The increased vigor and productiveness due to cross-fertilization may be illustrated by Dar The average ratio of height during the 10 generations was 100 to 77. The same vigor and superiority of the cross-fertilized plants was also shown in all other features, such as the number of capsules and seed, constitutional vigor, etc., in fully as marked a proportion as was shown in increased height. Similar superiority of cross-fertilized over self-fertilized plants has been demonstrated to occur in Mianulus, Digi talis, Verbascum, Popover, and many other plants, and the rule may be considered a fairly general one. However, the evidence available at present does not tell nearly so much in favor of the advantages of cross-fertilization as did that of Darwin's time.
Darsvues experiment with the common gar den pea, however, form an exception of some interest. Here the average height of the cross fertilized plants was 34.62 inches and that of the self-fertilized plants 39.68 inches, or in the pro portion of 100 to 115. The pea, however, is a plant that is normally self-fertilized, crossing rarely occurring. The plant has therefore be come adapted to self-fertilization, and does not lose vigor as a result. The lack of vigor shown by the cross-fertilized progeny would indicate that such plants which are normally self-ferti lized may have assumed this habit through' some benefit derived from the self-fertilization and would thus be injured as a result of crossing. Wheat, barley and oats, among the cereals, are almost wholly self-fertilized, crossing very sel dom occurring. As a result of experiments con ducted by Professor Hays, of the Minnesota Agricultural Experiment Station, it has been shown that the artificial crossing in wheat of individuals of the same race, or of different races, almost invariably results in decreased fer tility as a whole, although as a result of crossing distinct races, certain individuals with increased fertility can be selected.
It has been said by Nxgeli that "the conse quences of fertilization reach their optimum when a certain mean difference in the origin of the sexual cells is and by Fritz Muller, that "every plant requires, for the pro duction of the strongest possible and most pro lific progeny, a certain amount of difference between the male and female elements which unite. Fertility is diminished as well when this degree is too low (in relatives too closely re lated) as when it is too high (in those too little related).* Darwin says: "The offspring from the union of distinct individuals, especially if their progenitors have been subjected to very different conditions, have an immense advan tage in height, weight, constitutional vigor, and fertility over the self-fertilized offspring from one of the same parents.° Attention has been called by Willis to three factors in the gain resulting from cross-fertiliza tion, namely, a, fertility of mother plant; b, vigor of offspring; and c, fertility of offspring. The relative value of these factors varies with different plants. In the carnation, for instance, factor a of cross-fertilized plants was 9 per cent greater than in self-fertilized plants, b was 16 per cent greater, and c was 54 per cent greater; in tobacco, factor a was 33 per cent less than in self-fertilized plants, but factor b was 28 per cent greater, and factor c 3 per cent greater. Even when the fertility of the mother-plant is greatly reduced by hybridizing with a distinct species and the hybrids them selves are sterile or very unfertile they never theless often show extraordinary vigor, that is, with corn and practically the same results ob tained. McCluer states that: "The corn grown from the crossed seed was in nearly all cases clearly increased in size as a result of crossing,' and that "nearly all the corn grown a second year from the crosses is smaller than that grown the first year, though most of it is yet larger than the average size of the parent varieties."
In maize, the loss of vigor caused by close inbreeding was found in experiments conducted by the writer, in conjunction with Mr. C. P. Hartley, to be very marked. Seeds of Hickory King, a race grown commonly in the Eastern States, produced by inbreeding with pollen of the same stalk, yielded the next year at the rate per 100 stalks of 46 ears, weighing 9V3 pounds. Seeds of the same race, in every way compara ble, but produced by crossing different seedlings, yielded under the same conditions at the rate per 100 stalks of 82 ears, weighing 275/2 pounds. In attempting to fix hybrids of Hickory King b is often greater in hybrids than in pure-bred plants, but factors a and c show usually less.
In plant breeding the importance of the in creased vigor resulting from cross-fertilization is very great. In cases where there is no par ticular object in keeping the varieties pure, a marked increase in yield may be obtained by using crossed seed. The practical value of this fact is indicated by results obtained at the Illi nois Experiment Station by Morrow and Gard ner in crossing various races of corn. Of 15 cross-bred corns tested, 12 gave a decided in crease in yield over that of the parent sorts, ranging from 2 to 86 per cent in individual cases. In three cases a decrease in yield of from 8 to 20 per cent resulted. In the 15 cases taken together an average increase in yield of about 16 per cent was secured. In some cases the cross-bred corns were grown to the second generation without crossing and showed a decidedly larger yield than the parent varieties. A number of crossing experiments of a similar nature had previously been made by McCluer crossed with Cuzco or Peruvian corn, some ears were inbred with pollen from the stalks bear ing them, while others were pollinated from other hybrid seedlings of the same parentage. The hybrids of the second generation, where the seed was inbred with pollen from the same stalk, showed great loss of vigor, being small in stature and almost totally sterile; while those produced from seed which was inbred with pollen from a different seedling were much more vigorous and productive, seeming to have lost hut little by this process of inbreeding. In the majority of cases crossing distinct sorts improves the vigor and results in greatly in creased yield. By selecting varieties which have increased yields uniformly when crossed and crossing these for seed corn, it seems cer tain that the average yield can be greatly in creased. Securing seed corn from a cross of any two races desired is not a difficult or ex pensive process, being easily accomplished by planting the two desired sorts in alternate rows and removing the tassels, as soon as they op pear, from the one to be used as the female parent. The ears that form on the rows from which the tassels have been removed will have been crossed with pollen from the variety from which the tassels have not been removed. The seed corn should therefore be selected from the ears produced on the detasseled rows. The field planted to the two varieties, as above de scribed, to secure crossed seed should be some what isolated from other cornfields, and should be of sufficient size to produce the necessary , quantity i of seed. The only extra expense in curred n producing seed corn in this way is the cost of de-tasseling the alternate rows, as ears will form on both as usual.