VARIATION AND HEREDITY 23 



grains wrinkled and white, each combination appearing 

 in twenty-five per cent of the whole. 



It follows that a nearly equal number of the germ 

 cells of the double hybrid first obtained bear each of 

 the four possible combinations of characters. The dis- 

 tribution of one character does not depend on the 

 presence or absence of the other characters. If a germ 

 cell is yellow, it is an even chance whether it is smooth 

 or wrinkled. The inheritance of the two pairs of 

 characters goes on independently. 



But this independence is not found universally. In 

 some cases two characters are present together if present 

 at all ; they are coupled. In other cases it is found 

 that one character, if present, prevents the acquirement 

 of the other ; something in the characters must be 

 inconsistent, so that they cannot exist together, or can 

 so exist only with difficulty. 



Again, some characters can only manifest themselves 

 in presence of certain other characters, without the 

 existence of which they remain latent. On these lines, 

 the well-known phenomenon in domestic species of 

 reversion to ancestral wild types has been explained by 

 Bateson, in terms of Mendelian conceptions. 



On the other hand, some qualities seem to be in- 

 compatible with others, so that if one is present the 

 other is either absent or is converted from a dominant 

 into a recessive character. The possession of horns in 

 sheep is connected with the sex-factor. In the Dorset 

 horned breed both sexes have horns, in the Suffolks 

 neither. If a Dorset be crossed with a Suffolk, the 

 resulting rams are horned and the ewes hornless, while 

 in the next generation all varieties appear. Thus the 



