even of insects. It seems probable that aquatic birds and water-beetles have been the means of distributing aquatic plants and freshwater mollusks, which are remarkable for their wide diffusion; and the spawn of amphibians and fresh-water fishes may he conveyed from one body of fresh water to another by the same means.
Lastly, man is often unintentionally the means of conveying both plants and animals from one region to another. The foreign plants found growing on ballast heaps near every civilized port are instances of this, and so, also, are the plants which have sprung from seeds introduced with imported grain and merchandise. The whole costal region of North America is overrun with European weeds. Whereever European ships have gone the rats and other vermin of the Old World have accompanied them, and hundreds of species of exotic injurious insects are known in all agricultural districts.
Obstacles to Diffusion of Animals.— For all land plants and land animals the most obvious and effective barrier is a wide expanse of ocean; and where the expanse is very wide it is seldom passable except with the aid of man. For land mammals the ocean is an absolutely impassable barrier, and hence native mammals are always absent from oceanic islands (that is, islands that have never been connected with the mainland); and this barrier is almost equally effective for serpents and amphibians, which also are nearly always wanting where there are no native mammals. Lizards are more frequently found indigenous on oceanic islands, though their means of transit from the mainland is unknown. Arms of the sea and broad rivers are likewise generally impassable for the creatures mentioned, though some of them have greater powers of swimming than is generally supposed. The tiger, the jaguar, the bear and the bison are capable of swimming the widest rivers; pigs have been known to swim ashore when carried out to sea to a distance of several miles; and even a boa constrictor, it is said, has swum to the island of Saint Vincent from the South American coast — a distance of 200 miles.
Mountains, and especially high mountains, are also frequently effective barriers to the migration of land plants and animals; but in some cases they enable plants and animals of a cold climate to spread along their summits into latitudes where, in the plains, the climate is too hot for them. Again, deserts act as a barrier to the majority of plants and animals; forests are a barrier to the camel, hare, zebra, giraffe, etc.; treeless regions to apes, lemurs and many monkeys; plains to wild goats and sheep. Broad rivers also act occasionally as barriers to distribution, and that, strange to say, even in the case of some species of birdb.
Another important barrier is that of climate; but climate merely limits the range of a species or group within a continuous area, for example, through limiting the food supply by restricting vegetation. The range of insects is peculiarly liable to be limited in this way, certain insects being attached to particular species of plants, or at least to genera or families; and for this reason insects, in spite of the exceptional facilities for dispersal which many of them enjoy, are remarkable, as a rule, rather for the restriction of their areas of distribution than for their wide diffusion. Various other minor factors might be mentioned.
But a more generally operative organic barrier consists in the fact of a region being already fully occupied by a native flora and fauna, so that there is no room for newcomers. Hence it happens that seeds may be wafted in plenty from one country to another without a single plant growing from these seeds being able to establish itself; and there may even be, as in South America, a free communication with another region while the fauna remains strikingly distinct, simply because that portion of the American continent is already completely stocked with a fauna perfectly adapted to the physical conditions there prevailing.
The barriers to the spread of marine creatures are not so numerous as in the case of terrestrial forms. The freedom of communication between one part of the ocean and another makes it impossible to mark out any marine zoogeographical regions, though many seas and coasts are distinguished by characteristic fishes and other marine creatures. There is also in the ocean a vertical distribution, the limits of which are determined by depth and its conditions. The principal barriers for fish are temperature and the intervention of land. Thus, the Isthmus of Panama is at present a complete barrier for fishes requiring warm seas.
Geological Evidence.— If all the barriers to migration had existed in all past time as they are now, it would be quite impossible to explain the present distribution of plants and animals on the supposition that kindred groups have had a common birthplace. But the solution of the problems of distribution is to be found in the fact that all the barriers are liable to change. Of changes of sea and land geology supplies us with abundant evidence. Portions of the mainland now continuous were at one time severed by arms of the sea; and islands have been formed by the severance of portions of land that once belonged to the mainland. Such islands are known as continental islands, and the study of their faunæ and floras is one of peculiar interest in connection with geographical distribution. These faunæ and floras show, as might be expected, a greater or less degree of correspondence with those of the mainland from which the islands have been cut off; and the resemblance is the closer the more recently the land connection has been destroyed. The relative date of the disunion is usually approximately indicated by the depth of the sea which now separates island and mainland, shallow seas dividing portions of land that have only recently been disconnected, and deeper seas separating those which have been longer apart.
The most remarkable case of isolation is presented by the Australian region, the fauna and flora of which are the most peculiar in the world. In the widest sense, this region includes not only the vast island of Australia itself, but also New Guinea and all the Malayan and Pacific islands to the east of a deep channel between the islands of Bali and Lombok — a channel the significance of which, as a boundary line for plants and animals, was first pointed out by A. R. Wallace, the great authority on animal distribution, and hence known as Wallace's Line. The peat featnre of this region (so far as animal distribution is concerned) is “the almost total absence of all the forms of mammalia which abound in the rest of the world, their place being taken by a great variety of marsupials.” This presents one of the best examples of what are known as discontinuous areas of distribution, and offers an illustration of the mode in which such discontinuity is usually brought about. The early severance of the Australian region from the Asiatic continent (in the Mesozoic Age) saved the Atrsiralian marsupials from the com petition which almost extinguished liie group elsewhere.
