HYBRIDISA TION. 8 Ji I 



function is in some variety-hybrids scarcely perceptible, in others but small ; in 

 general it is the more marked the more distant the genetic and sexual aflmity of 

 the parent- forms. When species-hybrids have the power of producing seeds by self- 

 pollination, and this is repeated in the progeny, their fertility generally diminishes 

 from generation to generation ; though this phenomenon probably depends less on 

 the sexual feebleness of hybrids than on the circumstance that their flowers have 

 probably been generally self-fertilised, instead of being pollinated by 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. 



' 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 difl'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 generations. An actual re- 

 version to one of the two parent-forms (with pure breeding-in) takes place especi- 

 ally 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). 



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

 parent-form, or with a hybrid of different 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 different origin. When a union is effected 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 ap- 

 proach 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 

 effect 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 varie- 

 ties) are combined ; Wichura has even united six different 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 different parent-forms that are united in 



