192 PLANT HYBRIDIZATION BEFORE MENDEL 



Pollination through one of the two parent species, however, 

 excludes self-fertilization, and the hybrid reverts back to this 

 parental species. The hybrid middle-forms between species have 

 accordingly no constancy, and disappear again after a short time. 

 According to the relationship of the parental forms, they appear 

 in three ways : 



(i) In the species with the most limited relationship: as a 

 middle-form, present in an extremely few quite infertile indi- 

 viduals, without transition-forms to the parental species. 



(2) In species with limited relationship: as a scanty middle- 

 form with restricted fertility, and with individual transition- 

 forms to one or the other of the two parent species. 



(3) In species with close relationship: as a more or less scanty 

 middle-form with partial fertility, and with numerous transition- 

 forms to both the parent species. 



7. There are other intermediate forms, which are distinguished 

 through greater individual numbers, and through complete fer- 

 tility and constancy. They appear in three ways : 



(1) As an isolated middle-form; the gaps between it and the 

 two principal species being mostly tilled up by scanty hybrid 

 transition-forms. 



(2) As two or several isolated middle-forms which lead by 

 degrees from one principal species to another ; the gaps between 

 these and between them and the principal forms being filled up 

 through limited hybrid transition stages. 



(3) An unnoticeable transition series between the two principal 

 species, in which all the members are represented by numerous 

 and completely fertile individuals. The hybridity of these con- 

 stant intermediate forms is apparently evidenced by their occur- 

 rence solely in company with the parental forms. 



In one passage (4c, p. 229), Nageli remarks upon the fact of 

 heterosis in species-hybrids, i.e., the fact that species-hybrids show, 

 in the whole vegetative sphere in the widest sense, 



"... a striking tendency to vegetative luxuriance ; in this respect they 

 ordinarily transcend the two parental varieties." 



A statement of a rather general character regarding species- 

 crosses is made as follows (4e, p. 260) : 



"when two species together form a hybrid, the characters in which 

 the parents differ from one another do not go over to it complete, but 



