THE BREAKDOWN OF CONTINUITY 



sorting them out in such a way as to produce the differentiation 

 between races in respect of interchange which already occurs 

 between the species stratnotiiHm and nictcloidcs, for example. 



TABLE 26 



DISTRIBUTION OF INTERCHANGE TYPES IN NATURAL 

 STRAINS OF DATURA STRAMONIUM AS REVEALED 

 BY CROSSING WITH A STANDARD TYPE (PT2) 

 (BLAKESLEE, et al., 1937) 



PT = Prime Type. 



In the cross-pollinating Campanula persicifolia, on the other hand, 

 one basic type occurs throughout Europe, and the numerous 

 interchanges occur singly as heterozygotes in individuals. By 

 crossing six of these successively a ring of I2 chromosomes can be 

 built up step by step. Hence it seems that interchanges which are 

 immediately fcced in an inbreeder (or by inbreeding) can be pre- 

 served floating for a great length of time in an outbreeder. 



With this comparison in mind we can understand what happened 

 to Oenothera, for there is a transition from the outbreeding western 

 species with floating interchanges, such as organensis of Arizona, to 

 the inbreeding eastern species, such as novae-scotiae, in which the 

 interchanges are fixed, but fixed as heterozygotes. It is evident that 

 the transition from outbreeding to inbreeding has been combined 

 with the fixation of the interchanges in the heterozygous condition. 



316 



