126 BREEDING 



are, dominance is not simply due to the fact that 

 the character in question belongs to the wild species ; 

 and that some other cause must be sought for. 



This is suggested by the second theory. The 

 reader will remember that, in order to explain the 

 result of crossing a pea with a maple seed-coat with a 

 pea with a purple-spotted one, we invented the theory 

 that, in this case, the pair of characters did not 

 really consist of two characters, like yellow and green, 

 but of a particular character, maple, as the dominant 

 member, and the mere absence of this character as 

 the recessive member of the pair. And the impres- 

 sion may have been given that this theory was only 

 invoked to explain an outstanding case. So far 

 from this being the case, this theory, which is called 

 the Presence and Absence hypothesis, is now being 

 applied to all the other Mendelian characters. In 

 many cases the fitness of this application is at once 

 obvious. In the case of eye-colour in man, for instance, 

 the duplex character which is dominant, is due to the 

 presence of the brown pigment, and the recessive 

 character, simplex, is due to the absence of this pig- 

 ment. Stated in its most general terms, this theory is 

 that the dominant character is due to the presence of 

 something, and the corresponding recessive character 

 is due to the absence of that something. It will be 

 perceived that this conception of the constitution of 

 the Mendelian pair of characters differs profoundly 

 from that which we should naturally derive from a 

 contemplation of, for instance, the seven pairs of 

 characters studied by Mendel. In all these seven 



