163 



Several of these families consisted of but one individual each, 

 and these may be left out of account as having no significance. The 

 most interesting is 09258 in which a bursa-pastoris parent produced 

 a single Heigeri offspring, thus showing that this parent was 

 heterozygous in respect to capsule-character. Six of the eleven 

 families derived from bursa-pastoris parents bred true to the 

 bursa-pastoris type of capsule, this being slightly in excess of 

 expectation on the assumption that this character is determined 

 independently by two genes, and considerably in excess of the 

 one-third which should have bred true if but one gene differentiated 

 the Heegeri capsules from the bursa-pastoris capsules of the 

 P x . However the number of families is wholly inadequate to permit 

 the attachment of any special significance to this closer agreement 

 with the requirements of the two-gene hypothesis. All of the 

 families from parents having Heegeri capsules have bred true to 

 the parental character, as they should do to agree with Mendelian 

 Interpretation. The four families which split into the two parental 

 types show the ratios, 4.67 : 1 ? 15.6 : 1, 24.0 : 1, and 63.5 : 1, all 

 of these ratios differing in the same direction but in quite various 

 degrees from the two available ratios 3 : 1 and 15 : 1. Two of 

 the families show a suggestive approximation to the expected 

 ratios, while the other two depart widely from the nearest 

 available ratio 15 : 1. All of these families as well as the three 

 families of the F 2 have shown a smaller proportion of Heegeri 

 plants than required by the hypothesis that the bursa-pastoris 

 capsules are determined independently by two genes. 



It appears to me that the explanation of these results is to 

 be sought in some modifying influence acting upon the normal 

 Mendelian processes. The ratio 4.67 : 1 may then represent a 

 modified ratio of 3:1, and the other three ratios may be referred 

 to the ratio 15 : 1. These three families taken together give 

 a ratio of 22.2 : 1, essentially identical with the observed ratio 

 21.9 : 1 in the F 2 , and showing almost exactly the same pro- 

 portional departure from 15 : 1, that 4.67 : 1 shows from 3 : 1, 

 for 4.67 : 3 = 23.3 : 15. 



The nature of the modifying cause or causes which may be 

 operating to produce these defective ratios need not be discussed 

 at length here, as the matter is capable of experimental treat- 

 ment and is being investigated ; but it may be pointed out that 

 either „selective fertilization" favoring the union of unlike gametes, 



11* 



