J. W. H. Harrison 15 



Let us digress here for a moment to consider the course of events 

 when inbreeding is persisted in within the limits of one species. During 

 the period between the years 1906-1912, for purposes quite remote 

 from the circumstances under discussion, I kept going a race of Zono- 

 soma orbicularia which, on account of the great rarity of the species, 

 could not be otherwise than inbred, and inbred extremely closely. At 

 first, despite the fact that I was rearing two and sometimes three broods 

 per annum, the race seemed to retain its wonted vigour; but, in 1910, 

 abnormal sex ratios were manifested coupled with a decided diminution 

 of both vigour and of sexual instinct, cases of reluctance to pair, even 

 with the conditions at the optimum, being quite usual. 



And the unequal sex ratios were in the direction of an excess of 

 females. Still, inbreeding was resorted to until, in 1911, several of 

 the broods were wholly female in composition, while yet certain of the 

 remainder contained a few males. Although these, in the main, refused 

 to pair, they supplied enough potent males to ensure the continuance 

 of their race until the ensuing year, when it ended in the production of 

 females only, involving, of course, the extinction of the stock. 



Now let us deliberate as to what this may mean. Only one inference 

 can be extracted from it, that the power of the male sex factors has, by 

 degrees, been diminished by inbreeding until at the end it has become 

 in its effects wholly negligible. In other words the sex potential of 

 the male gene (or rather of the chromosome bearing it) falls, brood by 

 brood, until it becomes practically zero and thus the final zygotes, as 

 regards sex level, are in the neutral position reflected in those zygotes 

 by what we call femaleness. 



What is the bearing of this on the results with which we are now 

 concerned ? 



A theory has been formulated above that the aberrant unisexual 

 broods are consequent upon the disparity in sex potential of the male 

 and female sex genes, or of the chromosomes in which they are located ; 

 in particular, it was deemed proved that the male potential of the 

 phylogenetically older P. pomonaria and P. lapponaria was great enough 

 to overwhelm the power of the female sex genes, or Y chromosome, of 

 the younger N. zonaria. But if, as a result of the inbreeding of the 

 former pair of species, the potential of the male element falls, it is obvious 

 that if its fall progresses with the degree to which we carry the in- 

 breeding, it may sink low enough to fail tb overcome the eflfect of the 

 female determiner of N. zonaria. Consequently, in certain zygotes, 

 there will occur no setting up of the pseudo-XX condition expressed 



