SECONDARY BALANCE 327 



This distribution is different from that characteristically found 

 in simple triploids already discussed where intermediate numbers 

 are eliminated. The favouring of seedlings with seven extra 

 chromosomes suggests that these are the seven that make up the 

 primary set whose existence is inferred on other grounds. 



In Nicotiana (x = 12) also the progeny of triploids having 12 

 bivalents and 12 univalents do not show the sharp elimination found 

 in simple triploids. In these cases the middle number is favoured 

 (Table 48, cf. East, 1933) : — 



Table 53 



Distribution of extra chromosomes of the triploid Nicotiana 

 paniciilata [n = 12) X N. rustica (n = 24) in the back-crosses to 

 its parents (Lammerts, 1929, 1931). 



♦ Through failure of reduction. Cf. Ch. XI. 



t Gametic selection on male side is determined by relationship to female parent as in QLnothera. 



These results may be explained as due to a lack of differentiation 

 in the chromosomes, or to the set of twelve being derived from an 

 earlier set of six by polyploidy. The latter view is the more 

 probable and is favoured by evidence of secondary pairing in 

 Nicotiana and of the general evolutionary relationship of chromo- 

 some numbers [v. Ch. VI). Thus Nicotiana with an apparent basic 

 number of 12 may be said to have a primary basic number of 6 

 from which the secondary basic number of 12 is in process of develop- 

 ing — without, however, any change in balance in the whole set. 



The second kind of evidence of change of balance in evolution 

 consists in the occurrence of intra-haploid homology, i.e., the 

 reduplication of materials (genes) within the haploid set. This 

 may be inferred from both genetical and cytological observa- 

 tions, which indicate that pairing occurs exceptionally between 

 chromosomes in the same haploid set in the absence of competition, 



