When these plants were again self-fertilized and the offspring of each plant separately sown, a new surprise awaited the observer. The progeny of the recessives remained pure recessive; and in any number of subsequent generations never produced the dominant type again, that is, never reverted to the original parent, whose qualities had failed to appear in the second generation. When the seeds obtained by self-fertilizing the plants with the dominant characteristics were sown, it was found by the test of progeny that the dominants were not all of like nature, but consisted of two classes--first, some which gave rise to pure dominants; and secondly, others which gave a mixed offspring, composed partly of recessives, partly of dominants. Once more, however, the ratio of heredity asserted itself and it was found that the average numerical proportions were constant--those with pure dominant [{212}] offspring being to those with mixed offspring as one to two. Hence, it was seen that the seventy-five per cent of dominants are not really of identical constitution, but consist of twenty-five per cent which are pure dominants and fifty per cent which are really crossbreds, though like most of the crossbreds raised by crossing the two original varieties, they exhibit the dominant character only.
These fifty crossbreds have mixed offspring; these offspring again in their numerical proportion follow the same law, namely, three dominants to one recessive. The recessives are pure like those of the last generation, but the dominants can, by further self-fertilization and cultivation of the seeds produced, be again shown to be made up of pure dominant and crossbreds in the same proportion of one dominant to two crossbreds.
The process of breaking up into the parent forms is thus continued in each successive generation, the same numerical laws being followed so far as observation has gone. As Mendel's observations have now been confirmed by workers in many parts of the world, investigating many different kinds of plants, it would seem that this law which he discovered has a basis in the nature of things and is to furnish the foundation for a new and scientific theory of heredity, while at the same time affording scope for the collection of observations of the most valuable character with a definite purpose and without any theoretic bias.
The task of the practical breeder who seeks to establish or fix a new variety produced by cross-breeding in a case involving two variable characters is simply the isolation and propagation of that one in each sixteen of the second generation offspring which will be pure as regards the desired combination of characters. Mendel's discovery, by putting the breeder in possession of this information enables him to attack this problem systematically with confidence in the outcome, whereas hitherto his work, important and fascinating as it is, has consisted largely of groping for a treasure in the dark. The greater the number of separately variable characters involved in a cross, the greater will be the number of new combinations obtainable; the greater too will be the number of individuals which it will be necessary to raise in order to secure all the possible combinations; and the greater again will be the difficulty of isolating the pure, that is, the stable forms in such as are similar to them in appearance, but still hybrid in one or more characters.
The law of Mendel reduces to an exact science the art of breeding in the case most carefully studied by him, that of entire dominance. It gives to the breeder a new conception of "purity." No animal or plant is "pure," simply because it is descended from a long line of ancestors, possessing a desired combination of characters; but any animal or plant is pure if it produces gametes--that is, particles for conjugation of only one sort--even though its grandparents may among [{214}] themselves have possessed opposite characters. The existence of purity can be established with certainty only by suitable breeding tests, especially by crossing with recessives; but it may be safely assumed for any animal or plant, descended from parents which were like each other and had been shown by breeding tests to be pure.
This naturally leads us to what some biologists have considered to be the most important part of his work--the theory which he elaborated to explain his results, the principle which he considers to be the basis of the laws he discovered. Mendel suggests as following logically from the results of his experiments and observations a certain theory of the constitution of germinal particles. He has put this important matter so clearly himself and with such little waste of words that it seems better to quote the translation of the passage as given by Professor Bateson, [Footnote 18] than to attempt to explain it in other words. Mendel says:--
[Footnote 18: Bateson: Mendel's Principles of Heredity. Cambridge. The University Press. 1902.]
The results of the previously described experiments induced further experiments, the results of which appear fitted to afford some conclusions as regards the composition of the egg and pollen-cells of hybrids. An important matter for consideration is afforded in peas (pisum) by the circumstance that among the progeny of the hybrids constant forms appear, and that this occurs, too, in all combinations of the associated characters. So far as experience goes, we find it in every [{215}] case confirmed that constant progeny can only be formed when the egg-cells and the fertilizing pollen are of like character, so that both are provided with the material for creating quite similar individuals, as is the case with the normal fertilization of pure species.
We must therefore regard it as essential that exactly similar factors are at work also in the production of the constant forms in the hybrid plants. Since the various constant forms are produced in one plant, or even in one flower of a plant, the conclusion appears logical that in the ovaries of the hybrids there are formed as many sorts of egg-cells and in the anthers as many sorts of pollen-cells as there are possible constant combination forms, and that these egg and pollen-cells agree in their internal composition with those of the separate forms.
In point of fact, it is possible to demonstrate theoretically that this hypothesis would fully suffice to account for the development of the hybrids in the separate generations, if we might at the same time assume that the various kinds of egg and pollen-cells were formed in the hybrids on the average in equal numbers.
Bateson says in a note on this passage that this last and the preceding paragraph contain the essence of the Mendelian principles of heredity. Mendel himself, after stating this hypothesis, gives the details of a series of experiments by which he was able to decide that the theoretic considerations suggested were founded in the nature of plants and their germinal cells.