168 OBSERVATIONS UPON MULING AMONG PLANTS, 



of the really specific difference between the two. In the case of 

 Lychnis and Agrostemma, though seeds were formed containing 

 apparently perfect embryos, not a single one germinated, showing 

 some weakness of constitution in the result of the union, of which 

 many instances occur in these researches. Union had certainly 

 taken place ; for had the seeds been due to the access of homo- 

 geneous pollen, there could be no reason why they should not 

 have germinated, assuming them to be perfectly developed. 

 The union of Rhododendron and Rhodora with Azalea, and of 

 Mr. Herbert's two divisions of JYerine, are instances of bigeneric 

 fecundation. The constitutional weakness of the result of the 

 former union of Rhododendron with the yellow or orange 

 Azalea has a close analogy with that of Lychnis and Agro- 

 stemma just mentioned. These perhaps are the only well- 

 authenticated instances of bigeneric union ; those of Cucumis 

 and Melo, Cheira?ithus and Matthiola, Brassica and Raphanus, 

 Lychnis and Saponaria, Pisum and Vicia, and some others, 

 all seem to be more or less uncertain in some part of their 

 history, into which we have not room to enter. 



On the whole then it appears that the fitness for union depends 

 not on the agreement of outward generic characters, but on the 

 inward nature of the procreative elements of individuals ; so that 

 two kinds of affinity exist, not necessarily proportionate or con- 

 current — an outward systematic and an inward sexual affinity. 

 At the same time we must remember that genera are confessedly 

 artificial, and that in groups undoubtedly belonging to one 

 common type, it does not follow that because one species will 

 hybridize, another will. In some cases where union will not 

 take place, the species belong to different natural groups, and 

 have been separated as such under generic names. The rough- 

 seeded CEnotherce will not unite with those which have smooth 

 seeds, the species of Datura with smooth fruit with those which 

 have thorny fruit, the species of Erica with cylindrical blossoms 

 with those which have bell-shaped corollae ; Primulce with a 

 valvular orifice, as P. acaulis, with those which, like Auriculce, 

 have the throat pervious; blue with yellow-flowered Linum; 

 and so in many other cases. 



The intimate agreement on which aptness for fecundation 

 depends is indeed usually concurrent with external resemblance, 

 but not necessarily so. This is what Mr. Herbert calls similarity 

 of constitution, and which our author terms sexual affinity, or 

 elective affinity (Wahlverwandschaft). The procreative fitness 

 then of a species, which indeed sometimes is confined to in- 

 dividuals, is not to be determined from outward signs, but from 

 direct and often repeated experiment. 



It should be noticed that agreement in the time of flowering 



