Elementary Zoology, Second Edition

by Vernon L. Kellogg

The third great barrier, the ocean, is perhaps the most obvious of all in its influence. It is only in rare cases that any land-animal can independently cross a great ocean. Thus the land-animals of Australia differ from those of all other countries, and those of Africa and South America have developed almost independently of one another. The ocean is, as already mentioned, also a barrier for fresh-water aquatic animals, and even marine fishes which live normally in shallow waters along the shore rarely venture across the great depths of mid-ocean.

The obstacles or barriers met with determine the limits of a species. Each species broadens its range as far as it can. It attempts unwittingly, through natural processes of increase, to overcome the obstacles of ocean or river, of mountain or plain, of woodland or prairie or desert, of cold or heat, of lack of food or abundance of enemies—whatever the barriers may be. The degree of hindrance offered by any barrier differs with the nature of the animal trying to pass it. That which forms an impassable obstacle to one species may be a great aid to the spread of another. "The river which blocks the monkey or the cat is the highway of the fish and turtle. The waterfall which limits the ascent of the trout is the chosen home of the ouzel."

Faunæ and zoogeographic areas.—The term fauna is applied to the animals of any region considered collectively. Thus the fauna of Illinois includes the entire list of animals found naturally in that State. The fauna of a schoolyard comprises all the animals found living naturally in the yard. The fauna of a pond includes all the animal inhabitants of the pond. (Flora is used similarly of all the plants in a given region.) The relation of one fauna to another depends on the character and effectiveness of the barriers between, and the physical character of the two regions. Thus the fauna of Illinois differs but little from that of Indiana or Iowa, because there are no barriers between the States, and they are alike physically. On the other hand the fauna of California differs much from that of the Eastern States because of the great barriers (the desert and the Sierra Nevada Mountains) which lie between it and these States, and because of the great differences in the physical and climatic conditions of the two regions.

The land-surface of the earth has been divided by zoogeographers into seven great realms of animal life, based on the distributional characters shown by these various regions. These realms are separated by barriers of which the chief are the presence of the sea and the occurrence of frost. These realms are named, from their geographical region, the Arctic, the North Temperate, the South American, the Indo-African, the Madagascar, the Patagonian, and the Australian. Of these the Australian alone is sharply defined. Most of the others are surrounded by a broad fringe of debatable ground that forms a transition to some other zone.

Habitat and species.—The habitat of a species of animal is the region in which it is found in a state of nature. It is currently believed that the habitat of any animal is the whole of that region for which it is best adapted. But this is not necessarily true. In fact in most cases it is not true. The trout naturally debarred from the rivers in Yellowstone Park by the waterfalls could live there well if the barrier could be passed. In the case of one stream it has been passed and the trout flourish above the fall. The success of the black and brown rats and the English sparrow in America, of the rabbit in Australia, of bumblebees and house-flies in New Zealand, all of which animals had a natural habitat not including these regions, but by artificial means have been carried over the barriers and into the new territory, prove that "habitat" is not necessarily coincident with "only fit region." Shad, striped bass, and catfish from the Potomac River have been introduced into and now thrive in the Sacramento River in California. In fact the whole work of the introduction and diffusion of valuable food-animals in territory not naturally included in the habitat of the species is based on our knowledge that the habitat of a species is often determined by physical barriers rather than by exclusive fitness of environment. Within the natural habitat the environment is fit for the species' existence, outside of it the environment may be fit.

But there occur numerous instances where a species successful in leaving its original habitat is unsuccessful in attempting to maintain itself on new ground. Man has introduced various animals from one country to another. The English sparrow (naturally debarred from this country by the ocean barrier), brought to America from Europe, has covered its new territory rapidly and maintains itself with brilliant success. But the nightingale, the starling and skylark which have been repeatedly introduced and set free are unable to maintain themselves here.

Species-extinguishing and species-forming.—Accompanying the constant slow migrating of species into new habitats and the constant slow changing of environment and conditions everywhere is to be seen a constant modification of the fauna of any region due to the inability of some species to maintain their ground, the predominating increase of others, and the modifying or adaptive changing of others into new forms. In 1544 the black rat of Europe was introduced into America and it soon crowded out the native rats, being in its turn crowded out by the European brown rat (the present common rat in buildings), introduced about 1775. Here we have the original native species unable to maintain itself in competition with introduced forms.

With a change of environing conditions, certain species are unable to maintain themselves. With the destruction of the forests going on in parts of our country the great host of wood-creatures, the bears, squirrels, the wood-birds and insects, can no longer maintain themselves, and grow rare and disappear. Man often also influences the status of a species by checking its increase either by actual slaughter, as with the bison and passenger-pigeon, or by making adverse changes in its environment, as by destroying forests, or putting the plains under cultivation.

In the discussion of "species-forming" (see p. [408]) it was shown that adaptation may lead to the altering of species, and to the formation of new ones (under the influence of natural selection). With the gradual change of conditions, or with the facing of new conditions because of an unusual migration to or invasion of new territory, those individuals of the species exposed to the new conditions must adapt themselves in structure and habit in order to meet successfully the new demands. By the cumulative action of natural selection these adaptive changes are emphasized; and this emphasis may come to be so pronounced that the part of the species represented in this newly acquired territory, if isolated from the original stock, is so altered as to be quite distinct in appearance from the old. If these changed individuals are also physiologically distinct from the old stock, i.e. are unable to mate with them, a new species is established. As already mentioned, the peopling of islands from mainlands is an excellent and readily observable example of the phenomena referred to in the third law of distribution.