784 
that the frequency v of Réntgen rays may be 
obtained from either of the equations 
, Ty 
OF 9) 
v—2 Sel02 = Sq OR 
where 7'v is the mean energy and Tm the maxi- 
mum energy of the electrons emitted when 
Réntgen rays fall on a metal. For these 
high frequencies v, may be neglected compared 
with v. 
Our results are favorable to a theory of the 
photoelectric effect of the type of Einstein’s* 
combined with the hypothesis that the differ- 
ence in the work P for different substances is 
determined by the contact difference of po- 
tential. O. W. RicHARDSON 
Kart T. Compton 
PALMER LABORATORY, 
PRINCETON UNIVERSITY 
PRELIMINARY NOTE ON THE OCCURRENCE OF A 
SEX-LIMITED CHARACTER IN CATS* 
THE problems offered by so-called “ sex-lim- 
ited characters” have lately been attacked by 
several investigators who have found in many 
of the cases a possible explanation of the 
observed phenomena by considering one of the 
sexes a Mendelian homozygote for the “sex- 
producing” factor, while the other sex is con- 
sidered a heterozygote. 
The sex-producing factor is commonly 
designated by X, its absence by —. Thus 
one sex would be homozygous, XX, and the 
other would be heterozygous, X —. Certain 
eases have been found in which experimental 
results indicate that the female is homozy- 
gous, XX, while the male is heterozygous, 
X —, while in other cases the facts are best 
explained on the hypothesis that the female is 
heterozygous, the male homozygous. Interest 
increases as sex-limited characters are found 
in the higher animals, the inheritance of 
which follows one or the other of these form- 
ule. 
It has long been known that “tortoise 
shell” (a blotching of black and yellow, or 
1 Ann. der Physik., Vol. 17, p. 146, 1905. 
1From the Laboratory of Genetics, Bussey In- 
stitution. 
SCIENCE 
[N.S. Vou. XXXV. No. 907 
blue and cream) occurs in cats, in a vast 
majority of cases in the female sex. Don- 
caster’ (1904) attempted to ascertain whether 
tortoise shell could be considered as a sex- 
limited character whose appearance conformed 
to the then existing hypotheses of sex-inherit- 
ance. He came to the conclusion that “ tor- 
toise” was merely the female form of hetero- 
zygote obtained in a cross between orange 
(yellow) and black animals. The male form 
of “heterozygote” was orange in certain 
crosses. Thus he found that (1) orange fe- 
male X black male gives tortoise females and 
orange males, but the reciprocal cross (2) 
black female X orange male gives tortoise, 
black (and probably orange) sexes not stated. 
This last-named cross is crucial, for in it is 
contained the evidence that the male “ hetero- 
zygotes” between orange and black are not 
always orange, but may be black. The writer 
has, in a very small way, carried on this cross. 
Thus four black females crossed with the same 
orange male have given a total of 15 young; 
of these 7 were males, all being black, and 8 
were females, all being tortoise; no “ orange” 
animals appeared. Here there is evidence 
that the cross of orange male X black female 
produces male offspring, all of which are 
black, while Doncaster’s evidence shows the 
reciprocal cross to produce male offspring, all 
of which are orange. 
We must, therefore, suppose a reversal of 
dominance to occur in the reciprocal crosses 
unless we can use the hypothesis of sex-lim- 
ited inheritance. 
If we adopt, tentatively, the hypothesis that 
the female is a homozygote, XX, and the male 
is a heterozygote, X —, and if we suppose 
that black, B, is always coupled with the sex- 
producing factor, X, we should conclude that 
the black female is of the gametic constitu- 
tion, BB, and that the black male is of the 
composition B—. 
The yellow male lacks the factor for the 
production of black pigment in the coat and 
is of the gametie composition Y —, while the 
2Proc. Camb. Phil. Soc., XIII., Pt. I., p. 35, 
1904. 
