i82 BREEDING 



This type of mating is not one of any practical 

 value, but its existence should be borne in mind, 

 because superficially the results of it are indistin- 

 guishable from the result of mating two dominants 

 (DD and DD) in those cases in which the character 

 of the hybrid is the same as that of the dominant ; 

 and if this fact is not borne in mind the false con- 

 clusion might be drawn from the results of breeding, 

 that a dominant character had become fijs:ed when 

 it really had not. Take, for instance, the duplex 

 eye in man ; the result of mating a hybrid duplex 

 with a pure one is, as far as we can see from the 

 immediate result of the mating, the same as that of 

 mating two pures, namely, the production of nothing 

 but duplexes ; the fact that half of them are hetero- 

 zygous, or hybrid, and half homozygous, or pure, 

 can only be determined by subsequent breeding. And 

 so long as DR is mated with DD, and for however 

 many generations, the apparent result will be the 

 same as mating DD with DD. What, then, it may 

 be asked, does it matter whether DD is mated with 

 DR or with DD, if the result is the same ? The 

 answer is that it does not matter provided that the 

 union of DR x DD can be ensured, but as this is a 

 laborious matter (involving as it does the testing 

 of both forms before they are mated) the union 

 DR X DR is sure to occur sooner or later ; and then 

 the fat is in the fire, and a quarter of the generation 

 produced consists of recessives. 



Mating of type No. 6 is represented in the four- 

 square Table on the next page. 



