the dominant character, but also individuals which presented the recessive character. Such a fact also was known in a good many instances. But Mendel discovered that in this generation the numerical proportion of dominants to recessives is on an average of cases approximately constant, being in fact as three to one. With very considerable regularity these numbers were approached in the case of each of his pairs of characters.

"There are thus in the first generation raised from the crossbreds seventy-five per cent dominants and twenty-five per cent recessives.

"These plants were again self-fertilized, and the offspring of each plant separately sown. It next appeared that the offspring of the recessive remained pure recessive, and in subsequent generations never produced the dominant again.

"But when the seeds obtained by self-fertilizing the dominants were examined and sown it was found that the dominants were not all alike, but consisted of two classes: (1) those which gave rise to pure dominants, and (2) others which gave a mixed offspring, composed partly of recessives, partly of dominants. Here also it was found that the average numerical proportions were constant, those with pure dominant offspring being to those with mixed offspring as one to two. Here it is seen that the seventy-five-per-cent dominants are not really of similar constitution, but consist of twenty-five which are pure dominants and fifty which are really cross-breds, though, like the cross-breds raised by crossing the two original varieties, they only exhibit the dominant character.

"To resume, then, it was found that by self-fertilizing the original cross-breds the same proportion was always approached, namely: 25 dominants, 50 cross-breds, 25 recessives,

or 1D 2DR : 1R.

"Like the pure recessives, the pure dominants are thenceforth pure, and only give rise to dominants in all succeeding generations studied.

"On the contrary the fifty cross-breds, as stated above, have mixed offspring. But these offspring, again, in their numerical proportions, follow the same law, namely, that there are three dominants to one recessive. The recessives are pure like those of the last generation, but the dominants can, by further self-fertilization, and examination or cultivation of the seeds produced, be again shown to be made up of pure dominants and cross-breds in the same proportion of one dominant to two cross-breds.

"The process of breaking up into the parent forms is thus continued in each successive generation, the same numerical law being followed so far as has yet been observed.

"Mendel made further experiments with Pisum sativum, crossing pairs of varieties which differed from each other in two characters, and the results, though necessarily much more complex, showed that the law exhibited in the simpler case of pairs differing in respect of one character operated here also.

"In the case of the union of varieties AB and ab differing in two distinct pairs of characters, A and a, B and b, of which A and B are dominant, a and b recessive, Mendel found that in the first cross-bred generation there was only one class of offspring, really AaBb.

"But by reason of the dominance of one character of each pair these first crosses were hardly if at all distinguishable from AB.

"By letting the AaBb's fertilize themselves, only four classes of offspring seemed to be produced, namely:

"AB showing both dominant characters.

"Ab showing dominant A and recessive b.

aB showing recessive a and dominant B.

"ab showing both recessive characters a and b.

"The numerical ratio in which these classes appeared was also regular and approached the ratio

9AB3Ab: 3aB : 1ab.

"But on cultivating these plants and allowing them to fertilize themselves, it was found that the members of the

namely, AB's, Ab's, aB's, and ab's,

and the average number of members of each class will approach the ratio 1:33 9 as indicated above.

"The details of these experiments and of others like them made with three pairs of differentiating characters are all set out in Mendel's memoir."

Perhaps the most striking thing about Mendel's work is the singularly suggestive and luminous interpretation which he gave of just why the pea characteristics were transmitted exactly as they were; why, in general, the peculiar numerical ratio between dominant and recessive should be, and why it should persist so uniformly. This interpretation or explanation is now well known in biology as the theory of the "purity of the germ cells," or, as Cuenot has called it, the theory of "gamètes disjoints," or "la disjonction des charactères dans les gamètes des hybrides" (the separation of characters in the germ cell of hybrids), the Spaltungsgesetz of de Vries.

This interpretation is simply that in the young of the first generation after a cross-mating, although because of dominance but one of the contrasting pair of parental characters will show itself in the body make-up, yet when these young form their germ cells the two parental characteristics will be represented, but only one in any one germ cell; that is, in the case of Mendel's peas that the pollen cells and ovule cells produced by the cross-bred young would carry each one of the alternative or mutually exclusive parental varietal characters. If this were the case and if, on an average, the pollen cells and ovule cells were evenly divided as to the two characteristics, then by miscellaneous or random mating (mating according to the law of probabilities) between these cells we should get in the developed young just such conditions with regard to the contrasting characteristics as Mendel actually did get in his peas. For twenty-five per cent of the pollen grains representing the dominant character would unite with twenty-five per cent of the ovule cells representing the dominant character, twenty-five per cent of the recessive pollen grains with twenty-five per cent of the recessive ovule cells, and the remaining fifty per cent of each kind with each other; that is, of every four pollen grains and every four egg cells we should get by random pollination 1 pollen dominant X 1 ovule dominant; 1 pollen recessive X 1 ovule recessive; 1 pollen dominant X 1 ovule recessive; 1 pollen recessive X 1 ovule dominant. This condition would bring it about. that the fully developed young would show the contrasting characteristics (remembering the dominance of one of the characteristics in those cases in which dominant and recessive are united), in this condition: 3D, 1R. Which is exactly what occurred in Mendel's peas, and has since been noted to occur

