68 
R. Ruggles Gates. 
same explanation as the duplicate condition of the factor for red in 
CE. rubricalyx. There are, as previously pointed out 1 , at least two 
ways in which such a result is likely to have come about. 
(1) Through a locus in a second chromosome having independently 
undergone the same change or mutation. (2) An individual which 
would normally be homozygous for one factor for red (RR') might* 
by a mismating of the chromosome pairs in fertilization or after, 
become heterozygous for two factors for red (RrR f r'). In this way 
an individual giving a 15 : 1 family of offspring could arise from a 
homozygous member of a 3 : 1 family. It is necessary to assume 
that such a regrouping of chromosomes took place at fertilization 
or soon after, so that the nuclei of the whole germ track 2 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 in Drosophila that eye-colours so closely alike as 
to be indistinguishable except by the expert, may arise in different 
chromosomes, as independent mutations. 3 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 
1 Gates 1915b. 
2 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. 
3 In the same way, the factor for red midribs described by Heribert 
Nilsson (1912) in a derivative of the vSwedish race of CE. lamdrckiana appears to 
be different from that in the Amsterdam race. 
