vm WILD AND DOMESTIC VARIETIES 85 



In respect of the evolution of its now multitudinous 

 varieties, the story of the sweet pea is clear and straight- 

 forward. These have all arisen from the wild by a pro- 

 cess of continuous loss. Everything was there in the 

 beginning, and as the wild plant parted with factor after 

 factor there came into being the long series of derived 

 forms. Exquisite as are the results of civilization, it is 

 by the degradation of the wild that they have been. 

 brought about. How far are we justified in regarding 

 this as a picture of the manner in which evolution works ? 



There are certainly other species in which we must 

 suppose that this is the way that the various domesti- 

 cated forms have arisen. Such, for example, is the case 

 in the rabbit, where most of the colour varieties are re- 

 cessive to the wild agouti form. Such also is the case 

 in the rat, where the black and albino varieties and the 

 various pattern forms are also recessive to the wild agouti 

 type. And with the exception of a certain yellow variety 

 to which we shall refer later, such is also the case with the 

 many fancy varieties of mice. 



Nevertheless there are other cases in which we must 

 suppose evolution to have proceeded by the interpola- 

 tion of characters. In discussing reversion on crossing, 

 we have already seen that this may not occur until the F2 

 generation, as, for example, in the instance of the fowls' 

 combs (cf. p. 65). The reversion to the single comb 

 occurred as the result of the removal of the two factors 



