Results of backcrosses on rabbits and mice have led to the formulation of a more adequate theory of size inheritance than the Lang multiple factor hypotheses. Many genes exert in addition to their known effects an influence on the growth of the body as a whole. Some act as accelerators, others as inhibitors, each exerting its influence to a different degree. Because of this lack of uniformity in direction and magnitude, no adequate formula for the number of gene differences involved can be formed. In addition there is some theoretical evidence that non-chromosomal structures may share in the genetic determination of size.