MENDELISM 67 



sometimes to be most strongly attracted by egg-cells 

 of different constitution to its own, and sometimes male 

 and female reproductive cells of the same constitution 

 refuse altogether to unite. 



A case of somewhat the same nature as that of the 

 yellow mice has lately been described with regard to one 

 of the mutants of Lamarck's Evening Primrose., It 

 seems that the pollen-grains bearing certain characters, 

 and the egg-cells bearing others, shrivel up and die, so 

 that the inheritance of certain characters must take 

 place through either pollen-grain or egg-cell only. It 

 is believed that the particular form in question is really 

 a hybrid which " breeds true," owing to the fact that 

 the pollen-grains and egg-cells which reach maturity 

 have necessarily different constitutions. 



A very puzzling case of what is termed " sex-limited " 

 inheritance has been noted with regard to the peculiar 

 pathological condition in man known as " bleeding." 

 An affected person bleeds profusely and continuously 

 from even the smallest wound, owing to the fact that 

 the blood, for some obscure reason, does not coagulate. 

 The condition affects only men. The sons of bleeders 

 are not themselves affected, and the children of the 

 sons also do not show the condition. It appears only 

 in the sons of daughters of bleeders. Here, then, we 

 have a condition which affects only the male, and is 

 inherited only through the female. The mode of in- 

 heritance appears to be Mendelian, but the reason of 

 the complication is not known. 



We ma,y here mention the question of the causation 

 of sex itself. It will be evident that the proportions 

 of the sexes usually found, viz. 50 per cent, of each, are 

 what result when a hybrid dominant is mated with a 

 recessive. It appears likely that one of the sexes is 

 produced by the presence of a dominant factor in the 

 hybrid condition, while the other sex represents the pure 

 recessive. As to which sex is the dominant, the evidence 

 from different kinds of organisms is contradictory. The 

 state of affairs probably differs in different types. But 

 the question is admittedly not yet fully understood. 



