vi REVERSION 59 



cumbent plant with long internodes. This is the 

 ordinary tall sweet-pea of the wild Sicilian type, 

 reversion here, again, being due to the bringing 

 together of two complementary factors which had 

 somehow become separated in the course of evolution. 



To this interpretation it may be objected that 

 the ordinary sweet-pea is a plant of upright habit. 

 This, however, is not true. It only appears so 

 because the conventional way of growing it is to 

 train it up sticks. In reality it is of procumbent 

 habit, with divergent stems like the ordinary Cupid, 

 a fact which can easily be observed by any one who 

 will watch them grow without the artificial aid of 

 prepared supports. 



The cases of reversion with which we have so 

 far dealt have been cases in which the reversion 

 occurs as an immediate result of a cross, i.e. in the 

 F : generation. This is 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 F 2 

 generation. Such a case we have already met 

 with in the fowls' combs. It will be remembered 

 that the cross between pure pea and pure rose gave 

 walnut combs in F p while in the F 2 generation a 

 definite proportion, I in 16, of single combs 

 appeared (cf. p. 30). 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 

 F 2 ; 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 



