No. 586] THE INHERITANCE OF DOUBLENESS 



633 



It must be admitted, however, that (3) is not in itself 

 improbable if (1) is true, in view of Miss Saunders's 

 evidence. A similar difference between races, with re- 

 spect to linkage, occurs with * ' cream ' ' flower-color, 

 which is partially linked with doubleness in the sulfur- 

 white races, but completely linked in the pure-cream 

 races. The essential difference of the viability-hypoth- 

 esis, as here presented, relates to (1); the demon- 

 strated lower viability of the singles, evidently the basis 

 of (2), makes possible the simplification of (1). 



If we accept this viability-hypothesis, there seem to be 

 two general possibilities as to the origin of the double- 

 throwing races. One is that the mutation by which Ss 

 (double-throwing) races arise from SS (pure single) 

 races involves a simultaneous or consequent alteration 

 in the remaining S factor (or the production of a lethal 

 factor completely linked with S), by which the presence 

 of S becomes incompatible with pollen-formation. 

 Second, it may be that the particular race or races in 

 which our double-throwing forms originated had an S 

 factor originally different from that of the pure single 

 races which have been used in crossing with double- 

 throwers— that is to say. an S factor originally incompat- 

 ible with the formation of good pollen in an Ss plant — 

 or else that they originally possessed the lethal factor 

 suggested. If the second supposition is correct, such 

 pure single races may be found,— races which in crossing 

 with double-throwers never give the F 2 ratio 3 singles: 1 

 double, but only approximately 1 single :1 double. 



With Petunia, if we ignore the new seed-producing 

 double (Francis, 1913), which has a distinct type of 

 flower, the general case would seem to be similarly 

 simple. Here, as is well known, the doubles are pro- 

 duced only when singles are pollinated by doubles, the 

 ordinary doubles having stamens but not pistils, or, at 

 most, non-functional rudiments of pistils. In this case 

 the doubleness factor (D) is plainly dominant, and is 

 perhaps to be considered an inhibitor; the single, then, is 



