140 Shull. 



element, because, as we have already seen, when two chromosomes, each 

 possessing the determiner C, pass into the same germ-cell, the sister 

 germ-cell receives their two homologs from which C is absent. Then 

 by the union of two germ-cells of the latter type an individual will be 

 produced in which the gene C is omitted, and whose soma can not ex- 

 hibit, therefore, the characteristic reaction in which C plays an essential 

 role. Here then is an easy explanation of the origin of recessive mutants 

 without the actual destruction or loss of determiners. Applied to the 

 specific case of Bursa, the very same operation which doubled the de- 

 terminer for the triangular capsule in Bursa bursa-pastoris, may have 

 given rise also to Bursa Heegeri. 



Of the four cases of duplicate determiners thus far demonstrated, 

 namely, the presence of a ligula in oats, red pericarp-color in wheat, 

 yellow endosperm -color in maize, and the triangular capsule in 

 Bursa, two are color- characters, and two are structural characters. 

 The color-characters do not suggest in any clear way their probable 

 method of origin; the phenomena they present may be primary or 

 secondary; they are about equally well explained on the basis of 

 any of the above described schemes. The simplicity of such a char- 

 acter makes it about as easy to imagine that the duplication is due 

 to repeated mutations as to a rearrangement of the genes. In the 

 two structural characters, however, I believe it to be clearly indi- 

 cated that for the ligula of Avena the duplication of determiners is a 

 primitive condition, and that for the triangular capsule in Bursa it is 

 a derivative condition. Comparative morphology offers the first evidence 

 in favor of this fundamental difference between these two cases, for the 

 ligula is almost universally present in the grasses and so must have been 

 present in the ancestors of Avena, while a triangular capsule is formed, 

 so far as I am aware, in no Crucifer other than Bursa, and therefore 

 this form of capsule was probably absent in the ancestors of this 

 genus. A further indication that the duplication in the case of the 

 ligula was the primitive condition is given by the large number of 

 duplicate genes whose existence has been made probable, four having 

 been indicated in one cross, though only two have been adequately 

 demonstrated. The crucial test of the hypothesis is a cross between 

 the liguleless oat, "Jaune geant a grappes", or one of its recessive 

 derivatives, with an undoubted wild oat. Owing to the large number 

 of duplicate determiners for the ligula likely to be found in such a 

 cross, it must be carried out on an unusually large scale; indeed, 

 completely decisive analysis may prove to be impossible, owing to the 



