Distribution

Distribution, or Chorography, is that division of Zoology which deals with the geographical distribution of animals. It has a double value, as it throws much light both on the physical conditions and geography of past ages as well as on the lines along which the evolution of various groups of animals has proceeded. The distribution of land and water animals depends on such different conditions that it is best to consider them separately. Taking the land animals first, we soon see that the distribution does not depend simply on distance, which in fact has but a comparatively slight influence. Thus, if we compare the faunas of England and Japan we find many very striking resemblances between them; if, on the other hand, we compare those of Bali and Lambok, two adjacent islands in the Malay Archipelago, we find the faunas fundamentally different. England and Japan were probably once both connected to the great European and Asian continents, whereas Bali and Lambok are separated by deep sea and in ail probability have never been united: the one has always belonged to the continent of Asia and the other to Australasia. The two main factors that control the distribution of any group are the power of migration and locomotion possessed by its members and the date of the appearance of the group in the life history of the world. Thus, with the former, we should expect animals that can fly to be most widely distributed; next to this would come those whose eggs or larvae would be most likely to be carried about by birds or on floating timber. Those to which sea-water is most fatal would naturally be most restricted. Thus if we take the case of an oceanic island it is natural to find that its principal inhabitants are birds, rats that have escaped from passing ships, other small mammals and snakes that have floated across on timber, insects whose eggs or larvae have been carried by birds, etc.; animals, such as slugs, to which sea-water is absolutely fatal, are the rarest.

The date of the evolution of a group has an important bearing, because, if an old one, the main lines in its present distribution may have been laid down at a time when oceans and continents were differently arranged. Hence, if the world be mapped out into zoological provinces, different groups would yield very different divisions. For example, the best known arrangement of zoological provinces is that made by Mr. Sclater, and is based on birds. He divides the world into six provinces, the Palaearctic (Asia, north of the Himalayas, Europe and Africa, north of the Sahara), the Neearctic (North America, except Mexico), the Ethiopian (Africa, south of the Atlas Mountains), the Neotropical (Central and South America), the Oriental (India and South-east Asia), and the Australasian. But the birds are probably the most recently developed group, and, therefore, in spite of their powers of flight, it is natural to expect them to agree most closely with existing geographical conditions. But if we take an old group such as the Tortoises (Chelonia) we only get five provinces. The first includes all Asia, Europe, North Africa, and North and Central America; the second consists of the rest of Africa; the third of South America and Madagascar; the fourth of Australasia, except. New Zealand, which forms the fifth. The Lizards, again, have a very different distribution. The whole of the New World forms but one province; Africa, Europe, and Northern Asia form a second; India and South-east Asia form a third; Australia, New Guinea, etc., a fourth; while Madagascar and New Zealand constitute the fifth and sixth. The Snakes, Mammals, Amphibia, etc., also give different divisions, and thus the evidence of any one group is not sufficient to demonstrate that continental distribution has not always been the same as it now is.

An instance of the limitations on the distribution of a group caused by its later introduction is afforded by the absence of snakes from Ireland. The severance of England and Ireland was effected at a time when the climate had not fully recovered from the arctic cold of the Glacial period; hence though most animals could live in the country it was too cold for the snakes, who only entered the British area after the Irish Sea had separated Ireland.

In regard to marine animals, as the conditions of life are more equable, they have a wider range. We have to consider the range both in space and depth. The latter, or the bathymetrical range, depends on the supply of light and heat. Light can only penetrate to a limited depth, and the water below is dark, except for the light derived from the phosphorescence (q.v.) of the animals. Similarly, below a certain depth the temperature of the water is constant at about 39° Fahrenheit; therefore, animals such as reef corals, which require a high temperature, cannot live below fifteen to thirty fathoms. The conditions in the deeper parts-of the great ocean-basins are not very favourable to life, and therefore the animals that live in them are those that have been driven downward by natural selection from the more favourable shallower zones, where they were unable to gain a living; they have, as a rule, a somewhat bizarre aspect, and frequently possess features which were probably once possessed by the ancestors of the group. The deep-sea fauna has, therefore, frequently been regarded as a primitive one, but it is more probable that these characters have been reacquired and are secondary. The shallower zones have been divided into several divisions, known as the Coralline, the Laminarian, etc., but these terms have fallen into disuse. The marine zones now accepted are the littoral (down to 100 fathoms), the continental (from 100 to 1,000 fathoms), and the deep sea (below 1,000 fathoms).

In regard to distribution in space the main influence is as to whether the group is fixed or free; in some mollusca, such as the Scallops (Pecten), which are locomotive, the species have a world-wide distribution, while in other closely allied forms, such as the oyster, they are restricted within narrow limits. In many cases fixed forms have free-swimming larvae and thus gradually become widely scattered. The general aspect of the littoral fauna is given it by the modifications assumed to enable its members to withstand currents and tides; thus they are fixed and flexible such as the Zoophytes; fixed, rigid, and strong as the Corals; low and flat, with plenty of power of attachment, as in Starfish, Sea-urchins, Crabs, and flat-fish; or they may burrow through sand and mud, as Cockles, Sandworms, etc.

