tinually occurring as mutations as shown by the many controlled observations on the fruit fly and other forms of life. In crossfertilized organisms, and particularly in domesticated animals and plants, crossing keeps these covered over and out of sight by combining them with normal factors. Many of these recessive weaknesses are not distinct and visible characters as are the

Figure 27.

Two genetically different dwarf types give tall plants when crossed, due to the fact that the normal growth factor which each lacks is supplied by the other.

chlorophyll deficiency or dwarfness in corn but nevertheless they weaken the organism in some way. When such crossbred races are inbred, the heterozygous combinations are reduced and the resulting individuals which are homozygous to a greater and

greater degree, as the inbreeding is continued, show the recessive weaknesses and are either unable to reproduce themselves or are reduced in size and rate of growth to a point below that of the original stock. The inbred individuals each receive some of the hereditary factors for vigorous growth. Some receive more than others as a chance allotment and are therefore better able to survive the inbreeding process. Others are so weakened that they perish. On account of the way in which the hereditary mechanism operates it is extremely improbable that any one individual will receive all the more favorable growth factors, and in actual practice inbred strains of corn are all reduced by inbreeding. It is theoretically possible to obtain individuals which possess an unusually large share of the more favorable growth factors or even all of them and for that reason show no reduction from inbreeding. Darwin obtained self-fertilized races of Ipomea and Mimulus which were more vigorous than the naturally cross-fertilized variety at the start. Cummings reports self-fertilized strains of squash that are as productive as the original variety and much more uniform in type. King has obtained inbred rats after long-continued brother and sister mating that are fully as vigorous as the material with which she started. The fact that no such result has been obtained with corn shows how dependent this plant has become upon cross-fertilization to maintain production.

THE TRANSITORY NATURE OF HYBRID VIGOR.

The increased growth resulting from crossing is quickly lost in the following generations when the hybrid individuals are bred among themselves or again inbred. In other words, hybrid vigor is a temporary manifestation which ordinarily cannot be fixed and made permanent in sexually reproduced offspring. The reason for this is readily appreciated when the illustrations previously given. are followed into the later generations. The cross of the golden and dwarf corn gives all normal tall green plants in the first hybrid generation. Seed from these hybrid plants, either selfed or inter-crossed, always gives in the next generation all the possible combinations of characters that went into the cross. In this particular case the golden plants also lacked the ligule which is the small extension of the leaf sheath surrounding the stalk above the leaf blade. Liguleless plants hold their leaves in a characteristically upright position close to the stalk. In the second generation of this cross of liguleless golden by dwarf, eight different kinds of plants are produced. These are shown in figure 28. Due to the recombination of Mendelian units, this generation is extremely variable, and while some of the tall, green, liguled plants may be as vigorous and productive as the first crossed plants this generation as a whole averages much less productive. By further inbreeding, eight distinct pure-breeding combinations of these three characters

can be obtained and within each type still further minor differences could be established. Crossing any two of these types gives increased growth and restores the normal condition provided the factors for normal growth are all present in one or the other type. In the same way the vigorous and productive crosses between inbred strains of corn fall off in size and yield in the second generation and are much more variable. This always results whether the first crossed plants are self-fertilized or are inter-crossed among themselves. If the inbred strains are uniform and fixed in their type the first generation hybrid plants are germinally all alike so

Figure 28. The second generation offspring from the crossing of golden liguleless by dwarf. Eight different combinations of these three characters are obtained by Mendelian segregation and recombination.

that it is easily understood why self-fertilization and inter-crossing give the same result. To test this out two inbred strains were crossed after 14 generations of self-fertilization. A number of the hybrid plants were self-fertilized and an equal number were interpollinated. The seed of these two lots was planted in alternate rows, replicated three times. The self-fertilized plants averaged 76.2.57 inches in height in comparison with the intercrossed plants which averaged 73.8+.70. In production of grain they stood respectively 22.2+1.2 and 22.0±2.4 bushels per acre. In neither case are the differences significant.

INBREEDING AFTER CROSSING.

When the second generation plants are allowed to intercross naturally no further reduction in vigor is expected. Variability and yield should remain at the same level thereafter until natural or artificial selection eliminates certain strains. But when the second generation plants are self-fertilized there is a further reduction in size, and if the inbreeding is continued the decline in size and vigor and in variability proceeds in approximately the same way as when the parental strains were first inbred. This is shown in figures 29, 30 and 31.

In this demonstration of inbreeding after crossing, two inbred strains, self-fertilized for eight generations, were crossed and the first generation plants again self-fertilized. In the second generation a single plant was again chosen as the progenitor and pollinated in the same way, and this was continued for eight successive

Figure 29. The result of inbreeding after crossing. Two inbred strains at the left, their first generation hybrid adjoining, followed by seven successive generations self-fertilized.

generations. Seed was saved from each year's selfing up to the fifth generation. Since corn seed will not retain its germination satisfactorily for more than six years, single plants were again selffertilized the fifth year in each generation and this seed was used from then on. All eight inbred generations were grown in 1923 along with the two parental strains as shown in the accompanying illustrations. This demonstration has been grown each year since the original cross was made and the yields obtained in the different years are given in table VII. Production has varied rather widely from season to season and from generation to generation. This is due in part to the character of the individual plants chosen for progenitors. A very noticeable drop takes place from the first to the second generation amounting to over 30 per cent. as an average of the six years. Kiesselbach tested the first and second generations of eight hybrid combinations of different strains during two seasons and obtained an average of 52.2 and 27.8 bushels per acre respectively for the two generations, to be com

pared with 41.7 bushels for the original corn from which the inbred strains were obtained. He secured his seed for the second generation by pollinating several first generation plants with composite pollen from 15 sib plants. The reduction from the first to the second generation of nearly 50 per cent. is even greater than in our case where the plants were self-fertilized. Kiesselbach also grew a third generation from seed of interpollinated plants. The comparative yields obtained for the first, second and third generations were 51.5, 29.4, and 25.6 bushels per acre. The reduction from the second to the third as would be expected from this mode of pollination is small compared with the drop from the first to the second. Continued inter-pollination should cause no further decrease in yield unless particularly unfavorable strains are isolated.

The average height of these successive self-fertilized generations compared with the first generation hybrid and the parental strains is shown graphically in figure 32. There is a continued

TABLE VII.

The production of grain in bushels per acre, of two inbred strains of corn and their hybrid and the Fi to the F. generations successively selffertilized.

reduction in each generation, but the decrease is much less during the last three generations than in the first four. From the first to the fifth generation there is a decline of 27.2 inches in stature and from the fifth to the eighth 8.6 inches. The rate of growth as measured by the daily gain in height is also steadily reduced as shown in figure 33, the decline being greater during the first stage of inbreeding than in the last. The differences between the last two generations in all measurable characters, including yield, height, length of ear and rate of growth, are so small that it seems evident that the reduction in size and vigor is rapidly approaching an end. The last two generations are so similar in appearance that they cannot be distinguished in the field. In tassel type, foliage character, position of the ear on the stalk, and in the size and conformation of the ears these two generations are practically identical.

The reduction in variability from the first to the eighth generation was very noticeable in the field. One of the parent strains has green silks, the other red. The first generation hybrid plants