In precisely similar fashion is the developmental series of hybrids exhibited when three kinds of differentiating characters are conjoined in them. The hybrids form eight various kinds of egg and pollen cells—ABC, ABc, AbC, Abc, aBC, aBc, abC, abc—and each pollen form unites itself again on the average once with each form of egg cell.
The law of combination of different characters which governs the development of the hybrids finds therefore its foundation and explanation in the principle enunciated, that the hybrids produce egg cells and pollen cells which in equal numbers represent all constant forms which result from the combinations of the characters brought together in fertilisation.
Experiments with Hybrids of other Species of Plants.
It must be the object of further experiments to ascertain whether the law of development discovered for Pisum applies also to the hybrids of other plants. To this end several experiments were recently commenced. Two minor experiments with species of Phaseolus have been completed, and may be here mentioned.
An experiment with Phaseolus vulgaris and Phaseolus nanus gave results in perfect agreement. Ph. nanus had together with the dwarf axis simply inflated green pods. Ph. vulgaris had, on the other hand, an axis 10 feet to 12 feet high, and yellow coloured pods, constricted when ripe. The ratios of the numbers in which the different forms appeared in the separate generations were the same as with Pisum. Also the development of the constant combinations resulted according to the law of simple combination of characters, exactly as in the case of Pisum. There were obtained—
| Constant combinations | Axis | Colour of the unripe pods. | Form of the ripe pods. |
| 1 | long | green | inflated |
| 2 | " | " | constricted |
| 3 | " | yellow | inflated |
| 4 | " | " | constricted |
| 5 | short | green | inflated |
| 6 | " | " | constricted |
| 7 | " | yellow | inflated |
| 8 | " | " | constricted |
The green colour of the pod, the inflated forms, and the long axis were, as in Pisum, dominant characters.
Another experiment with two very different species of Phaseolus had only a partial result. Phaseolus nanus, L., served as seed parent, a perfectly constant species, with white flowers in short racemes and small white seeds in straight, inflated, smooth pods; as pollen parent was used Ph. multiflorus, W., with tall winding stem, purple-red flowers in very long racemes, rough, sickle-shaped crooked pods, and large seeds which bore black flecks and splashes on a peach-blood-red ground.
The hybrids had the greatest similarity to the pollen parent, but the flowers appeared less intensely coloured. Their fertility was very limited; from seventeen plants, which together developed many hundreds of flowers, only forty-nine seeds in all were obtained. These were of medium size, and were flecked and splashed similarly to those of Ph. multiflorus, while the ground colour was not materially different. The next year forty-four plants were raised from these seeds, of which only thirty-one reached the flowering stage. The characters of Ph. nanus, which had been altogether latent in the hybrids, reappeared in various combinations; their ratio, however, with relation to the dominant characters was necessarily very fluctuating owing to the small number of trial plants. With certain characters, as in those of the axis and the form of pod, it was, however, as in the case of Pisum, almost exactly 1 : 3.
Insignificant as the results of this experiment may be as regards the determination of the relative numbers in which the various forms appeared, it presents, on the other hand, the phenomenon of a remarkable change of colour in the flowers and seed of the hybrids. In Pisum it is known that the characters of the flower- and seed-colour present themselves unchanged in the first and further generations, and that the offspring of the hybrids display exclusively the one or the other of the characters of the original stocks[43]. It is otherwise in the experiment we are considering. The white flowers and the seed-colour of Ph. nanus appeared, it is true, at once in the first generation [from the hybrids] in one fairly fertile example, but the remaining thirty plants developed flower colours which were of various grades of purple-red to pale violet. The colouring of the seed-coat was no less varied than that of the flowers. No plant could rank as fully fertile; many produced no fruit at all; others only yielded fruits from the flowers last produced, which did not ripen. From fifteen plants only were well-developed seeds obtained. The greatest disposition to infertility was seen in the forms with preponderantly red flowers, since out of sixteen of these only four yielded ripe seed. Three of these had a similar seed pattern to Ph. multiflorus, but with a more or less pale ground colour; the fourth plant yielded only one seed of plain brown tint. The forms with preponderantly violet coloured flowers had dark brown, black-brown, and quite black seeds.