Experimentally, therefore, the theory is justified _that the pea hybrids form egg and pollen cells which, in their constitution, represent in equal numbers all constant forms which result from the combination of the characters when united in fertilisation_. The difference of the forms among the progeny of the hybrids, as well as the respective ratios of the numbers in which they are observed, find a sufficient explanation in the principle above deduced. The simplest case is afforded by the developmental series of each pair of differentiating characters. This series is represented by the expression _A_ + 2_Aa_ + _a_, in which _A_ and _a_ signify the forms with constant differentiating characters, and _Aa_ the hybrid form of both. It includes in three different classes four individuals. In the formation of these, pollen and egg cells of the form _A_ and _a_ take part on the average equally in the fertilisation; hence each form [occurs] twice, since four individuals are formed. There participate consequently in the fertilisation-- The pollen cells _A_ + _A_ + _a_ + _a_ The egg cells _A_ + _A_ + _a_ + _a_. It remains, therefore, purely a matter of chance which of the two sorts of pollen will become united with each separate egg cell. According, however, to the law of probability, it will always happen, on the average of many cases, that each pollen form _A_ and _a_ will unite equally often with each egg cell form _A_ and _a_, consequently one of the two pollen cells _A_ in the fertilisation will meet with the egg cell _A_ and the other with an egg cell _a_, and so likewise one pollen cell _a_ will unite with an egg cell _A_, and the other with egg cell _a_. Pollen cells _A_ _A_ _a_ _a_ | \ / | | \ / | | x | | / \ | | / \ | \|/ \/ \/ \|/ Egg cells _A_ _A_ _a_ _a_ The result of the fertilisation may be made clear by putting the signs for the conjoined egg and pollen cells in the form of fractions, those for the pollen cells above and those for the egg cells below the line. We then have _A_/_A_ + _A_/_a_ + _a_/_A_ + _a_/_a_.