It has been maintained that the deep sea is uninhabited and that the forms obtained from it are either dead animals that have fallen to the bottom or are picked up by the net near the surface. The operations of recent deep-sea exploring expeditions, such as the Albatross, working with closed nets, leave no doubt that there is a true abyssal fauna.

In 1855 Mr. A. Wallace stated the following propositions: - "Large groups, such as classes and orders, are generally spread over the whole earth, while smaller ones, such as families and genera, are frequently confined to one portion, often to a very limited district. In widely-distributed families the genera are often limited in range; in widely-distributed genera well-marked groups of species are peculiar to each geographical district. When a group is confined to one district and is rich in species, it is almost invariably the case that the most closely-allied species are found in the same locality or in closely adjoining localities, and that, therefore, the natural sequence of the species by affinity is also geographical. Most of the larger and some smaller groups extend through several geological periods. In each period, however, there are peculiar groups, found nowhere else, and extending through one or several formations. Species of one genus or genera of one family occurring in the same geological time are more closely allied than those separated in time. As generally in geography no species or genus occurs in two very distant localities without being also found in intermediate places, so in geology the life of a species or genus has not been interrupted. In other words, no group or species has come into existence twice." From these facts he deduced the law that "every species has come into existence coincident both in space and time with a pre-existing closely-allied species." In other words, the succession of plants and animals in geological time and their geographical distribution can only be explained on the theory of descent or evolution, and when so explained the facts of succession and distribution are hardly less important as indicative of affinity or pedigree than are those of embryology and comparative anatomy.

As to geological succession, we can only here make a few general remarks. The farther we go back the more unlike is the general assemblage of plants and animals to that which now exists. There has certainly been on the whole a steady advance in organisation, and an increase in variety and complexity, from the earliest geological periods to the present day. In each group, generalised types, combining, that is, the characters of two or more types which have subsequently become distinct, have preceded the more specialised or differentiated types. Thus the insects from the Palaeozoic rocks have been grouped inan order, the Paleeodictyoptera, combining the characters of the Neuroptera, Orthoptera, and Hemiptera, whilst such Eocene Ungulata (q.v.) or hoofed animals as Palaeotherium and Anoplotherium, or such Carnivora as Cynodictis, combine the characters of horse, tapir, and rhinoceros, of pigs and ruminants, or of dogs and cats, these more specialised groups of mammals seldom occurring below the Miocene. The imperfection of the palaeontological record, arising partly from the non-preservation of most land animals or of those destitute of hard parts, and partly from the subsequent destruction or metamorphosis of so many feet of fossiliferous strata by denudation or by heat, is so great as to prohibit all argument from negative evidence. Thus we know four species of fossil mammals from Triassic rocks, none from the Lias, four from the base of the Bath Oolite, and none from any part of the Jurassic series until we come to the Purbeck, and then in one small area one bed has chanced to yield us 25 species. Some of the Triassic forms, all of which are Marsupialia (q.v.), are herbivorous, others are carnivorous; so that the primitive mammal was presumably Palaeozoic. Similar reasoning led Darwin to the conclusion that before the curiously varied series of Invertebrata that occurs in the Lower Cambrian (q.v.) system could have lived, "long periods elapsed as long as, or probably far longer than, the whole interval from the Cambrian age to the present day, and that during these vast periods the world swarmed with living creatures." Botanists subdivide the Northern division into three floras, the Arctic-Alpine, the Temperate, and the Mediterraneo-Caucasian. The Arctic-Alpine occupies the northern parts of both Old and New-Worlds, with many interesting detached areas, whither it was carried in Glacial times, includes saxifrages, gentians, cranberries, rhododendrons, and primroses. The Temperate includes the zone of needle-leaved pines and firs, succeeded southward by that of deciduous catkin-bearing broadle ived trees, and is marked by the cultivation of apples, potatoes, wheat, barley, and oats, by numerous Compositae, such as asters, in America, and by Uinbelliferee, such as the giant cow-parsnip, in Siberia. The Mediterraneo - Caucasian flora is largely evergreen, including myrtles, laurels, evergreen oaks, a fan-palm, and the area of the cultivation of maize, almonds, olives, figs, grapes, and oranges. The Tropical division, characterised by large evergreen trees, palms, and bamboos, includes the Indo - Malayan or Oriental flora, that of ginger, rice, mango, tea, and teak; the Tropical African, that of date, and oil-palms, baobabs and spinous euphorbias; and the Tropical American, that of the ivory-palm, rose-wood, caoutchouc, and mahogany. The Southern division includes the Australian flora, that of gum-trees and Epacris; the South African, that of pelargoniums, heaths, everlastings and carrion - scented stapelias; the Andine, represented from Tasmania and Victoria to Western America, the flora of Fuchsia and Calceolaria; and other scattered floras.