HYBRIDISATION. 9T9 



generation ; though this phenomenon probably depends less on the sexual feebleness 

 of hybrids than on the circumstance that their flowers have probably been generally 

 fertilised with their own pollen, instead of being pollinated from other flowers or 

 other individuals of the same hybrid. Nageli's rule holds true in the general way, 

 that the male organs of species-hybrids are functionally weak to a higher degree 

 than the female organs, although the rule is not without exceptions. 



9. * Hybrids usually vary less in the first generation, the less the degree of 

 affinity between their parent-forms ; species-hybrids therefore less than variety- 

 hybrids ; the former are often characterised by a great uniformity, the latter by a 

 great variability. When hybrids are self-fertilised, the variability increases in the 

 second and succeeding generations the more completely it was absent from the first; 

 and three diff"erent varieties arise more certainly the less the affinity between the 

 parent-forms; viz. one corresponding to the original (hybrid) type, the two others 

 bearing a greater resemblance to the two parent-forms. But these varieties show 

 but little constancy, passing easily into one another, at least in the earlier genera- 

 tions. An actual reversion to one of the two parent-forms (with pure breeding-in) 

 takes place especially when the parent-forms are very nearly related, as in variety- 

 hybrids and those from species that approximate to varieties. When this reversion 

 occurs in other species-hybrids, it appears to be limited to those cases where one 

 of the parent-species exercised a preponderating influence in the hybridisation.' 

 (Nageli, /. c.) 



10. When a hybrid is made to unite with one of its parent-forms, or with 

 another parent-form, or with a hybrid of diff'erent origin, the product is termed a 

 ' derivative hybrid ;' and this may again on its part unite with one of the parent- 

 forms or with a hybrid of diff'erent origin. When a union is eff"ected between a 

 hybrid and one of its own parent-forms, and the hybrid thus obtained unites again 

 with the same parent-form, and so on through several generations, the derived 

 progeny approach more and more nearly in their characters to those of this parent- 

 form, until they come to resemble it in all respects. According as one or the other 

 of the parent-forms is taken, a larger or smaller number of generations are required 

 to eff"ect the perfect reversion ; and this behaviour has been reduced by Nageli to a 

 numerical expression (formula of heredity), which indicates in numbers the amount 

 of influence exercised by a species in reference to the hereditary transmission of 

 its qualities in hybridisation. In proportion as the derivative hybrid approaches 

 one or the other of its parent-forms, its hybrid nature gradually decreases, and its 

 fertility at the same time increases. 



When a hybrid unites with a new parent-form or with a hybrid of another 

 species, a derivative hybrid results in which three, four, or more species (or varieties) 

 are combined ; Wichura has united as many as six diff'erent species of Willow in one 

 such derivative hybrid. Hybrids of this kind, which may conveniently be termed 

 * combined hybrids,' usually follow the same rules with reference to their form and 

 other characters as hold good in the case of simple hybrids. Combined hybrids 

 become less fertile the larger the number of diff'erent parent-forms that are united in 

 them ; and they are usually very variable. Wichura showed, from his own observa- 

 tions and those of Gartner, that hybrid pollen produces a greater variety of forms 

 in its progeny than does the pollen of true species. 



