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other introduced fishes in the lower Colorado River system. The establishment of the red shiner is attributed to escapements from the Arizona Fish Farms near Blythe, California. 182. Hubbs, C. L. 1955. Hybridization between fish FISH species in nature. Systematic Zoology SPAWNING 4(1):1-20. GENETICS This report deals with the analysis of hybridi- zation in fish. Examples of experimental and natural hybridization is discussed along with the role of environmental factors attributing to hybridization. 183. Hubbs, C. L. 1958. Isolating mechanisms in BEHAVIOR the speciation of fishes. In: Blair, POPULATION W.F. (ed.) 1961. Vertebrate speciation. FISH University of Texas Press, Austin. pp. 5- HABITAT 23. GENETICS Many species that may be crossed readily by artificial fertilization or by cohabitation in aquaria seldom, if ever, hybridize in nature. Behavioral and habitudinal factors are no doubt often effective. Hybridization often occurs when two species previously separated are brought together in nature. Unbalanced numbers favor hybridization, disturbances of habitats, either natural or artificial, often lead to cross breeding. In general, genetic isolation increases with hyletic differentiation. The intergrades or hybrids tend to be swamped out by populaton pressure and competition, and are often reduced by reason of some small degree of lowered viability or lowered fertility. Isola- tion is undoubtedly a common and probably the usual antecedent of differentiation, segregation and speciation apparently may often be effected sympatry. This seems to be particularly true in lacustrine loci of explosive evolution. 184. Hubbs, C. L. 1961. Isolating mechanisms in GENETICS the speciation of fishes. In: W. F. BEHAVIOR Blair (ed.). Vertebrate speciation. DISTRIBUTION University of Texas Press, pp. 5-23. FISH 78