Cnanges in the climatic barrier have also had an important influence on geographical distribution; and it is by such changes, combined with changes in the continuity of land in the north polar regions, that the affinities between the floras of Japan and eastern North America must be explained. (See Land-Bridges) When these affinities were first pointed out by Asa Gray, that distinguished botanist divined the true explanation — namely that in former geological epochs, a genial climate must have prevailed even within the polar circle, so as to allow of the existence of a remarkably uniform flora, suitable to such a climate, all round the pole in very high latitudes; and that as the climate became colder in the North this flora was driven southward, and became differentiated according to the differences of climate in the more southerly latitudes to which it advanced. Hence the eastern parts of America and Asia, as they correspond pretty much in climate, came to correspond also more closely than other tracts in the same latitude in the character of their floras. The soundness of this surmise was afterward confirmed by the discovery of abundant plant remains of the Miocene Age, indicating a warm climate in Greenland, Spitsbergen, and elsewhere. The effects on distrrbution of the changes of climate belonging to the Glacial Period or Ice Age may only be alluded to here.
Zoogeographical Regions.— As the result of all the processes of dispersal across the various barriers to migration, and of the changes in these barriers, we have the present distribution of plants and animals, which is such as to enable us to divide the terrestrial surface of the globe into more or less well-marked regions. For mammals, the regions adopted by Wallace are nearly the same as those first suggested by Sclater as applicable to the distribution of birds for in spite of the exceptional facility which birds have for crossing barriers impassable by mammals, Wallace finds that the distribution of mammals (which afford the best means of marking off zoogeographical regions) corresponds with that of birds to an extent that one would not perhaps have previously anticipated. But with regard to these regions it must be remembered (1) that it is impossible in most cases to draw any very clearly marked boundary line between one region and another; (2) that the degree of divergence between different regions is different in different cases; and (3) that, when any two regions are compared, we have not the same degree of divergence between different groups of the animal kingdom, or between animals and plants belonging to the two regions. Obviously, the degree of correspondence depends largely on the facilities for dispersal, and largely also on the geological age of different groups; and both of these are varying factors. These considerations being premised, we may now state briefly the limits of the six zoological regions adopted by Wallace, as given in his “Island Life.” In the space to which the present article is necessarily restricted it is impossible to give even, the most fragmentary sketch of the characteristic life of the different regions, for which the reader must be referred to the works cited at the end of the article.
(1) Palæarctic Region, including Europe and north temperate Asia and Africa to the northern borders of the Sahara.
(2) Ethiopian or Palæotropical Region, consisting of all tropical and South Africa, together with Madagascar and the Mascarene Islands.
(3) Oriental Region, comprising all Asia south of the Palæarctic limits, and the Malay Islands as far as the Philippines, Borneo and Java.
(4) Australian Region, the Papuan Islands, Australia, New Zealand, and the islands of Oceanica. Celebes might be referred almost with equal right to this or the previous region. New Zealand is treated by Wallace as a highly peculiar sub-region of this great region.
(5) Nearctic Region, comprising all temperate and arctic North America, including Greenland, and extending on the south to an irregular line running from the Rio Grande del Norte on the east to a point nearly opposite Cape Saint Lucas on the west.
(6) Neotropical Region, the American continent south of this line, together with the West Indian Islands, sometimes callled Neogæa.
Heilprin and others advocate the union of the Nearctic and Palæartic regions under the name of Holarctic, and introduce three transitional tracts (the Mediterranean, embracing southern Europe, northern Africa and western Asia, south of the Caspian and west of India, but exclusive of the southern half of Arabia; the Sonoran tract, embracing the northwest of Mexico; and the Austro-Malaysian tract, embracing Celebes and the smaller islands lying between it and New Guinea and Australia). Otherwise his major faunal divisions of the globe are similar to those of Wallace.
On plant distribution the most important recent works are those of Engler and Drude. Engler attempts to trace the history of the vegetable kingdom since the Tertiary period, and comes to the conclusion that already in the Tertiary period four “floral elements” (Florenelemente) could be distinguished, namely:
(1) The Arcto-tertiary element, characterized by an abundance of conifers and numerous genera of trees and shrubs now prevalent in North America, or in extratropical eastern Asia and in Europe.
(2) The Palæotropical element, characterized by the presence of the families and subfamilies dominant in the tropics of the Old World; and still more by the absence of certain families, groups and genera found in the territory of the Arcto-tertiary elitnent.
(3) The Neotropical or South American element, which, according to Engler, must have had in Tertiary times much the same character as that now possessed by tropical Brazil and the West Indies.
(4) The old Oceanic element, consisting of forms which possessed the power of traversing considerable stretches of ocean and developing further on islands.
The modern provinces of the vegetable kingdom are subordinated by Engler to these great divisions. Drude, in the first place, distinguishes the oceanic (marine) flora from the terrestrial forms, and the latter he divides into three great groups, and these again into 14 floral domains. See Plant Geography.
Bibliography.— Sclater, ‘Geographical Distribution of Birds,’ in ‘Journ. Linn. Soc.’ (Zool.), Vol. II (1875); Wallace, ‘Geographical Distribution of Animals’ (New York 1876); Wallace, ‘Island Life’ (New York 1885); Heilprin, ‘Geographical and Geological Distribution of Animals’ (New York 1887); Beddard, ‘Text-book of Zoogeography’ (London 1895); Engler, ‘Entwicklungsgeschichte der Pflanzenwelt’ (Leipzig 1879-82); Drude, ‘Die Florenreiche der Erde’ in ‘Petermann's Mitteilungen’ (1884).