J. W. H. Harrison 7 



autumnata, in the Lepidopterous family Geometridae the males, as 

 regards sex, are homozygous and the females heterozygous. Conse- 

 quently the males of the whole of the species must be represented 

 as of constitution XX ; similarly that of the female is XY^. Therefore 

 the gametes of the male are all of type X, and of the female X and Y, 

 appearing in equal numbers. Naturally then, on fertilisation, whether 

 the gametes taking part proceed from the same or diverse species, two 

 types of zygotes are generated, one type being of composition XX and 

 the other of formula XY. In pure species the former are the males, 

 and the latter manifest themselves as the females ; and the same holds 

 true, Avhen no sex disturbance occurs in the products, of the hybrids. 

 When such interference does appear, the males still remain of the 

 type XX whilst the affected zygote, no longer to be recognised as a 

 female, nevertheless bears the sex chromosome combination peculiar to 

 that sex. It follows immediately from this that, when the sex genes 

 in any given zygote are not those of one and only one species, a mere 

 setting up of the XY system does not in itself suffice to ensure that 

 that zygote should develop into an organism of female type. Or, in 

 terms of the chromosome theory (and almost certainly the actual 

 explanation) of heredity and sex, whether we regard the so-called 

 sex chromosomes as synonymous in whole or in part with, the sex 

 genes, the possession of an X chromosome inherited from a phylo- 

 genetically older male, and a Y from a younger female, does not indicate 

 that the individual carrying them is a female. An immediate deduction 

 from this is that the male sex gene or chromosome of the older form 

 is able, not only to neutralise, but to overpower, either completely or 

 partially, that from a female of later development. Whenever this 

 overpowering takes place wholly, we have a complete conversion of 

 zygotes of build XY' into males. Such an occurrence happens in the 

 hirtaria x zonaria and accompanying crosses. In other words, it seems 

 that the sex chromosome designated "X" of the older insect has been 

 able to supply enough sex chromatin, or is powerful enough in itself 

 (the two statements are not synonymous) to set up for the hybrid a 

 system ending in the complete passage of a female into a male. Or, 

 symbolically, considering a zygote of form XY' produced in the hybrid- 

 isation of phylogenetically old males with young females, the X is 

 potent enough to destroy the effects of the Y present and also to set 

 up in its place a modified (and therefore better written XX) XX system 



^ X is used to designate the male gene or factor and Y the female, whilst X' and Y' are 

 used for similar genes in the case of a second species. 



