784 LECTURE XLIV. 



formed into one another; an actual reversion to one of the two parent-forms 

 (the breeding being purely in and in) occurs chiefly when the parent-forms are very 

 nearly related, as in the case of hybrids between varieties and species which resemble 

 the varieties. When it occurs with other species-hybrids it appears to be confined 

 to those cases where one species has exerted a dominant influence in the hybrid 

 fertilisation ' (Naegeli). 



(10) If a hybrid is sexually united with one of its parent-forms, or with another 

 parent-form, or with a hybrid of different descent, a dcrivalive hybrid arises, which in its 

 turn again can be united with one of the parent-forms or with hybrids of other descent. 

 If the union of a hybrid with one of its parent-forms is accomplished, and if the 

 derivative hybrid thus obtained is again united with the same parent-form, and this 

 continued through several generations, the derived descendants acquire more and more 

 the peculiarities of the one parent-form, and at last completely resemble this, the 

 derivative hybrid reverting to the parent-form employed in the process. According 

 as the one or the other of the two parent-forms is used, more or fewer genera- 

 tions are necessary for the derivative hybrid to become like the one parent-form; 

 from this behaviour Naegeli has deduced certain numerical expressions (formulae of 

 inheritance) which give in figures the amount of the influence of one species with 

 respect to the inheritance of the properties in hybridisation. In proportion as the 

 derivative hybrid approximates to the one parent-form its hybrid nature diminishes 

 more and more, and its fertility especially is increased. 



If a hybrid is sexually united with a new parent-form or with a hybrid of another 

 species, there arises a derivative hybrid in which three, four, or more species (or 

 varieties) are fused; Wichura has combined even six different species of Willow into a 

 derivative hybrid. Such hybrids, which may probably be better designated compound 

 hybrids, follow, with respect to their form and other relations, in general the rules 

 given for the simplest hybrids ; the compound hybrids are so much the more sterile 

 the more diff"erent parent-forms are united in them, and they are also generally very 

 variable. Wichura showed from his own and Gartner's observations that the progeny 

 from the pollen of the hybrids are more variable (more multi-formed) than those from 

 the pollen of true species. 



In the preceding, hybridisation has only been regarded from its theoretical 

 side : that it has practical value also is shown by the innumerable hybrids of 

 cultivated flowering-plants which have long adorned our gardens, and which are 

 increased every year by new forms. Perhaps the most important of all hybrid plants 

 practically however are the hybrid forms of the Vine ; except the W" illows {Salix) 

 there is scarcely any other genus of plants whose species are so easily crossed as 

 those of the genus Vitis. The Vine cultivated in Europe, Vitis znnifera, comes from 

 western Asia: numerous other species grow wild in N. America. Since the time when 

 the Vine-insect {Phylloxera) began to devastate the vineyards, especially in France, the 

 American vines and their hybrids, together and with our old cultivated species, have 

 obtained great practical importance, because several of them withstand the attacks of 

 the Vine-insect, and present to the Vine-growers a means for further culture. I\Iy 

 friend A. INIillardet, Professor of Botany in Bordeaux, has been engaged for ten years 

 with the study of the devastation caused by the Vine-insect in the French vinejards, 

 and has especially investigated the question of the resistance of the American species 



