I. Leitch 193 



only one change. That, together with the faihire of selection to show 

 any further splitting in the X or M gi-oup, seems a sufficient indication 

 that only one fixctor has been altered in E to produce the mutation M. 



However, this modification must not be taken in any restricted 

 sense. There is every reason to interpret the change in the broadest 

 fashion. In the first place, as will be evident from the curves of distri- 

 bution, and as is strikingly evident in handling the material, the M 

 type produces usually fewer beans than the original E type or the 

 X type. But there is certainly a correlation between the number of 

 beans produced by a plant and their size, and whether the difference 

 between the types be regarded as a difference in size or in fertility, 

 primarily, is worth considering, though the point may not be of great 

 importance. Further the M type is much more liable to be attacked b}^ 

 fungus than the E and X types. No attempt has been made to deter- 

 mine the reason for this, but it indicates that the change from E to M 

 must be regarded as a change in the reaction of the whole organism. 



If the change from E to M could be regarded as a sudden discrete 

 transformation of one factor into a modification functioning as a new 

 factor, the hybrids between E and M would be doubly heterozygous and 

 they would be expected to split into a variety of types including some 

 such type as X and possibly a type still longer than M. If the foctor 

 altered be called A and it be considered as modified to B, then the 

 hybrids would have the composition AaBh, and to explain what really 

 does happen, it would be necessary to postulate complete coupling 

 between A and B. In addition it would be necessary to assume that 

 forms containing neither factor are indistinguishable from aaBB (the 

 M form) ; the X form then would be the double homozygote with both 

 factors ; or alternately, if presence of factors be taken to mean additional 

 size, and B be regarded as modified into A, it must be assumed that 

 AABB is indistinguishable from A Abb. And even so, the difference 

 between the two heterozygous forms, one of which is phaenotypically 

 like E and the other intermediate between E and 31, remains to be 

 accounted for. This may prove to be a very far-fetched attempt to 

 reconcile the results with Mendelian results in general and until other 

 work on similar material has thrown more light on the subject, it is not 

 a very profitable discussion. The one point stands out, that the theory 

 of loss of factors here fails to account for the nature of the mutation. 



With regard to the difference between the behaviour of the groups 

 of heterozygous plants in general and the two aberrant, similar groups (1 

 and 9, Professor Johannsen has suggested the possibility of cytological 



