in Hybridisation 65 



of the combination series, 4 71 the number of individuals 

 which belong to the series, and 2 71 the number of unions 

 which remain constant. The series therefore embraces, if 

 the original stocks differ in four characters, 3 4 = 81 of classes, 

 4 4 = 256 individuals, and 2 4 =16 constant forms ; or, which 

 is the same, among each 256 offspring of the hybrids there 

 are 81 different combinations, 16 of which are constant. 



All constant combinations which in Peas are possible by 

 the combination of the said seven differentiating characters 

 were actually obtained by repeated crossing. Their number 

 is given by 2 7 = 128. Thereby is simultaneously given the 

 practical proof that the constant characters which appear in 

 the several varieties of a group of plants may be obtained in 

 all the associations which are possible according to the 

 [mathematical} laws of combination, by means of repeated 

 art ific ial fertilisatio n. 



As regards the flowering time of the hybrids, the ex- 

 periments are not yet concluded. It can, however, already 

 be stated that the period stands almost exactly between 

 those of the seed and pollen parents, and that the con- 

 stitution of the hybrids with respect to this character 

 probably happens in the same way as in the case of the 

 other characters. The forms which are selected for experi- 

 ments of this class must have a difference of at least twenty 

 days from the middle flowering period of one to that of the 

 other; furthermore, the seeds when sown must all be placed 

 at the same depth in the earth, so that they may germinate 

 simultaneously. Also, during the whole flowering period, 

 the more important variations in temperature must be taken 

 into account, and the partial hastening or delaying of the 

 flowering which may result therefrom. It is clear that this 

 experiment presents many difficulties to be overcome and 

 necessitates great attention. 



B. 5 



