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FISHES AND FISHERIES OF NEVADA 73 ( \ /I y ? j• ? '?r4? '??ASI wr?- re # tt?! 41j"r `"?tral The the We understand that birds and mammals readily migrate or move from one place to another, and that the larger of these, at least, eaaly cross mountain, desert and river barriers. We know that entire n- mw?ities of plants move about also, but at a much slower rate--so slowly, indeed, that changes in plant communities are not gener;dly detectable in a lifetime unless these changes are man-caused, such as fire, over-.-razing or agricultural land-uses. Yet if we could ascend in a balloon, moor it motionless several mules above the earth and speed time up so that each year becomes a second, we would see vegetation creeping actively below us much as a large herd of cattle or sheep would Riove in grazing. In a similar very slow and visually undetectable manner, streaans are movie.- about also. A stream of water is always actively cutting its channel headward to its own source, deepening its valley near its head- waters and cutting less slowly as it loses its power of flow over low- lands, where it may actually be aggrading or building up land surfaces rather than tearing them down. Its entire picture is one of constant, tireless, inexorable change. In mountains, where waters are swifter and have more cutting poster, we see the typical picture of streams heading on both sides of a divide and eutting back towards the crest so that headwaters of opposing streams continually come closer together. Eventually one cuts into the other and "captures" its source waters, which may entirely divert the captured stream or markedly decrease its volume of flow. The fact that drainage systems existing on opposite sides of a main- tain range have usually been isolated for long perids of time before the first stream captures occur means that fish faunas of the two suites will differ from each other ill many respects, dependin.- upon the lenth of time involved as well as upon the origin of each fauna. 6-Past Times (1) PHYSIOGRAPHIC HISTORY OF NEVADA 1'hc first affinities of Nevada fish faunas were oceanic ones, and call INS diMposed of briefly. For all but a small part of the 300 million plus years of the PAeo- toie Era, most of the area now called Nevada was eoustautly a"der warm, shallow seas whieh connected in various directions and at various times with oceans to the west, south and oecasioually the east. Durhig the Ili million years of the following Mesozoic Era, the picture was one of decreasing marine conueetious. The two (hest 1wrimis of this era, the Triatssie and .Jurassic, represent a combined time lapse of 55 million years in which at least parts of the State were rovcred by marine waters. lu the oldest, tl?e Triassic, for some 30 Million years half, or more than half. of Nevada was under water, with Kedway eonuections to larger bodies of marine water both north and w?nth. During the duratisic Period, the next 25 million years saw a gradual and voutinual decline of surface marine waters, caused by a correspond- in91y gradual eneroachment of land from the east until, by the end of A