Much emphasis is placed on rate of gain in attempts to improve meat animals. However, little attention is given to composition of the carcass, except as it is indicated by the conformation of the live animals, largely because carcass data can be obtained only for progeny or relatives of the animals chosen for breeding purposes. Rate of gain and composition of carcasses are both determined by the growth rates of the constituent tissues. Hence, if there is more heritable variation in fat deposition, for example, than in growth of bone and muscle, selection for faster total gains would increase the rate of fat deposition more than it would the rate of bone and muscle growth, and fatter carcasses would result.