PAIRS OF CHARACTERS 137 



useless. But if it stimulates an investigation of 

 the nature of characters described as Mendelian, it 

 is justified. 



How far the truth of this hypothesis is borne 

 out by the available evidence is an entirely different 

 question, which will now be discussed. There is a 

 large number of pairs of characters to which this 

 theory obviously applies. There are also a number of 

 cases, like cotyledon shape and colour, which this 

 theory correctly describes, though the manner of 

 application is not at first sight easy to see. But 

 there is also a considerable number of cases which 

 constitute obstacles in the way of its general appli- 

 cation. For instance, there are certain cases where 

 apparently the same character is dominant in one 

 animal, or plant, and recessive in another. For 

 instance, the possession of horns is dominant in 

 sheep, but recessive in cattle, to the hornless con- 

 dition ; white in pigs appears to be dominant to 

 black, but recessive to it in sheep ; and, lastly, the 

 dissection of the margin of the leaf is dominant 

 to the entire condition of the edge in the nettle, 

 and recessive to it in Chelidonium. It is doubtful 

 in this last case whether the condition of entirety 

 is of the same nature in the two cases ; but the 

 possession of horns in cattle and sheep certainly 

 would seem to be. 



Then there is a further set of cases, in which the 

 dominant character seems to consist in the absence 

 of something and the recessive in the 'presence of 

 that something. For instance, the possession of 



