SEX 375 



unalterably upon unit factors. The hereditary factors or genes are 

 usually regarded by geneticists as unmodifiable except by sudden muta- 

 tions. Failure to distinguish between the modification of genes and the 

 modification of the interaction of genes during ontogenesis has led many 

 to the view that the sex of the individual must be rigidly fixed at fertiliza- 

 tion in digametic and dioecious forms, or at reduction in the case of dice- 

 cious gametophytes like those of Sphcerocarpos. It is very difficult to 

 reconcile any inflexible theory of this nature with the great diversity of 

 situations known without resorting to hypotheses of somatic segregation 

 of factors, alterations in dominance, and other assumptions not well 

 supported by observational evidence. Such difficulties are encountered 

 in the common hermaphroditic condition of gametophytes, which are 

 haploid; in the possibility of causing the development of the second 

 sex in gametophytes normally unisexual (Onoclea) ; and especially in the 

 numerous cases of sex intergrades, in which it is possible not only to con- 

 trol the relative amounts of maleness and femaleness in hermaphroditic 

 forms, but also to produce all intermediate grades between male and 

 female individuals, and furthermore to reverse the sex in unisexual forms, 

 even in certain species with sex-chromosomes (moths and probably 

 pigeons). 



If, in accordance with the ideas of many biologists, sex is held to 

 be a quantitative, "fluid" character associated with a continuous series 

 of physiological states which may pass into one another, the way is open 

 for the explanation on a common basis of all cases of hermaphroditism in 

 both haploid and diploid individuals, of sex intergrades, and of the experi- 

 mental modification of sex. At the same time the influence of the sex- 

 chromosomes or even of smaller factors within them may be allowed. 

 As Riddle (1917) states, organisms have had the problem of producing 

 germs of two metabolic levels, and in some cases this has led to the 

 establishment in the two sexes of two amounts of chromatin or even of two 

 different chromosome complements. The sex-chromosomes, or units 

 contained within them, act with others in the maintenance of two 

 diverse levels of metabolism in the gametes and in the offspring, and with 

 these levels are correlated the two conditions which we distinguish as 

 male and female. Even if the sexes in such cases do not differ in the 

 quality of their chromatin, they at least differ quantitatively in this 

 respect, in agreement with the theory that sex is a quantitative character. 

 The chromosome difference being only one factor in a complex system 

 producing the two sexual states, and no single element in this system 

 being the sole determiner of sex, it is not impossible that the effect of this 

 one factor should be annulled by sufficiently altering the other factors 

 and thus modifying the action of the factorial system as a whole. The 

 same is to be said of other characters also. What an organism inherits 

 is not simply this or that character or sex, but rather a tendency to 



