hand of man has carried the golden trout to other little mountain torrents, where it thrives as well as in the one where its peculiarities were first acquired.

Other cases of this nature are found among the blind fishes of the caves in different parts of the world (Fig. 172).

FIG. 172.-Fishes showing stages in the loss of eyes and color. A, Dismal Swamp fish (Chologaster avitus), ancestor of the blind fish; B, Agassiz's

In general, caves are formed by the erosion or wearing of underground rivers. These streams are usually clear and cold, and when they issue to the surface those fishes that like cold and shaded waters are likely to enter them. But to have eyes in absolute darkness, in which no use can be made of them,

cave fish (Chologaster agassizi); C, cave blind is a disadvantage in the struggle for life.

fish (Typhlichthys subterraneus).

Hence the eyed species die or withdraw, while those in which the eye grows less from generation to generation, until its function is finally lost, are the ones which survive. By such processes the blind fishes in the limestone caves of Kentucky, Indiana, Tennessee, and Missouri have been formed.

rather when specimens showing intergradation of characters are known, the word sub-species is used. The word variety has much the same meaning when used for a subdivision of a species, but it is a term defined with less exactness. Thus the common fox (Vulpes pennsylvanicus) is a distinct species, being separate from the arctic fox or the gray fox or the fox of Europe. The cross fox (Vulpes pennsylvanicus decussatus) is called a sub-species, as is the silver fox (Vulpes pennsylvanicus argentatus), because these intergrade perfectly with the common red fox.

To processes of this kind, on a larger or smaller scale, the variety in the animal life of the globe is very largely due. Isolation and adaptation give the clew to the formation of a very large proportion of the "new species" in any group.

153. Effect of barriers. It will be thus seen that geographical distribution is primarily dependent on barriers or checks to the movement of animals. The obstacles met in the spread of animals determine the limits of the 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. Were it not for these barriers, each type or species would become cosmopolitan or universal. Man is pre-eminently a barrier-crossing animal. Hence he is found in all regions where human life is possible. The different races of men, however, find checks and barriers entirely similar in nature to those experienced by the lower animals, and the race peculiarities are wholly similar to characters acquired by new species under adaptation to changed conditions. The degree of hindrance offered by any barrier differs with the nature of the species trying to surmount it. That which constitutes an impassable obstacle to one form may be a great aid to another. The river which blocks the monkey or the cat is the highway of the fish or the turtle. The waterfall which limits the ascent of the fish is the chosen home of the ouzel. The mountain barrier which the bobolink or the prairie-dog does not cross may be the center of distribution of the chief hare or the arctic bluebird.

154. Relation of species to habitat. 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 creature is the region for which it is best adapted.

But the reverse of this is often true. There are many cases in which a species introduced in a new territory, through the voluntary or involuntary influence of man, has shown a marvelous adaptation and power of persistence. The rapid spread of rabbits and pigs as wild animals in Australia, of horses and cattle in South America, and of the English sparrow in North America, of bumble-bees and houseflies in New Zealand, are illustrations of this. Not one of these animals has maintained itself in the wild state in its native land as successfully as in these new countries to which it has been introduced. The work of introduction of useful animals illustrates the same fact. The shad, striped bass, and cat-fish from the Potomac River, introduced into the Sacramento River and its tributaries by the United States Fish Commission, are examples in point. These valued food-fishes are nowhere more at home than in the new waters where no species of their types had ever existed before. The carp, originally brought to Europe from China, and thence to the United States as a foodfish, becomes in California a nuisance, which can not be eradicated, destroying the eggs and the foodstuff of far better fish.

In all mountain regions waterfalls are likely to occur, and these serve as barriers, preventing the ascent of trout and other fishes. On this account in the mountains of California, Colorado, Wyoming, and other States, hundreds of lakes and streams suitable for trout are found in which no fishes ever exist. In the Yellowstone Park this fact is especially noticeable. This region is a high volcanic plateau, formed by the filling of an ancient granite basin with a vast deposit of lava. The streams of the park are very cold and clear, in every way favorable for the growth of trout; yet, with the exception of a single stream, the Yellowstone River, none of the streams was found to contain any fish in that part of it lying on the plateau. Below the plateau all of them are well stocked. The reason for this is ap

FIG. 173.-Osprey Falls, on Gardiner River, Yellowstone Park. These falls prevent the ascent of trout, which are common in the river below the falls, to that part of the river (on the plateau) above the falls.

parent in the fact that the plateau is fringed with cataracts which fishes can not ascend. Each stream has a cañon or deep gorge with a waterfall at its head, near the point where it leaves the hard bed of black lava for the rock below (Fig. 173). So for an area of fifteen hundred square miles within the Yellowstone National Park the streams were without trout because their natural inhabitants had never been able to reach them. When this state of things was discovered it was easy to apply the remedy. Trout of different species were carried above the cascades, and these have multiplied with great rapidity.

The exception noted above, that of the Yellowstone River itself, evidently needs explanation. An abundance of trout is found in this river both above and below the great falls, and no other fish occurs with it. This anomaly of distribution is readily explained by a study of the tributaries at the head waters of the river. When we ascend above Yellowstone Lake to the continental divide, we find on its very summit that only about an eighth of a mile of wet meadow and marsh, known as Two Ocean Pass (Fig. 174), separates the drainage of the Yellowstone from that of the Columbia. A stream known as Atlantic Creek flows into the Yellowstone, while the waters of Pacific Creek on the other side find their way into the Snake River. These two creeks are connected by waterways in the wet meadow, and trout may pass from one to the other without check. Thus from the Snake River the Yellowstone received its trout, and from the Yellowstone they have spread to the streams tributary to the upper Missouri.

This case is a type of the anomalies in distribution of which the student of zoogeography will find many. But each effect depends upon some cause, and a thorough study of the surroundings or history of a species will show what this cause may be. In numerous cases in which fishes have been found above an insurmountable cascade, the cause is seen in a marsh flooded at high water, connecting one