VI REVERSION 6s 



perhaps the commonest mode of reversion, but instances 

 are known in which the reversion that occurs when two 

 pure types are crossed does not appear until the F2 

 generation. Such a case we have already met with in the 

 fowls', combs. It will be remembered that the cross be* 

 tween pure pea and pure rose gave walnut combs in Fi, 

 while in the Fg generation a definite proportion, i in 16, 

 of single combs appeared (cf. p. 32). Now the single 

 comb is the form that is found in the wild jungle fowl, 

 which is generally regarded as the ancestor of the domestic 

 breeds. If this is so, we have a case of reversion in F2 ; 

 and this in the absence of the two factors brought together 

 by the rose-comb and pea-comb parents. Instead of the 

 reversion being due to the bringing together of two com- 

 plementary factors, we must regard it here as due to the 

 association of two complementary absences. To this 

 question, however, we shall revert later in discussing the 

 origin of domesticated varieties. 



There is one other instance of reversion to which we 

 must allude. This is Darwin's famous case of the oc- 

 casional appearance of pigeons reverting to the wild blue 

 rock (Columba livia), when certain domesticated races are 

 crossed together. As is well knov^n, Darwin made use of 

 this as an argument for regarding all the domesticated va- 

 rieties as having arisen from the same wild species. The 

 original experiment is somewhat complicated, and is 

 shown in the accompanying scheme. Essentially it lay in 



