204 Double Flowers and Sex-Linkage in Begonia 



postponed until reduction has been already undergone. The problem 

 is not unlike that so often raised by the differentiation of parts com- 

 posing a meristic series. In one Lizard the nth vertebra carries the 

 pelvis and undergoes special modification. In another Lizard the ver- 

 tebra thus differentiated is the n 4- mth in ordinal series. Morphologists 

 have long discussed whether in allotting homologies among vertebrae 

 we should be guided by the differentiations, or by the ordinal positions. 

 When once the true nature of segregation and differentiation is under- 

 stood the question is seen to lose all significance \ and having no precise 

 meaning is incapable of being answered. For the individuality of the 

 segments is not respected or maintained in variation, nor are differ- 

 entiation and nuQierical change necessarily interdependent. We may 

 easily satisfy ourselves that the numbers may vary and that within con- 

 siderable though unascertained limits the functions and differentiations 

 of the segments may be redistributed. I can scarcely doubt that we 

 must similarly interpret the series of divisions and differentiations of 

 which the life-cycles consist. 



In the Tables we represent the plants as of five classes. Singles are 

 those in which the male flowers have not been seen to have more than 

 the four normal petals. The slightly petalodic class have generally one 

 or two, though occasionally rather more extra petals or petalodic anthers. 

 These two classes cannot be quite strictly instituted, and plants having 

 flowers of both kinds are common. The half-double class ranges from 

 the slightly petalodic to the really double, but nevertheless it is a fairly 

 uniform class. Doubles and full doubles are not essentially distinct, but 

 the termfidl was applied only to flowers in which the petals were very 

 numerous and close. 



As was stated above, peculiar and transitional forms are common. 

 In particular some difficulty is caused by structures consisting of female 

 and male flowers imperfectly resolved from each other^. Such flowers 

 can generally be recognized by examination of the bracts, but when 

 this condition of imperfect resolution is combined with some degree 

 of petalody the degree of doubling cannot be determined with much 

 confidence. 



Since double flowers stand terminally, that is to say in the male 

 position, we supposed at the beginning of these experiments that double 

 flowers were necessarily petalodic males. Happening however to examine 



^ See Problems of Genetics, p. 66. 



2 Noticed by Bond, Jour. Gen. iv. 1915, p. 341. 



