in Hybridisation 355 



different characters in hybrid union is independent of the 

 other differences in the two original parental- stocks. 



\tn represent the number of the differentiating characters 

 in the two original stocks, 3" gives the number of terms of 

 the combination series, 4 n the number of individuals which 

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

 remain constant. The series therefore contains, if the 

 original stocks differ in four characters, 3 4 = 8i classes, 

 4 4 = 256 individuals, and 2 4 =i6 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 

 artificial fertilisation . 



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

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

 be stated that the time stands almost exactly between those 

 of the seed and pollen parents, and that the constitution of 

 the hybrids with respect to this character probably follows 

 the rule ascertained in the case of the other .characters. 

 The forms which are selected for experiments of this class 

 must have a difference of at least twenty days from the 

 middle flowering period of one to that of the other; further- 

 more, the seeds when sown must all be placed at the same 

 depth in the earth, so that they may germinate simul- 

 taneously. 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. 



If we endeavour to collate in a brief form the results 

 arrived at, we find that those differentiating characters, 

 which admit of easy and certain recognition in the experi- 

 mental plants, all behave exactly alike in their hybrid 



232 



