m. 



Gen; 



DIV. ii PHYSIOLOGY 321 



gated into pure red and pure white sexual elements. In the process of 

 fertilisation the union producing a hybrid, red x white (white ? x red $ , 

 red 9 x white $ ), will occur twice as frequently as the union red x red 

 or white x white, which give rise to pure forms. 



2. RULE OF DOMINANCE. The characters in which the parents 

 differ do not, however, always blend so that the hybrid exhibits 

 an intermediate character. More usually the hybrids completely 

 resemble in this respect either the paternal or maternal parent, the 

 character of the one parent being dominant in the hybrid while the 



A urtica A 



^^ Dodartii + pilulifera J Ik 



iP A 



pilulifera ^fl B^ Dodartii 



J| IfcGen. 

 ^^^^^ i 



^/^^ "^^^ *^^^^^ ^^^^ 



ill) lit) iiii ilil 



FIG. 269. The hybrid between Urtica pilulifera and Urtica Dodartii in three generations. 

 (Diagram. After COBHEXS.) 



other remains latent. This is the case, for example, in hybrids 

 between Urtica pilulifera with serrate leaves and U. Dodartii (Fig. 

 269). The hybrids have all serrate leaves like U. pilulifera. so 

 that in the second generation the proportion of serrate-leaved to 

 entire-leaved individuals is per cent 75:25 (3:1). Only 50 per 

 cent of the serrate-leaved individuals are, however, of hybrid nature 

 and continue to show a similar splitting of characters in the next 

 generation ; 25 per cent have become pure U. pilulifera. It is 

 impossible to predict which characters will prevail in any cross, and 

 the question can only be settled by experiment ; usually the phylo- 

 genetically younger character appears to be dominant. 



With regard to the above example of dominance CORRENS ( 83 ) has 



Y 



n. 



Gen. 



