E. R. Saunders 151 



By intercrossing therefore the gap between the half-hoary form, 

 itself an intermediate, and the glabrous type was partly bridged, and 

 we had the now expanded series fully hoary, half-hoary, quarter-hoary, 

 glabrous, of which both intermediate grades were characterised by the 

 presence of a new factor not met with in either of the extreme types. 

 In the light of the indications afforded by these results as to the nature 

 of the relation of this new factor to. some components of the 4-factor^ 

 group conditioning full hoariness, it was tempting to hazard a forecast 

 as to the expectation in cases of union with other components of this 

 group, assuming the simplest kind of inter-relation uncomplicated by 

 disturbing causes such as coupling, repulsion and the like. It was not 

 overlooked however that this new factor, provisionally regarded in the 

 absence of definite evidence pointing to a different view as a new and 

 distinct determiner for surface character, and hence denoted by a sepa- 

 rate symbol, might prove on fuller investigation to yield results showing 

 that it should more justly be regarded as a modified (? weakened) 

 variant of one or other of the two original determiners for hoariness, 

 rather than as a third factor distinct from either. That we must accept 

 this latter view as the correct one has now become clear, the later work 

 having shown that one of the two determiners for full hoariness must 

 be held to stand in a special allelomorphic relation to its substitute in 

 the intermediate grades. It follows that the prediction based on the 

 assumption of a simple Mendelian relationship involving three distinct 

 factors is not realised in those unions in which this multiple allelo- 

 morphism comes into play, and that the theoretical ratios tentatively 

 put forward in the earlier account require modification in accordance 

 with this new fact-. Furthermore we find, as was not unsuspected in 

 view of the allelomorphic relation between the special factor peculiar 

 to the partially hoary forms and the corresponding factor for full hoari- 

 ness, that partial hoariness, like full hoariness, is only manifested when 

 the individual contains in addition the two factors for sap colour', a 

 point which the earlier experiments had left undecided. It can now 

 in fact be shown that the colour couple acts as a limiting factor for 

 hoariness in all grades included in the hoary group. In the absence of 

 either or both colour components the most that we find is an occasional 



1 Consisting of the two pairs HK and CB, of which the former {HK) has been Rhown 

 to stand in a causative, the latter [CR) in a permissive, relation to the manifestation of 

 hoariness. 



* See Jouriuil of Genetics-, loc. cit. p. 157. 



3 I.e. the permissive pair referred to above in footnote 1. 



