46 THE FAMILY AND THE NATION 



from both sides, and, being pure-bred with regard to 

 that character, will show it openly. 



The more complicated phenomena which have been 

 traced in plants and animals when two characters are 

 coupled, or are incompatible, have also appeared in the 

 study of mankind, and, when more is known, will 

 probably explain cases of inheritance which are now too 

 involved for elucidation. 



Night-blindness, an inability to see in a faint light, 

 affects men more often than women. It is transmitted 

 by affected men, but not by unaffected men. It is, how- 

 ever, often transmitted by unaffected women. Apparently 

 normal women, sisters of affected men, may transmit the 

 peculiarity to some of their sons, but only if they marry 

 a night-blind husband can they give it to their daughters. 



This sex-limited descent is to be compared with the 

 inheritance of horns in sheep, which we have described 

 above. In sheep, horns are dominant in rams and 

 recessive in ewes. If the rams contain the factor at all 

 they exhibit it ; hence they can transmit the character 

 if they themselves show it, but not otherwise. In the 

 ewes, some other factor opposes the development of 

 horns, and horns only appear if the tendency to form 

 them is born in the ewe from both its parents. Some 

 ewes who do not themselves show horns may never- 

 theless transmit them to their male lambs. Similarly, 

 night-blindness must be regarded as a dominant char- 

 acter in men, but a recessive character in women. 



The characters hitherto considered have been definite 

 and simple — a man is or is not brachydactylous or night- 



