14 Mutations and Evolution. 



that such a regrouping of chromosomes took place at fertilization 

 or soon after, so that the nuclei of the whole germ track 1 would have 

 the same chromosome grouping. The difficulty with this view, and 

 it appears now to be a fatal one, is that it seems necessary to 

 assume that the other chromosomes with which these two " red " 

 chromosomes are mated will differ from them in other factors as 

 well and will therefore introduce new differences in the offspring. 

 Morgan has shown \nDrosophila that eye-colours so closely alike as 

 to be indistinguishable except by the expert, may arise in different 

 chromosomes, as independent mutations. 2 Hence it appears more 

 probable that these factors for red in (E. rubricalyx, which are as 

 yet quite indistinguishable, may have arisen in the same way 

 through successive parallel mutations in different chromosomes of 

 the same race. 



Shull (1914b), however, uses the second hypothesis, mismating 

 of chromosomes, to account for the origin simultaneously of the 

 duplicate condition for triangular capsule form in Capsella bursa- 

 pastoris and the mutant C. Heegeri with round capsules. If, in 

 the reduction division, the pair of chromosomes containing each a 

 determiner for triangular capsule pass into the same daughter 

 nucleus, this would produce an individual with duplicate factors for 

 capsule form, while the other germ cell, lacking both these deter- 

 miners, would later give rise to the mutant Heegeri. His modified 

 suggestion that both these conditions, (1) duplicate factors for 

 triangular capsule, (2) the origin of a mutant lacking both these 

 factors and hence round, could come about through the detachment 

 of the determiner for triangular capsule from the end of one 

 chromosome and its attachment to the end of the next, seems more 

 likely ami more in accord with our present knowledge. The difficulty 

 with it as a general explanation is that one cannot suppose that all 

 duplicated factors have been conveniently located on the end of a 

 chromosome, especially where a number have been described for 

 the same organism, as is the case with wheat, maize and tobacco. 

 The most likely hypothesis of the origin of most duplicate factors 

 is then the independent origin of each through a chemical alteration 

 of a locus in a different chromosome. 



1 The term germ track, the English equivalent of the German Keimbahn, 

 is used to represent the line of cells or cell divisions following each other from 

 the fertilised egg to the pollen mother cells or the eggs. As de Vries pointed 

 out (Intracellular Pangenesis), we may thus think of a pedigree of cells derived 

 chiefly from apical cells and forming a connected system. 



1 In the same way, the factor for red midribs described by Heribert 

 Nilsson (1912) in a derivative of the Swedish race of (E. lamarckiana appears to 

 be different from that in the Amsterdam race, 



