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Organisms show adaptation in their more important characters, while many of their minor characters fail to show their utility. There are definite tendencies to mutation in particular directions, and there is abundant paleontological evidence of trends toward increasing modification in particular directions. Qualities so appearing may be indifferent in their beginnings, but may through this orthogenesis become sufficiently useful or hurtful to affect selection. Such trends, when they affect physiological qualities, are likely to bring about an unbalanced, distorted physiological condition and be peculiarly hurtful. Probably this has been one of the chief causes of the disappearance of species. Orthogenesis as thus interpreted is but the handmaiden of natural selection which, acting upon all qualities thus developed to major proportions, will cause them to disappear if ill-adapted. At the same time advantageous trends will be encouraged in the struggle for existence and the direction of evolution be turned toward further adaptation. Adaptation is the most salient result of evolution and natural selection its great cause.
The American Naturalist