It is impossible to give more than a very brief outline of the vast amount of work which has been done on this subject. Broadly speaking, two methods have been used, selection and hybridisation. Of these selection is the simpler, but even selection is by no means the simple matter it might appear to be. Let us examine for a moment the various characters of a single wheat plant which determine its capacity for yielding grain. The average weight of one grain, the number of grains in an ear, the number of ears on the plant, are obviously all of them characters which will influence the weight of grain yielded by the plant. Many experimenters have examined thousands of plants for these characters, often by means of extremely ingenious mechanical sorting instruments, and have raised strains of seed from the plants showing one or more of these characters in the highest degree. The results of this method of selection have as a rule been unsuccessful, no doubt because the size of the grain, the number of grains in the ear, and the number of ears on the plant, are so largely determined by the food supply, or by some other cause quite outside the plant itself. They are in fact in most cases acquired characters, and are not inherited. This method of selection results in picking out rather the well nourished plant than the well bred one. Again it is obvious that the weight of grain per acre is measured by the weight of one grain, multiplied by the number of grains per ear, multiplied by the number of ears per plant, multiplied by the number of plants per acre. Selecting for any one of these characters, say large ears, is quite likely to diminish other equally important characters, say number of ears per plant.