. Fishes. Fishes. CHAPTER VIII BARRIERS TO DISPERSION OF RIVER FISHES JHE Process of Natural Selection. — We can say, in general, that in all waters not absolutely uninhabit- able 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 condition of water, of bottom, of depth, of speed of current, but finds some species with characters adjusted to it. These adjustments are, for the most part, of long stand- ing; and the fauna of any single stream has as a rule been produced
. Fishes. Fishes. CHAPTER VIII BARRIERS TO DISPERSION OF RIVER FISHES JHE Process of Natural Selection. — We can say, in general, that in all waters not absolutely uninhabit- able 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 condition of water, of bottom, of depth, of speed of current, but finds some species with characters adjusted to it. These adjustments are, for the most part, of long stand- ing; and the fauna of any single stream has as a rule been produced by immigration from other regions or from other streams. Each species has an ascertainable range of distribu- tion, and within this range we may be reasonably certain to find it in any suitable r,':.^<<'-<*y:'iWAx^ 1 ".fc t, ^y- Fig 74.—Slippery-dick or Doncella, Halichoeres biviUalus Bloch, a fish of the coral reefs, Key West. Family Lahridce. But every species has beyond question some sort of limit to its distribution, some sort of barrier which it has never passed in all the years of its existence. That this is true becomes evident when we compare the fish fauna of widely separated rivers. Thus the Sacramento, Connecticut, Rio Grande, and I oft. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Jordan, David Starr, 1851-1931. New York, H. Holt and Company
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