204 THE MENDEL JOURNAL 



presence and absence theory of Professor Bateson and Professor 

 Punnet. 



On Mr. Keeble's suggested hypothesis naaleness and femaleness 

 are independent allelomorphs, each with its presence and absence 

 designated thus : "Mm" and "Ff." Proceeding on the usual plan, the 

 scheme presented contains among Ifi individuals nine hermaphro- 

 dites with varied possibilities, three males, three females, and one 

 sexless. Amongst the nine hermaphrodites is one zygote, in 

 which both sexes are doubly present, and symbolically represented 

 as M M F F, and four of the type M m F f, where each contains 

 only a half-dose of the two sexes. Naturally the question arises 

 as to the actual sex which either type of zygote, i.e., M M FF 

 and M m F f , will manifest. Both contain maleness and female- 

 ness in equal amount. Other theories of sex regard maleness 

 as being dominant in some cases, and in others femaleness. 

 But Dr. Keeble, faUing back upon the familiar fact that double 

 begonias which normally bear female flowers, may be induced 

 to develope male flowers upon starvation of the plants, postu- 

 lates the conception that it is external conditions, such as nu- 

 trition, which determines the definitive sex of zygotes of either 

 these two types. He thinks that in general the number of males 

 and females among zygotes of the MMFF and MniFf type will be 

 about equal, though wide departures may be expected under 

 conditions which favour the development of one sex more than 

 another. The question is one which experiment alone can 

 decide. 



Among the remaining hermaphrodite types, i.e., Mm F F 

 and M M F f , the former, it is supposed, will produce female 

 organs and the latter male. 



Four of the remaining types will be unisexual, containing only 

 maleness or femaleness, and may be represented as MM f f, 

 M m f f, ra m F F, m m F f. The one remaining type will be 

 sexless, i.e., mm f f . Naturally it must be supposed that 

 M M f f and M jn f f will bear male organs and m m F F, m m F f 

 female organs whatever the nature of the external conditions. 

 The m m f f type, if viable, will be sterile. 



Dr. Keeble proceeds to show that many interesting problems, 

 such as prepotency, partial steriHty, homosporous and heteros- 

 porous ferns (the latter bearing spores which produce both male 

 and female or hermaphrodite prothalh, and the latter having 

 two sets of spores, one large which produce female prothaUi, 

 and the other small which produce small male prothalU), the high 

 rate of mortality accompanying spore formation, and the absence 

 of sexual reproduction in certain algfe, may receive an intelligible 

 explanation on the basis of his hypothesis, in part elaborated by 

 the collaboration of his colleagues, Miss Rayner, Miss Pellen, and 

 Mr. Jones. 



