326 CHROMOSOMES IN HEREDITY: PHYSIOLOGICAL 



absolute one. Certain kinds of unbalance, like certain gene- 

 mutants, are hardly inferior to the normal condition. The difference 

 between balance and unbalance must therefore be expressed in 

 relative terms thus : The balanced combination of genes has been 

 exposed to natural selection. The unbalanced combination has 

 not ; it is at random, and since the possibilities of random com- 

 bination are vast (most organisms probably having some thousands 

 of differentiated genes) and the number of successful combinations 

 possible in any one environment are few, the chances of a new 

 unbalanced type being as successful as the normal are remote. 



These arguments may be inverted, and we then see natural 

 selection from a different point of view, which is not without value. 

 We may say that natural selection consists in the elimination of 

 unbalanced forms from the body of proportion-mutants and gene- 

 mutants in a species. 



(e) Secondary Balance. From these considerations it follows 

 that the possibility is always open of variation occurring by a 

 change in proportion, either by reduplication of segments of 

 chromosomes or of whole chromosomes. If such a new proportion 

 were successful it would yield a new secondary balance having a 

 different phenotypic expression from the primary, ancestral, type. 

 Evidence of this probably universal type of variation is only 

 beginning to appear, for the reason that the necessary inferences 

 are elaborate ones. 



Where the change of balance is concerned with whole chromosomes 

 its occurrence seems to be confined to polyploids. Thus the 

 i8-series arises in Dahlia secondarily to the 8-series and the 17-series 

 in the Rosaceae secondary to the 7-series (v. Ch. VI). The primary 

 balance in the 17-series is shown by the critical test of triploid 

 progenies. Thus in the progeny of triploid-diploid crosses the 

 following distribution of chromosome number types is found in 

 Pyrus Mains (Moffett, 193 1 ; cf. Nebel, 1933 h) : — 



Chromosomes in excess 



of 34: I 2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17 



Nos. of 



seedlings --14251024 i i i i — — — — 



