J. W. H. Harrison 9 



carrying a force parallel with positive electrical potential, and the Y 

 as bearing a similar analogy to negative. From this view of sex, the 

 male is essentially a creature of high sex potential as carrying two like 

 factors, whereas a female, with two equal unlike factors, is one of neutral 

 or zero sex level. To illustrate (not that the figures have any real 

 value), suppose we assign to an X chromosome of hirtaria a sex 

 potential of + 50, and to a F chromosome of the same origin the value 

 — 50. Then, on fertilisation, the sex level of a zygote destined to be 

 males would be + 100, whereas that of the females would be 0. 



But there are many grades or stages between and 100, from which 

 it appears possible that all males are not necessarily exactly alike ; nor 

 are the females uniformly the same. There must be some scope, even 

 if not great, for oscillation around the values and 100. Our next 

 task will be to determine what would happen if by suitable manipulation 

 we obtained a series of organisms lying around the 50 level and shading 

 off in both directions, one lot toward zero and the other to 100 ; clearly 

 the former are tending to be females and the latter- males. This 

 suggests that the forms close to the 50 value are to be regarded as 

 neither males nor females ; in other words, they must be intersexes. 



Granting that such is the case, and that such potential differences 

 are at the root of sex determination, and granting also, as an allow- 

 able deduction from the experimental evidence in the hirtaria- zonaHa 

 crosses, that the values to be assigned to the sex genes vary with 

 the species, then it ought to be perfectly feasible, by some happy 

 hybrid combination of species, to produce insects endowed with a sex 

 potential which, as far as the hybrid is concerned, stands at the mean 

 in question. Careful study of the experimental data yielded by the 

 crosses named above makes it practically certain that, as we pass away 

 phylogenetically from hirtaria, the powers or potential of the male sex 

 factors diminish, and that of the female varies correspondingly. Hence, 

 crosses between two Bistonine forms, utilising males of phylogenetically 

 older insects and females of later origin, may give us confirmation of 

 these views. We chose, as a start, forms such as pomonaria male (the 

 older form) and isabellae female, and crossed them. In the actual ex- 

 periments, the result was that the sexes emerged in equal numbers. 

 Certainly from that we cannot extract any facts likely to throw light 

 on the problem, and the reason is plain; isabellae is a form of very 

 recent development from pomonaria, and their sex values have not 

 diverged to any appreciable extent. Laying aside this experiment as 

 not being of importance in so far as concerns our present quest, we took 



