all had red silks. The second, third and fourth generations segregated for this color while the remaining generations were all uniformly colored. Height of plant, position of the ear on the stalk, form of tassel and all structural details were noticeably uniform in the parents and the first hybrid generation. The plants in the generations from the second to the fifth were quite variable but later became more and more uniform until in the last two generations they showed as little variation as either of the parental strains.

The inbred strain which resulted from this second period of selffertilization differs from both parental strains. In tassel, ear, and character of the foliage it is quite unlike either but is noticeably susceptible to smut like one of the parents. In other words,

Figure 30. Inbreeding after crossing. Representative plants from the generations shown in figure 29.

Mendelian recombination has taken place so that the details of structure are altered. Apparently this inbred strain has about the same number of favorable growth factors, and for that reason it is no better or no worse than the parental stocks that went into the vigorous and productive hybrid from which the new strain was derived a few generations before.

For all practical purposes the reducing effect of self-fertilization in this particular case has ceased at the sixth inbred generation. This closely parallels the course of events when the parental strains were first inbred. Theoretically the loss of vigor follows the rule of halving the remaining difference in each generation. If we take an individual heterozygous for a single Mendelian pair of factors such as Aa we expect in the next generation fifty per

cent. of the plants homozygous for this pair of factors and having the composition AA or aa; the other fifty per cent. will on the average still be heterozygous for this factor pair; i. e. Aa in composition. In choosing a single self-fertilized individual for the progenitor the chances are even that it will be homozygous or heterozygous. This holds for any number of factor pairs and since each pair when once alike must remain so thereafter in selffertilization the number of mixed pairs is steadily reduced by half in each generation. Starting with an individual 100 per cent. heterozygous, the following generations would be on the average 50, 25, 12.5, 6.25, 3.125, 1.5625, etc.

Naturally the progeny of any heterozygous individual will vary greatly in composition. Some will be nearly or completely homozygous while others will be nearly or completely heterozygous with respect to all factor pairs. For that reason the result of any process of inbreeding depends entirely upon the composition of the individual plants which are chosen as progenitors. It is theoretically possible to obtain individuals in each generation which are as heterozygous as their parents and others that are completely homozygous. For that reason inbreeding may cause no reduction in size, vigor or variability, or complete reduction may take place in a single generation. The chances that such a result will be obtained, however, are extremely remote. Actually the reduction follows the rule of halving the remaining difference very closely so that it is evident that a very large number of factors play a part in hybrid vigor. How many such factors there are, we have no way of estimating at the present. Many factors which bring about visible differences possibly have no effect upon vigor but apparently the number of them which are essential to normal development in corn is exceedingly great.

THE ATTAINMENT OF COMPLETE HOMOZYGOSITY.

Whether complete fixity of type, absolute homozygosity, is possible of attainment by continuous self-fertilization has been previously discussed. (Jones 1924.) The experimental results show that small germinal differences may remain after many generations of inbreeding. Two lines separated from one in the third generation and then continued separately for several generations gave a marked increase in size when crossed, although not as great as in the case of lines separated at the beginning, showing that two self-fertilizations had not produced much uniformity in germinal constitution. The four original Leaming strains were continued as single lines up to the eighth generation. At that time they were all remarkably uniform and apparently fixed in their type. Then each line was separated into two lines which were continued separately thereafter for eight or more additional generations. At that time two of the paired lines had remained exactly alike. No visible differences in any respect could be seen.

One of the pained lines differed only in color of the seeds, one being noticeably brighter in color in some seasons. As the growing conditions were alike for all plants this slight difference can not be accounted for in any other way than as an heritable difference. The other paired line differed noticeably in many respects. One of the members was taller, the leaves were broader and lighter colored and the ears were larger, the seeds broader and duller in color.

Crossing these paired lines gave significant increases in all measurable characters in the one strain whose paired lines were visibly different. The other strains all showed slight but apparently significant increases in some characters. The two strains whose paired lines showed no visible differences were again tested after fourteen generations of self-fertilization in the following way. The two strains which were distinct from the beginning were crossed and gave the usual vigorous and uniform hybrid plants. A

number of these were self-fertilized and an equal number were inter-pollinated by sib plants. A careful test failed to show any differences in size or productiveness in the plants grown from these two lots of seed. If the parental strains were not germinally alike within themselves, intercrossing the first generation hybrid plants would not cause such a decrease in heterozygosity as self-fertilization. The fact that no difference was shown indicates that the parental strains were completely homozygous for all factors which influence growth vigor. However, this test is not a very delicate one and final proof awaits the crossing of the paired lines which have been separated in the seventeenth generation and will be carried along for several additional generations.

MUTATIONS IN CORN.

Complete homozygosity may be impossible to attain because of spontaneous variations, mutations, occurring from time to time.

During the seventeen years in which the four inbred Leaming strains have been under observation only two apparent germinal changes have been recorded. Until a fairly high degree of uniformity was reached, after six generations, various abnormalities occurred singly or in greater numbers in the rather small progenies that were grown. Presumably these were, at least in the great majority of cases, merely segregations from a heterozygous complex. But new characters appearing after uniformity is obtained which have not been noted previously have every indication of being mutations. Two such have been observed in different lines. One produced in the thirteenth generation a single self-pollinated ear segregating for defective seeds. All of the lines had been examined for the new character during three previous generations, without noting anything of this kind, and since the character

Figure 32. Graph showing the height of the two parental strains and the generations from the Fi to Fs.

segregated as a single Mendelian recessive when out-crossed, there is every reason to assume that a germinal change took place shortly before its appearance. Among approximately a thousand plants of another line, self-fertilized more than ten generations, which has always produced white cobs, four ears were found with light red cobs. The cobs of this strain are flattened and the plants are otherwise easily identified. The red cob plants were examined at harvest and noted to be typical for the strain in all respects except cob color. Neither of these changes could have been due to out-crossing. Stray pollen from any outside source immediately results in vigorous plants twice as large as the inbred plants ever grow and the crossed plants are completely changed in type. Since the mutant plants were in other respects typical plants of the strain and were no larger they could not have resulted from out-crossing.

Two additional changes have occurred in other inbred material

such that they have every indication of being recent germinal alterations. One strain after five generations produced for the first time striped, variegated plants which bore no pollen or seed. They occured in later generations in about 25 per cent. of the offspring from normal plants. Another strain after nine generations gave small narrow-leaved dwarf plants which were quite distinct from the normal plants. They produced a small amount of pollen and when out-crossed to normal plants they reappeared in later generations showing that the change was heritable.

These four apparent mutations are all that have been noted in a large number of uniform strains which have been under observation for many years. Hayes and Brewbaker record the production of chlorophyll deficient seedlings in four lines out of 953 which had

Figure 33. Graphs showing rate of growth (average daily gain in height) for the same generations as in the preceding illustrations.

not shown such abnormalities previously. In these cases the appearance of the abnormalities may have been due to delayed segregation, since the lines had not been reduced to uniformity and constancy. While it is evident that corn does mutate, the frequency of these changes is so low that inbred strains, when once reduced to uniformity, are stable for all practical purposes. Some care will be needed to maintain self-fertilized lines true to type, and when recessive abnormalities appear those progenies which show them will have to be discarded.

THE VALUE OF INBREEDING.

This review of the effects of inbreeding and crossing upon corn has been given in considerable detail because the facts learned from