Analysis of the Geotropic Orientation of Young Rats. Ix by W. J. Crozier and G. Pincus

With lines of rats well inbred it has been shown 1 that the relationship between orientation angle 0 and slope of surface a in the geotropic creeping of young rats is specific and quantitatively reproducible. I t is also found that the capacity of individuals to exhibit variation in geotropic response (orientation) is likewise a reproducible function of the genetic constitution of these rats. The capacity to exhibit variation of response is measured by the functional connection between relative variations of 0 and the magnitudes of the performance, 0, which in turn depend upon the intensities of the exciting force due to the acting gravitational component. I t was shown that the "proportionate modifiable variation" is a constant which differs for each of several lines tested. The proportionate modifiable variation is taken as the fraction of the total relative variation of performance which is dependent upon (or modified by) the intensity of excitation, and thus upon 0 (Crozier and Pincus, 1931-32 a). The evidence for the invariant character of the capacity to vary the performance 0, as a function of the impressed intensity of excitation, is derived from several sources. The percentage of modifiable variation is independent of the size of the sample of individuals or of measurements of 0, provided the sample is really homogeneous in this respect (Crozier and Pincus, 1931-32 a, b, etc.). I t is also quite unaffected by experimental modit~cations of the curve connecting 0 and ~, such as are brought about by forcing the young rats to carry additional loads (Crozier and Pincus, 1931-32 b) or by causing them to creep more rapidly when adrenin is administered (Crozier and Pincus,