CHAPTER VIII 



WILD FORMS AND DOMESTIC VARIETIES 



In discussing the phenomena of reversion we have 

 seen that in most cases such revei"sion occurs when 

 the two varieties which are crossed each contain 

 certain factors lacking in the other, of which the 

 full complement is necessary for the production 

 of the reversionary wild form. This at once suggests 

 the idea that the various domestic forms of animals 

 and plants have arisen by the omission from time 

 to time of this factor or of that. In some cases 

 we have clear evidence that this is the most 

 natural interpretation of the relation between the 

 cultivated and the wild forms. Probably the species 

 in which it is most evident is the sweet-pea {Latkyrus 

 odoratus). We have already seen reason to suppose 

 that as regards certain structural features the Bush 

 -variety is a wild lacking the factor for the pro- 

 cumbent habit, that the Cupid is a wild without 

 the factor for the long internode, and that the 

 Bush Cupid is a wild minus both these factors. Nor 

 is the evidence less clear for the many colour 

 varieties. In illustration we may consider in more 

 detail a case in which the cross- between two whites 



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