. The origin and evolution of life, on the theory of action, reaction and interaction of energy. uthor by W. K. Gregory and Erwin S. Christman. 164 THE ORIGIN AND EVOLUTION OF LIFE types which increasingly depend upon lateral motions of the body for propulsion and thus tend to lose the lateral fins and finally to assume (E) anelongate, eel shape, en-tirely finless, for pro-gression along the bot-tom; (C) the bottom-living forms, in whichthe body becomes later-ally broadened, the headvery large relatively andcovered with protectivedermal armature, themovements of the ani-mals becoming slowerand
. The origin and evolution of life, on the theory of action, reaction and interaction of energy. uthor by W. K. Gregory and Erwin S. Christman. 164 THE ORIGIN AND EVOLUTION OF LIFE types which increasingly depend upon lateral motions of the body for propulsion and thus tend to lose the lateral fins and finally to assume (E) anelongate, eel shape, en-tirely finless, for pro-gression along the bot-tom; (C) the bottom-living forms, in whichthe body becomes later-ally broadened, the headvery large relatively andcovered with protectivedermal armature, themovements of the ani-mals becoming slowerand slower as the dermaldefenses develop. Thislaw applies to all thevertebrates, includingman, namely: the de-velopment of armor ispari passu with the lossof speed. Conversely,the gain of speed neces-sitates the loss of ar-mor. Smith Wood-ward^ has traced similar radiations of body form in the historic evolution of each of the great groups of fishes. The interest of this fivefold law of body-form radiation is greatly enhanced when we find it repeated successively under Smith Woodward, A., UPPERSSILURIAN. PALEOGEOGnAPHY, UPPER SILURIAN (SALINA) TIMEAFTER SCHUCHERT, ,NE DEPOSITS ^- CONTINENTAL DEPOSITS ^,V SALT DEPOSITS Fig. 45. North America in Upper SilurianTime. During this period of depression of the Appala-chian region and elevation of the western half ofthe North American continent occurred themaximum evolution of the most primitive armoredfishes, known as Ostracoderms, which werewidely distributed in Europe, America, and theAntarctic. After Schuchert, 1916. EARLY ARMORED FISHES 165
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