5 o6 THE POPULAR SCIENCE MONTHLY. 



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 Yellow- 

 stone. 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 cur- 

 rent, 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 spe- 

 cies 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 bar- 

 riers 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 meas- 

 ured." In many groups anatomical characters are not more pro- 

 found 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 



