Young guinea pigs while progressing on an inclined surface orient upward in a way such that the path of progression is at a mean angle θ to the intersection of the plane with the horizontal. The angle θ increases as the angle α of the inclination of the plane increases. The results of this experiment indicate that in principle the formulation of similar behavior in the cases of young rats and mice holds essentially for young guinea pigs, and further supports a general muscle tension theory of the limitation of geotropic orientation. The relation of θ to log sin α is sigmoid in character and cos θ is a nearly rectilinear function sin α. It is notable however that in this case the functions are in reality compound, being made up of two curves with a break occurring at a slope of α = 45°. Observation of a blinded guinea pig in light upon an inclined plane reveals the fact that in the neighborhood of 45° the mode of progression changes. Below this angle the animal walks with the feet on the two sides of the body moving forward alternately, while above α = 45° the animal hops in such a way that both front feet and both hind feet move forward together. This change in the mode of progression clearly involves a change in the organization of muscular tensions, and in all probability accounts for the change in the relation of the values of θ to the magnitudes of the slope. The behavior of the mean θ's is closely paralleled by that of their P.E.'s, an automatic check being in this way given upon the significance of the measurements.

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