CHAPTER VIII 



WILD FORMS AND DOMESTIC VARIETIES 



IN discussing the phenomena of reversion we have 

 seen that in most cases such reversion 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 culti- 

 vated and the wild forms. Probably the species in 

 which it is most evident is the sweet pea (Lathyrus odora- 

 tus). We have already seen reason to suppose that as 

 regards certain structural features the Bush variety is 

 a wild lacking the factor for the procumbent habit, that 

 the Cupid is a wild without the factor for the long inter- 

 node, 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 de- 

 tail a case in which the cross between two whites resulted 



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