Speciation of Fishes

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Abstract
Environment is as indispensable in speciation as are changing gene ratios. Parallel variation demonstrates this point, and is exemplified by the general tendency of fishes to have distinctive proportions and an increased (rarely a decreased) number of segments in colder or more saline waters; to be stream-lined in large, swift rivers, less so in creeks; to have less inferior mouths and more poorly developed sense organs in springs than in creeks; of cyprinodonts to lose pelvic fins in isolated waters; to develop many long gill-rakers in lakes; to be terete and small-finned in waters of low productivity and deep-bodied and large-finned in rich waters; to have smaller eyes but better developed tactile organs in the more silty waters; to hypertrophy the sense organs in the deep sea; to become large-eyed and red in the twilight zone of moderate depths; to lose eyes and pigment in caves (an almost universal relation, explainable on the basis of repeated mutation); of lampreys to become non-parasitic and degenerate in creeks. Consideration of such parallel variations is essential to an explanation of specia- lion, which is an interplay between the adaptation of the organism and the selection of habitat. Natural selection is an essential basis of speciation as well as general phylogeny. "Blended" inheritance is typically shown when races, spp. or genera of vertebrates are crossed; this also is a fact essential to an explanation of speciation. Phases but not systematic forms of vertebrates are due to characters showing simple Mendelian segregation. Marked parallelisms are shown between specific and racial differences, direct environmental effects, age variations, and sexual dimorphism. Similar if not identical biochemical processes are probably involved in all spheres of differentiation. "Speciation is no aimless wandering of genes through the organic world, but rather an orderly adjustment under the rigid control of the environment.".