A small farmer in the tropics often grows a range of crops to protect himself against failure in a bad season. It is suggested that guidance on the choice of species, and the proportions between them, can be gained by studying past cropping figures, such as those collected for FAO purposes. Two cases are distinguished for systems using two species, (a) when the crops serve much the same purpose and are interchangeable in use, and (b) when they serve different purposes and a certain minimum is required of each. Special attention is given to intercropping, in which the two species are grown in close association.