162 The Journal 
confirmed by burning yolks in the 
“‘bomb calorimeter,’ and measuring 
the amount of heat liberated in burning 
the stored materials. This very accu- 
rate method of investigation gives results 
which confirm the results of breeding, 
showing that eggs which produce males 
differ quantitatively from eggs which 
produce females. The results show too 
that the storage capacity of the eggs 
increases gradually during the progress 
of the season. This storage is highest 
at the time ‘developmental energy”’ is 
lowest, and both these coincide with the 
female-producing period. 
POST-MORTEM STUDIES 
The tenth and last kind of evidence 
brought from these studies is gained by 
post-mortem examination of the repro- 
ductive glands. Many species of birds, 
it will be remembered, have normally only 
one ovary, the right ovary regularly 
failing to develop, or degenerating 
rapidly after beginning to develop. 
This is true of pigeons and doves. But 
it is found that females hatched under 
conditions which accentuate the female- 
ness—for example, late in the season 
following crowded  egg-laying—fre- 
quently have the right ovary developed. 
From this Dr. Riddle concludes that 
“the same pressure which carries the 
eggs of spring from male-producing to 
female-producing levels, also carries the 
earlier female-producing level to an- 
other yet more feminine.” 
In short, Dr. Riddle thinks, from the 
many kinds of evidence here outlined, 
that the nature of sex lies in the nature 
of differences between levels of metabol- 
ism, that the two levels are normally 
associated with different amounts of 
chromatin, or different chromosome 
numbers. But he considers these differ- 
ential amounts or aggregates of chro- 
matin as merely a means of insuring two 
diverse metabolic levels, and thus the 
two sexes; and he asserts that ‘if a new 
metabolic level is forced upon the germ, 
as in our experiments, the sex of the 
resulting offspring must coincide with 
the sex that can develop from this level, 
and this quite regardless of whether the 
of Heredity 
level was established through a differ- 
ential chromatin relation or value, or by 
other means.” . . . “Males arise from 
germs at higher levels, females from the 
lower.” 
If it is merely a matter of level, and 
not a matter, as many have supposed, 
of some mechanical difference in struc- 
ture, it is obvious that one might sanely 
hope to reverse sex. All that would be 
needed would be to exert sufficient pres- 
sure of an appropriate sort. Normally 
the two different kinds of eggs remain at 
different levels; one regularly produces 
males, the other females. To exert the 
heavy pressure necessary, Dr. Whitman, 
and later Dr. Riddle, made wide crosses. _ 
The pressure thus exerted (speaking 
metaphorically, of course), is sufficient, 
while both birds are producing their 
“strongest germs,’ to force female- 
producing eggs to a male-producing 
level. But when the cross is not of 
more than generic value, and the birds 
are made to yield weaker and weaker 
germs through reproductive overwork, 
the earlier male-producing level is 
followed in the weaker germs by a 
lower female-producing level. 
A SKEPTICAL AGE 
Every year a number of individuals 
claim to have controlled or reversed sex. 
Consequently, the biological world has 
become decidedly skeptical on the 
subject. ‘‘The insufficiently controlled 
experiment, the novice and the quack 
are the trinity of evils that has begotten 
this widespread skepticism,’ says Dr. 
Riddle. 
At least one good effect it has had— 
any experiment which professes to show 
a control of sex is submitted to an 
extraordinarily stringent examination. 
Very few of the experiments stand this 
test, but there are a few, made by 
biologists of repute, which seem to have 
withstood criticism successfully, though 
in all, or in nearly all of these, it has 
been impossible to disprove one or 
another of the possibilities urged by 
the ‘‘chromosomists.’’ Most of these 
publications have appeared since Dr. 
Riddle’s first short statement? in 1911 
2 Paper read December, 1911, before the American Society of Zoologists at Princeton; ab- 
stract in Science, N.S., Vol. 35, pp. 462-463, March 22, 1912. 
