VIII WILD AND DOMESTIC VARIETIES 8i 



In respect of the evolution of its now multi- 

 tudinous varieties, the story of the sweet -pea is 

 clear and straightforward. These have all arisen 

 from the wild by a process 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 civilisation, it is by the degrada- 

 tion 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 

 domesticated forms have arisen. Such, for example, 

 is the case in the rabbit, where most of the colour 

 varieties are recessive 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 reces- 

 sive to the wild agouti type. And with the excep- 

 tions of a pied form and 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 

 interpolation of characters. In discussing reversion 

 on crossing, we have already seen that this may not 

 occur until the P., generation, as, for example, in the 

 instance of the fowls' combs (cp. p. 59). The rever- 

 sion to the single comb occurred as the result of the 

 removal of the two factors for rose and pea. These 

 two domesticated varieties must be regarded as each 

 possessing an additional factor in comparison with 

 the wild single-combed bird. During the evolution 



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