172 OBSERVATIONS UPON MULING AMONG PLANTS. 



and female procreative energy do not keep an equal pace with 

 each other, though this difference has no influence on the typical 

 form of the hybrids which arise from the union. Still the 

 shorter or longer period necessary for the transformation of 

 one pure species into another by hybrid fecundation seems to 

 depend in some measure on this difference. 



When two species, such as Nicotiana rustica and glutinosa, 

 do not admit of union with one another, or, as N. paniculata and 

 Tabacum, only of imperfect union, the union may be accom- 

 plished sometimes by a third species which stands in close elective 

 affinity with either of the first. Thus Nicotiana rmtico-panicu- 

 lata is completely fructified by the pollen of glutinosa, as also 

 paniculato-rustica with that of Tabacum. This affinity is called 

 by the author compensating or mediate affinity (Vermittelnden 

 Verwandschaft). The peculiarity in these unions is, that the 

 consequent hybrids are generally so like the type of the com- 

 pensating species that they can be considered merely as varieties 

 of this latter which furnished the fecundating pollen, and are 

 for the most part perfectly barren. 



Another example may be given from the genus Dianthus, in 

 which D. barbatus and caryophyllus unite imperfectly, whereas 

 D. barbato-chinensis easily hybridizes with caryophyllus S , 

 D. chinensis being in this case the intermediate member. 



It was observed before that the difference of the number of 

 seeds is proportionate to the degree of elective affinity. Our 

 author avails himself of this as a means of estimating this degree 

 in the several species. It may not be uninteresting to give a 

 table of the affinities of a single species, though we cannot enter 

 upon various questions which arise as to the propriety of this 

 mode of estimation. Taking therefore normal impregnation as 

 unity, we have — 



9 Dianthus barbatus 6 proprio polline 1,0000 



superbus 0,8111 



Japonicus 0,6666 



Armeria .... . . 0,5333 



barbato-Carthusianorum . . 0,3111 



Chinensis ...... 0,2600 



collinus 0,2333 



deltoides . . . . 4 . . 0,2222 



Chinensis latif. Schr. . . . 0,1354 



Carthusianorum . . . . 0,1111 



prolifer 0,0333 



virgineus 0,0111 



pulchellus 0,0096 



arenarius 0,0084 



diutinus 0.0033 



