Adult hybrid rats from the cross of races A x B show a total capacity to vary their geotropic performance which is identical with that of their B parents. The proportionate modifiable variability of geotropic orientation also agrees quantitatively with that for the B parents. These relationships are not disturbed by the action of adrenin, which leads to a distortion of the θ vs. α curve. Young rats of the F1 generation show a greater proportion of unmodifiable variation of geotropic orientation. It is pointed out that the present findings support the conclusion that the capacity of rats to exhibit variation of geotropic orientation is limited by their genetically determined composition and that the special condition in the young hybrids may be understood as due to a kind of temporary disharmony of developmental events.

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