32 Studies in the Hybrid Bistoninae. IV 



simplicity and conciseness in the study, let us assign arbitrary values 

 to the potential of the sex genes (or chromosomes) of the two insects. 

 Suppose we assume the powers of the male sex gene of hirtaria to be 

 of value 50 and, consequently, that of the female as — 50. Then, since 

 we have discovered by experiment that those of zonaria must be fixed 

 at a lower figure, let their values be 20 and — 20 respectively. On 

 these assumptions the XX' zygotes produced carry the high potential 

 of 70 and are therefore males, while the XY' carries the abnormally 

 low one of — 30 ; this, whatever it may be, certainly makes no 

 approach to maleness and consequently is a female; and its extra- 

 ordinary low sex level may be the cause of its experimentally proved 

 sterility. Again there is harmony between fact and theory and, what 

 is still more weighty, this harmony extends over all known crosses of 

 a similar type. 



Although my inclinations lie towards the acceptance of the scheme 

 developed above as being the most satisfactory explanation of the sex 

 phenomena, it is not the only one springing to my mind. 



In fertilising ova of one species with the sperm of another, it is 

 necessary to note that the mere mechanical passage of the spermatozoa 

 through the ova is not a matter of normal conditions of environment. 

 In particular, on account of the great disparity in size between those 

 of hirtaria on the one" hand and those of zonaria on the other, hirtaria 

 sperms have a much longer distance to travel than in ordinary acts of 

 fertilisation. This means greatly delayed fertilisation ; that such delay 

 did result was specially obvious in the hirtaria-graecaria eggs, for so 

 great a time intervened between the passing of the sperm through the 

 micropyle and the fusion of the nuclei that the ova collapsed precisely 

 as in the case of a non-fertilised egg. Postponement of this nature 

 implies over-ripe eggs ; and over-ripeness in the eggs of other groups 

 has ended in wholly male cultures. Similar causes, induced in varying 

 ways, may produce similar effects, and over-ripeness may explain the 

 aberrant sex results. 



There is also another set of circumstances possible through the 

 presence of strange spermatozoa in the ova. It may happen that 

 the effect of this is to cause the sex chromosomes to behave uniformly 

 so that the Y chromosome (or even both X and F) is constantly forced 

 to the opposite pole to the intruding sperm, and thus to be invariably 

 extruded with the last polar body. In the former event, the egg nucleus 

 would always contain a male determiner or chromosome, whilst the 

 latter would yield ova void of sex determination; in which case, fer- 



