THE GENETIC NATURE OF TAXONOMIC DIFFERENCES 263 



out in the F4 which still had the 10 pairs derived from biennis, and 

 with four setosa chromosomes which formed two pairs. This enabled 

 the plant to breed true., and it was ranked as a new species and given 

 the name C. artificialis; but it is clearly unbalanced in its setosa frac- 

 tion, rvvo of the setosa pairs being entirely lacking. 



In some genera, many polysomic forms occur, which one would 

 suspect of being unbalanced. For instance, in the Melanium section of 

 Viola^ the chromosome numbers do not fall into a definite polyploid 

 series, but many of the species can be regarded as modified poh^loids. 

 Thus we have 



Diploid . . AA V. Kitaiheliana in = 14 



Tetraploid . . AABB V. tricolor, V. alpestris 2n = 26 



Hexaploid . . AABBCC V. arvensis, V. rothomagensis 2n =34 



V. Kitaiheliana in = 36 

 Octoploid . . AABBCCCC V. nana, V. lutea in = 48 



It is remarkable that V. Kitaiheliana has two forms, classified in the 

 same species, but having chromosome numbers which are not even 

 multiples of one another. Viola canina is another phenotypically 

 constant species with very variable chromosome numbers, which range 

 from 16 to 25. Probably in such cases many of the chromosomes are 

 more or less completely inert. This is. also suggested by the fact that 

 hybrids between such species may be quite fertile although they are 

 obviously aneuploid and give gametes with variable numbers of 

 chromosomes. 



5. Non-polyploid Species 



Species whose haploid numbers differ by one or a small integer are 

 probably more often related by breakage or fusion of chromosomes, 

 which leaves the balance unchanged, than by complete loss or addition. 

 Straightforward fusion or fragmentation has often been inferred in 

 such cases,^ and a particularly striking example may be found in the 

 sex chromosome of the Orthoptera. In some groups this is separate, 

 but in others, e.g. in Mermiria, it is fused to an autosome but can still 

 be recognized from the fact that it shows precocious condensation, 

 staining quite deeply in early prophase when the other chromosomes, 

 including the autosome to which it is attached, are still diffuse. The 

 occurrence of perfectly simple fusion, and still more of simple frag- 

 mentation, conflict, however, with the large body of evidence dealing 

 ^ Clausen 1927, 1931a, h, ^ Navaschin 1932. 



