Page:Popular Science Monthly Volume 37.djvu/522

506 suited to its life, this fact would represent a great oversight on the part of Mother Nature. But with this is the curious fact that the Yellowstone itself, both above and below its falls, is well stocked with trout and with no other fish. This is an anomaly of distribution, but this anomaly disappears when we examine the continental divide as it appears at the head of the Yellowstone. At one point, the Two-Ocean Pass, only about an eighth of a mile of wet meadow and marsh separates the drainage of the Yellowstone from that of the Columbia. From the Columbia the Yellowstone has therefore received its trout. No doubt every anomaly of distribution would become perfectly simple could we only know all the facts which bear on the case.

In my studies of the fishes of America I have had occasion to especially investigate the barriers to their distribution, and the relative value of these as limiting the range of the different forms.

In general we may say that, with rare exceptions, in all waters not absolutely uninhabitable, there are fishes. The processes of natural selection have given to each kind of river or lake species of fishes adapted to the conditions of life which obtain there. There is no state of water, of bottom, of depth, of speed of current, but finds some species with characters adjusted to it. Each of these species has an ascertainable range of distribution, and within this range we may be reasonably certain to find it in any suitable waters.

But every species has beyond question some sort of limit to its distribution, some barrier which it has never passed in all the years of its existence. That this is true becomes evident when we compare the fauna of widely separated rivers. Thus the Sacramento, Hudson, St. Johns, and Rio Grande have not a single species common to any two of them. None' of them has any species peculiar to itself, and each one shares the greater part of its fauna with the water-basins nearest to it.

With the shore fishes, as with other water animals, the barriers are primarily the heights of the land and the depths of the sea—physical obstacles not to be crossed. Next in importance is the barrier of climate. With some forms of life this is absolute, for the palm and the banana are the index of the torrid zone as the dwarf birch and reindeer moss are the index of the frigid. "Plants" says Dr. Gray, "are the thermometers of the ages by which climatic extremes and climates in general are best measured." In many groups anatomical characters are not more profound or of longer standing than are the adaptations to heat and cold. Heat-loving animals are far more numerous in species than animals of cold climates, though the latter often make up by greater abundance of individuals. Barriers less important