112 BREEDING 



gives rise to the purple spots is situate in the 

 other. 



The second hybrid generation produced by the 

 self -fertilisation of the hybrids just described shows 

 at once what the pairs of characters involved in this 

 cross are. It consists of nine plants, the seed- coats 

 of which bear both " maple " and " purple spot," 

 three with " maple " seed- coats only, three with 

 " purple spot " only, and one with neither " maple " 

 nor " purple spot," but a pale homogeneous grey 

 coat, amongst every sixteen plants, on the average. 

 The cases of a 9 : 3 : 3 : 1 ratio in the second hybrid 

 generation, which have already been described, at 

 once show what the two pairs of characters are. In 

 these cases, it will be remembered, the individuals 

 which occur in the ratio of nine possess two dominant 

 characters ; the two lots, which occur in the ratio of 

 three, possess respectively the dominant member of 

 one pair and the recessive of the other ; whilst the 

 individuals, which occur in the ratio of one, possess 

 the recessive members of both pairs. The two pairs 

 of characters in the instance under consideration are, 

 therefore, " maple " (dominant) and the absence of 

 " maple " (recessive) ; and " purple spot " (dominant) 

 and the absence of " purple spot " (recessive). Cases 

 such as this have given rise to the theory that one 

 (the dominant) of the two characters, which make 

 a pair, consists in the presence of something, and 

 that the other (the recessive) consists in the absence 

 of that something. The theory has been called the 

 Presence and Absence hypothesis. The reader who 



