470 SCIENCE PROGRESS 



(3) Fertilisation takes place only between gametes of opposite 

 sex. 



(4) The sex which is brought into the zygote by the egg is 

 dominant. 



To illustrate the sufficiency of this theory, we will take the 

 result of mating the male lacticolor produced by previous crosses 

 with the female grossulariata, which is heterozygous. The off- 

 spring of this mating are grossulariata all males, lacticolor all 

 females. The gametes of the male parent are Lm and Lf in 

 equal numbers. The gametes of the female parent are Gm, Lf, 

 in equal numbers. Thus the zygotes are Gm Lf, and Lf Lm in 

 equal numbers; and as the sex is determined by the ovum and 

 grossulariata is dominant, the visible character of the offspring 

 are grossulariata all males and lacticolor all females. But this 

 is not all : it can be shown that Doncaster's assumptions are not 

 so arbitrary as they seem, but are really in harmony with the 

 views I have maintained in this paper. If we suppose, as I 

 have urged, that ova are necessarily female and sperms neces- 

 sarily male, there is no segregation of sex between the gametes 

 of a single parent ; but we may further suppose that one-half of 

 the ova are strong and one-half weaker — and similarly for the 

 sperms. We may then suppose that there is selective mating 

 between the gametes — a strong ovum attracting a weak sperm 

 and vice versa, and that the stronger gamete determines the sex 

 of the zygote. The result is then the same as on Doncaster's 

 assumption, that dominance attaches to the sex which is brought 

 into the zygote by the ovum. For the weak ovum will be 

 equivalent to what Doncaster calls the "male-bearing ovum"; 

 and to say that this ovum makes the zygote male is the same 

 as saying that in this case the sperm is dominant. On this view 

 the terms of the theory have a real meaning ; whereas to say that 

 the ovum bears the male character is to use words which do not 

 represent any conceivable reality, and is a self-contradiction. 

 There is this difference, however, between my interpretation and 

 Doncaster's hypothesis : that if we make the weak ovum corre- 

 spond to the male-bearing ovum, the selective union is between 

 gametes of the same sex, not of opposite sexes — the weak ovum 

 unites with the strong sperm, and the latter is dominant. Thus 

 the sex is not determined by the ova alone, or by the sperms 

 alone, but by whichever is stronger — that is more vigorous or 

 dominant. My hypothesis further explains why the recessive or 



