The most critical evidence for hybrid vigor in domestic animals is furnished by Roberts and Laible at the Illinois Experiment Station. A purebred Duroc-Jersey sow was mated in the same period of heat to a purebred boar of the same breed and also to a purebred Poland-China. Six of the pigs in the litter from this double mating were purebred and four were crossed, as could be easily told by their color. All were born on the same day, nurtured by the same mother, and fed the entire time on exactly the same rations. The crossbred pigs weighed 3.75 pounds each at birth, the purebreds 3.23. After six months, the only two females of the six purebreds that were alive weighed together 371 pounds, while the two crossbred females weighed 456 pounds, making a total difference for the two of 85 pounds. The two crossbred males weighed 485 pounds.

Because crossbreds do so well, there is always the danger that farmers will use these fine-appearing animals for breeding. If this were generally practiced, the pure breeds would soon disappear as such. The reasons why crossbred animals are not desirable for continued reproduction should be thoroughly understood. The dominance of favorable growth factors is in the first generation, where the parental races are distinct but uniform in their genetic constitution. In later generations Mendelian recombination permits the reappearance of weaknesses and undesirable qualities that are overcome by crossing. Many of the progeny of a crossbred wool and mutton sheep will have the inferior fibre and light clip of one parental race together with the lean body and bony carcass of the other. The breeding together of Berkshire and Tamworth or Poland-China and Chester White crossbreds brings forth four-legged creatures that are inferior in size and vigor to the poorest specimens in either purebred.

"Registered livestock has been rigidly selected towards definite standards. Each breed has certain qualities which make it valued above all others. These qualities are built up to a higher degree than those of any large number of animals resulting from any other system of breeding. The pedigree record insures that they will come fairly true to that type. But purebreds, when compared with the better classes of non-pedigreed livestock, are often somewhat lacking in vigor, rapidity of growth, and fecundity, three very important considerations from the standpoint of the man on the farm who is making his living by producing meat and milk, wool and eggs. The very qualities which close mating unavoidably brings in are counteracted by crossing. Prolificacy, hardiness, and rapid growth are the outstanding features of crossbreds. In hybrid vigor there is a very real help towards increased production. Why not use it?

"There are several reasons why crossing is not more generally practiced than it is. In the first place, the breeders and sellers of purebred livestock discourage crossing. Nearly all writers on animal breeding see so clearly the absolute necessity of maintaining the invaluable pure breeds of livestock which we now have, and of making them still better, that they minimize the real benefit to be derived from crossing the breeds for production purposes. They hesitate to say anything which to them looks like breaking down that system which has had everything to do with building up the livestock industry to the high plane on which it exists to-day. The time is at hand when a broader viewpoint should be taken. The foundation work of creating the breeds has been done and done well, but there is still a great deal lacking."1

A more general use of crossbreeding would mean a larger demand for purebred stock. Little is to be gained by crossing animals that are not closely selected and uniform in their breeding behavior. Furthermore, registered livestock, when used for this purpose, can be more closely inbred than it now is. Some loss in size and vigor will occur, but the resulting crossbreds will in most cases be improved in uniformity and will lack nothing in ability to grow. Less 1 Jones, Donald Forsha,"Genetics in Plant and Animal Improvement," pp. 521-522, John Wiley & Sons, Inc., New York, 1925.

attention needs to be paid to the fine points that determine the appearance of the animal and therefore its sale. Purebred animals that make a low score in the show ring often perform as well or better than the prize winners when used for crossbreeding.

At the present stage of our knowledge it is impossible to state with any high degree of accuracy the comparative value of the different strains of the major breeds when they are used for crossing. This can be determined only by trial, and it is important that the animals be tested and kept up to a definite standard of performance. Once a good combination is found, there is evidence to show that it will continue to give equally satisfactory results.

Finally, as Wentworth says: "Development of a system of crossbreeding would create a wider demand for purebred animals than we have at present, would eliminate much of the cost of development of purebreds since relatively few would ever have to pass good breeding condition, and would make improvement in livestock type more than ever the work of specialists. The general adoption of a system of crossbreeding would result in a better quality of breeding animals on the general farm, a better quality of commercial animals, cheaper production costs, and a more accurate reflection of market values in purebred values."1

In many parts of the world meat is a luxury and will probably become increasingly so. Since it takes several pounds of plant material to make a pound of meat, countries that are densely populated rely principally upon food from plants. The cereal grains combine nutritive value with high production and convenient storage. In Asia, rice has been the mainstay of the most people, just as in America maize, in Africa millo maize, and in Europe wheat, have been the leading crops. Wheat is the outstanding 1 Record of Proceedings, American Society of Animal Production, Annual Meetings, 1925-1926, p. 47.