Nägeli was the only prominent botanist among Mendel's contemporaries who knew of this study and he did not catch its significance. In the light of later discoveries it assumed great importance.

Mendel found that instead of thinking of the entire plant as a unit he had to consider each plant as made up of units. He found in the pea seven of these unit characters on which he could depend: (1) the form of the ripe seeds; (2) the color of the substance of the pea; (3) the color of the seed coat; (4) the form of the ripe pods; (5) the color of the unripe pods; (6) the position of the flowers; and (7) the length of the stem. Moreover, these characters appeared to go in pairs: the seeds were either smooth or wrinkled; in color, they were either yellow or green. If two plants, differing in respect to any of these characters, were crossed the next generation did not show a blend but, without excерtion, one but not the other of the two types. To the character that appeared he gave the name dominant; to the one which disappeared the name recessive. He seems to have thought of some sort of a contest within the germ cell, the stronger character manifesting itself. Mendel made some fifty-eight crosses on ten plants and found that yellow was dominant over green in every instance. He found that the smooth, round seed coat was always dominant over the wrinkled. Here was a puzzle. What had become of the character that disappeared? The hybrid plants were allowed to self-fertilize. They produced 8,023 seeds, of which 6,022 were yellow and 2,001 were green. This, be it noted, is a ratio of about 3 to 1. Of the yellow-coated seeds 519 were planted and allowed to go to seed; 166 of them produced yellow seeds only, while 353 produced both yellow and green. The hybrids of the round and wrinkled sorts produced 5,474 round seeds and 1,850 wrinkled, while the hybrids of the tall and short varieties produced 787 tall and 277 short plants; in both cases approximately 3 to 1. The recessive character is thus not lost or destroyed. It reappears again in a certain percentage of the offspring.

Correns

Correns found that if he crossed the white variety of the four-o'clock with the red variety all the hybrids were pink, but that the self-fertilized seeds from these pinks produced white, pink and red flowers in the ratio of I to 2 to 1. At first glance this appears to be different from the results obtained by Mendel but in reality it is the same, the principle being brought out more clearly by the fact that the hybrid is of different color from the pure dominant.

To show what happens in these two cases we may compare them in the following diagrams, taking two animals as our subjects. About 1836, black and white Spanish chickens were introduced into England. The crossing of birds of the two colors produced a finely mottled, gray bird known as the blue Andalusian. To the disgust of the chicken fanciers when birds of this blue generation were bred to each other and the eggs hatched only about half of the next generation were blue, an approximately equal number being either white or black. In the guinea pigs, however, the hybrid generation reproduce only the dominant black. In the diagrams the number, order, and sex are arbitrarily arranged.

It will be noted that the chickens show in the second filial generation the 1 to 2 to I ratio obtained by Correns with the four-o'clocks, while the guinea pigs seem to give the 3 to I ratio of the sweet peas grown by Mendel. By breeding the individuals of this second filial generation the true facts appear. The black chickens or black guinea pigs mated with pure black will give black offspring only, while the recessive whites mated with whites produce whites only. The blue hybrid chickens, or the other two guinea pigs, if interbred, reproduce both types of the originals as well as the hybrids. We may follow Mendel's work a step further and note what happens when plants are crossed which differ in two sets of characters, recalling that round is dominant over wrinkled, yellow over green. Four types of seeds are possible. Actual observation shows that we shall obtain nine yellow round seeds; three yellow wrinkled; three green round; and one green wrinkled. This ratio of 9 to 3 to 3 to 1 has been found to hold in other experiments.

Mende, was keen enough to see that the explanation of these results must lie in the nature of the germ cells and not in environmental conditions. He could not give a complete answer to the questions raised, but he did show that the factors causing these phenomena were separately heritable. The rediscovery of his work in connection with that of the botanists just mentioned gave a great stimulus to the study of heredity and the phenomena under discussion are called Mendelian in honor of their first discoverer. Again, as so often, it happens that an earlier student had seen the same thing without being impressed thereby. In 1820 John Gross of Devonshire, seeking a new variety of pea, had observed the results of crossing different types but had not caught the existence of any underlying law.

THE MECHANISM OF HEREDITY

If we assume, then, that there is in the chromosomes a determiner for the different characters and do not forget that each individual gets the chromosomes of his germ cells from

both parents, we may hazard an explanation of the process of heredity. Consider the case of the guinea pigs. The black pig came of pure black ancestry and we may say that he was duplex or homozygous in so far as color is concerned. The

white pig had a white ancestry and was likewise duplex.

Now when these pigs are mated every ovum of the mother carries a determiner for white and every sperm of the father a determiner for black. Hence, in so far as color is concerned, all the offspring are simplex or heterozygous, and all are black as black is dominant over white. When these hybrids are mated, however, the chances are that out of four possible combinations, black will join with black once; black with white, twice; and white with white, once. Such a result is based on many matings, of course, for one case is not decisive. A cent thrown into the air may come down "headsup" ten times in succession, but out of a large number of throws heads and tails will be about equal,

In as much as the pig at the left in the third generation is duplex black evidently it cannot produce white if mated with another pure black; while the one at the right is duplex white and cannot produce black again if mated with white. The two pigs in the middle are simplex and will therefore produce both black and white if mated.

Unit Characters

To Mendel, then, we owe the idea that the body is made up of a number of unit characters which to some extent at least may be separately inherited and put into varying combinations. Evidently the determiners must exist in pairs in as much as they come from both parents. The uniting of any given male cell with any given ovum being assumed to be a matter of chance, the percentage of any given type of offspring is solely a question of mathematical average. The larger the number of cases the more closely will the actual count approximate the ideal. On these questions a great deal of evidence has accumulated since 1900.

The Germ Cell

This evidence has supplemented some of the claims made as early as 1890 by one of the greatest of the successors of Darwin, August Weismann. After 1867 Weismann had undertaken to develop a suggestion made by Virchow in his Cellular Pathology of 1858. In as much as every individual starts life as a single cell, and in as much as these cells all come from those of earlier generations by the process of division, Weismann became convinced that it was to the germ cells, not to the body as a whole, that we must look for the facts of heredity and variation. He came, therefore, to think of the germ plasm as something independent of the body in which it was housed and which was passed along, generation after generation, practically unchanged. He thought that within the germ cells there must be determiners of some sort