In the process of fertilisation each pollen form unites on an average equally often with each egg cell form, so that each of the four pollen cells _AB_ unites once with one of the forms of egg cell _AB_, _Ab_, _aB_, _ab_. In precisely the same way the rest of the pollen cells of the forms _Ab_, _aB_, _ab_ unite with all the other egg cells. We obtain therefore-- _AB_/_AB_ + _AB_/_Ab_ + _AB_/_aB_ + _AB_/_ab_ + _Ab_/_AB_ + _Ab_/_Ab_ + _Ab_/_aB_ + _Ab_/_ab_ + _aB_/_AB_ + _aB_/_Ab_ + _aB_/_aB_ + _aB_/_ab_ + _ab_/_AB_ + _ab_/_Ab_ + _ab_/_aB_ + _ab_/_ab_, or _AB_ + _ABb_ + _AaB_ + _AaBb_ + _ABb_ + _Ab_ + _AaBb_ + _Aab_ + _AaB_ + _AaBb_ + _aB_ + _aBb_ + _AaBb_ + _Aab_ + _aBb_ + _ab_ = _AB_ + _Ab_ + _aB_ + _ab_ + 2_ABb_ + 2_aBb_ + 2_AaB_ + 2_Aab_ + 4_AaBb_[42]. [42] [In the original the sign of equality (=) is here represented by +, evidently a misprint.] 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 Axis Colour of Form of combinations the unripe pods. the ripe pods.