HYBRIDISATION. 
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,/...) 
10. 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 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 effect the perfect reversion ; and this behaviour has been reduced by Nägeli 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. 
W^hen 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 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 
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. 
