By considering the general energy balance between the energy of muscular contraction in the extremities, kinetic energy imparted to the animal and the energy lost in the lifting of the center of gravity and in the possible muscular viscosity, an expression is derived for the speed of an animal in terms of the constant which determine its shape and structure. The relation discussed previously, namely, that the speed is proportional to the square root of the length of the extremities, turns out to be only a first rough approximation in a rather special case. Reasons are given for the number of joints actually observed in animal extremities.