. L'origine et l'évolution de la vie, sur la théorie de l'action, de la réaction et de l'interaction de l'énergie. La famille les cornes apparaîtront de nombreuses années plus tard dans l'histoire de l'évolution que dans les autres branches, et après leur apparition dans de nombreux cas theymai montre une inertie singulière, ou le manque de dynamisme, sur une période de temps, qui est exactement en accord avec notre principe géneral (p. 149) que chaque personnage a sa propre vitesse à la fois dans le développement individuel et dans le développement racial. L'origine de nouveaux caractères proportionnels NotPredeterminée par Ancestry les titanotheres ex

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. The origin and evolution of life, on the theory of action, reaction and interaction of energy. the family the horns will appear many thou-sands of years later in the evolution history than in otherbranches, and after their appearance in many instances theymay exhibit a singular inertia, or lack of momentum, over along period of time, which is exactly in accord with our gen-eral principle (p. 149) that every character has its own rateof velocity both in individual development and in racial de-velopment. The Origin of New Proportional Characters NotPredetermined by Ancestry The titanotheres exhibit another very important principle, namely, that the linear proportions of the bones of the limbsare exactly adapted to the weight they are destined to carryand to the speed which they are destined to develop; in otherwords, the speed and the weight of all these great herbivoramay be very precisely estimated by ratios and indices of theproportionate lengths of the different segments of the limbs, upper, middle, and lower. These proportionate lengths are 266 THE ORIGIN AND EVOLUTION OF LIFE. not predetermined by the heredity-chromatin, because thesame law of limb proportion prevails in all heavy, slow-mov-ing mammals, whatever their descent; for example, this lawholds among the heavy, slow-moving reptiles, the Sauropoda(Fig. 97), as well as among the heavy, slow-moving mammals.The most beautiful adjustment of the proportions of thelimb segments to speed is observed in the evolution of the horses (Fig. 130). Here we seethat the upper segments (hu-merus, femur) are abbreviated, while the lower segments (fore-arm, lower leg, manus, and pes)are elongated. This is preciselythe reverse of the conditionsobtaining among the slow-mov-ing titanotheres and proboscid-ians (Fig. 131). Among thehorses, too, the same law pre-vails and governs the veryprecise adjustment of the ratiosof each of the limb segments, quite irrespective of ancestry.In the swift Hipparion of Amer-ica, for example, the

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