Recent Advances in the Study of Heredity. J 
assumption that the female is homozygous. It may be advanced 
that these specialised insects have acquired heterozygousness after 
acquiring parthenogenesis, that is, the power of producing males by 
spontaneous development after some sort of reducing division. But 
this is only a suggestion. There is little doubt, however, that, as a 
consequence of the acquisition of parthenogenesis, the male has 
degenerated from its original heterozygote condition to a virtual 
homozygote. Thus we are brought up sharply against the wall of 
evolution in our pursuit of Mendelian characters. Our surprise at 
the nature of the obstacle will diminish as we recall the importance 
of the Mendelian unit in other connections as a probable factor of 
evolution. 
In reviewing the correlations in cytological and sex-phenomena 
in the cases just discussed, we may be justly permitted to infer that 
the male is heterozygous for sex with maleness dominant, and the 
female a homozygous recessive. Further, femaleness is always 
associated with two x-elements, and maleness with only one 
x-element or with one x- and one y-element. The y-element, it is 
found, may be present or absent in closely allied forms, hence, 
whether rightly or not, less importance is attached to it as a sex- 
determinant than to the x-element. Wilson has drawn attention 
to the close correlation in these phenomena between the haploid 
condition and maleness, and the diploid condition and femaleness. 
Herein would appear to be one of those curious differences which 
exist between animals and plants ; for it appears that in plants the 
asexual spores are nearly always formed with the haploid condition 
of the chromosomes, and are nevertheless differentiated with 
regard to sex in certain Pteridophytes and all flowering plants. 
I now pass on to consider another series of facts which support 
the view that the male is a Mendelian heterozygote. I refer to the 
sexual changes induced in organisms by the entrance of parasities 
into their tissues. 
It is known that when certain fungi infest the region of the 
ovary in female plants of Carex dioica and Lychnis dioica, 
anthers develop. Analogous phenomena in animals have been 
carefully worked out; especially in the case of crabs and their 
parasites. G. Smith has observed 1 that when the Spider-crab is 
infested by parasites such as Sacculina, belonging to the group 
Rhizocephala, the sexual characters of both male and female undergo 
' Fauna and Flora of Gulf of Naples, Vol. XXIX., Rhizocephala. 