in many other cases recorded by post-Mendelian observers and experimenters. These records are of both plants and animals, and are fast multiplying.

Thus the so-called Mendelian laws of heredity refer to two phases of the problem of inheritance-viz.: (1) how inherited characters are actually distributed, and (2) the fundamental cause, lying in the germ plasm, for this particular kind of distribution. Like Galton's formula, Mendel's law expresses the regularity of heredity based on actual recorded statistics of inheritance; but it also gives a satisfying fundamental reason for this regularity. Biologists, with few exceptions, see in the establishment of the Mendelian principles of heredity in biologic science the greatest advance toward a rational explanation of inheritance that has been made since the beginning of the scientific study of the problem.

The extraordinary fact that Mendel's work lay practically unnoted for thirty-five years (actually the only reference to it in scientific "literature" in all that time seems to have been one by Focke in 1881 in Die Pflanzenmischlinge, p. 109), has been partly explained by Bateson as due to the driving interest felt through all that time by biologists generally in other phases of investigation; but it remains a curious commentary on the possibilities of the temporary obscurity that may be in store for even the best scientific work. The "discovery" of Mendel's work seems to have been made in 1900 by three investigators almost simultaneously, who also discovered independently the same important facts of the transmission behavior in inheritance of exclusive or alternative characteristics. These men are de Vries, Tschermak, and Correns, and their published papers not only verify Mendel's particular work on the peas, but confirm his principles or laws on the basis of much added experimentation and observation on other plants. In the last five years zoologists, notably von Gnaita working with mice, Cuenot, Darbishire, Davenport, Bateson and Castle with mice, rabbits, guinea-pigs and chickens, McCracken with certain beetles, and Toyama, Mrs. Bell and Kellogg with silkworms, have shown that Mendelian principles obtain in animal as well as in plant inheritance. For the results of all of these investigations in large measure confirm our confidence in the Mendelian principles of dominance and recessivity and of the purity of germ cells. But also in nearly all of these studies the investigators

have found some inconsistencies and have caught glimpses of other principles which, when finally grasped, will undoubtedly considerably limit the application of Mendel's laws, but will, almost certainly, not detract from their importance, nor lessen in any degree the high place in science that belongs to the patient, persistent, clear-minded Augustinian monk of the cloister gardens of Brünn.

One of the modifications of the Mendelian behavior of hybrids which has been shown to exist in certain cases, is that the young of the cross-mated parents may not all exhibit in the same degree the dominant characteristic, although in the subsequent generations the regular Mendelian three-to-one splitting up into dominant and recessive appearance may occur. The young of the first generation may include a very few individuals showing the recessive character, as de Vries found in mating two varieties of Papaver somniferum (ninety-seven per cent showed the dominant character, three per cent the recessive). Or the first generation may show a sort of pseudoblend condition, approaching but not duplicating exactly the dominant characteristic, as occurs when Hyoscyamus pallidus is crossed with H. niger (de Vries, "Die Mutationstheorie," Bd. II., p. 162.)

When silkworm moths of the race Shanghai, with white cocoons, are crossed with moths of the race Yellow Var. with rose-yellow cocoons, the hybrid offspring make straw-yellow cocoons of a tint just between the two parent tints. The coloring matter of the grapevine Aramon has the chemical formula C46H36020, and the coloring matter of the race Teinturier has the formula C44H40020: the hybrid offspring called PetitBouschet, of a crossing of these two, has coloring matter of the formula C45H38020, exactly intermediate. Mendel himself got as the result of a crossing between two pea races, one one foot in height and the other six feet, hybrids measuring from six to seven and one half feet high. These are specific cases of blended inheritance and there are many others known.

Also when the plant Mirabilis jalapa ?, with red flowers, is crossed with a male variety with white flowers the hybrid offspring exhibit red flowers (maternal type), white flowers (paternal type), and flowers streaked with the two colors. So when corn with blue kernels is crossed with corn with white kernels, a hybrid is obtained exhibiting on a single ear blue