In the first and fourth term the egg and pollen cells are of like kind, consequently the product of their union must be constant, viz. _A_ and _a_; in the second and third, on the other hand, there again results a union of the two differentiating characters of the stocks, consequently the forms resulting from these fertilisations are identical with those of the hybrid from which they sprang. _There occurs accordingly a repeated hybridisation._ This explains the striking fact that the hybrids are able to produce, besides the two parental forms, offspring which are like themselves; _A_/_a_ and _a_/_A_ both give the same union _Aa_, since, as already remarked above, it makes no difference in the result of fertilisation to which of the two characters the pollen or egg cells belong. We may write then-- _A_/_A_ + _A_/_a_ + _a_/_A_ + _a_/_a_ = _A_ + 2_Aa_ + _a_. This represents the average result of the self-fertilisation of the hybrids when two differentiating characters are united in them. In solitary flowers and in solitary plants, however, the ratios in which the forms of the series are produced may suffer not inconsiderable fluctuations[41]. Apart from the fact that the numbers in which both sorts of egg cells occur in the seed vessels can only be regarded as equal on the average, it remains purely a matter of chance which of the two sorts of pollen may fertilise each separate egg cell. For this reason the separate values must necessarily be subject to fluctuations, and there are even extreme cases possible, as were described earlier in connection with the experiments on the form of the seed and the colour of the albumen. The true ratios of the numbers can only be ascertained by an average deduced from the sum of as many single values as possible; the greater the number the more are merely chance elements eliminated. [41] [Whether segregation by such units is more than purely fortuitous could probably be determined by seriation.] The developmental series for hybrids in which two kinds of differentiating characters are united contains among sixteen individuals nine different forms, viz., _AB_ + _Ab_ + _aB_ + _ab_ + 2_ABb_ + 2_aBb_ + 2_AaB_ + 2_Aab_ + 4_AaBb_. Between the differentiating characters of the original stocks _Aa_ and _Bb_ four constant combinations are possible, and consequently the hybrids produce the corresponding four forms of egg and pollen cells _AB_, _Ab_, _aB_, _ab_, and each of these will on the average figure four times in the fertilisation, since sixteen individuals are included in the series. Therefore the participators in the fertilisation are-- Pollen cells _AB_ + _AB_ + _AB_ + _AB_ + _Ab_ + _Ab_ + _Ab_ + _Ab_ + _aB_ + _aB_ + _aB_ + _aB_ + _ab_ + _ab_ + _ab_ + _ab_. Egg cells _AB_ + _AB_ + _AB_ + _AB_ + _Ab_ + _Ab_ + _Ab_ + _Ab_ + _aB_ + _aB_ + _aB_ + _aB_ + _ab_ + _ab_ + _ab_ + _ab_.