in the same environment, i.e., the doses of maleness and femaleness in a given sex always bear practically the same relation to each other.  Hence the types are fixed and uniform. (6) But different races are likely to have a different strength of chemical sex-doses, so that when they are crossed, the ratios of maleness to femaleness are upset.  Often they are almost exactly equal, which produces a type half male and half female—­or 2/3, or 1/3, etc.  The proof of this theory is that it solved the problems.  Goldschmidt was able to work out the strengths of the doses of each sex in his various individuals, and thereby to predict the exact grade of intersexuality which would result from a given cross.

Riddle’s work on pigeons [5,6] brings us much nearer to man, and suggests the results noted by both Goldschmidt and Lillie.  As in the Free-Martin cattle, there is an apparent reversal of the sex predisposition of the fertilized egg.  As in the gypsy moths, different grades of intersexes were observed.  In the pigeons, it was found that more yolk material tended to produce a larger proportion of females.  The most minute quantitative measurements were made of this factor, to eliminate any possibility of error.

The chromosome mechanisms practically force us to suppose that about half the eggs are originally predisposed to maleness, half to femaleness.  A pigeon’s clutch normally consists of two eggs, one with a large yolk and one with a small yolk.  But the half-and-half numerical relation of males to females varies considerably—­i.e., not all the large-yolked eggs produce females or all the small-yolked ones males.

Wild pigeons begin the season by throwing a predominance of males, and the first eggs of the clutches also tend to produce males all along.  In both cases, the male-producing eggs were found to be the ones with the smaller yolks.  Family crosses also produce small yolks, which hatch out nearly all males.  Some pairs of birds, however, have nearly all female offspring.  Riddle investigated a large number of these cases and found the amount of yolk material to be large.  In other words, there seems to be a definite relation between the amount of yolk and sex.

A great number of clever experiments were carried out to find out if eggs originally predisposed to one sex were actually used to produce the other.  Selective fertilization with different kinds of sperm was impossible, since in these birds there is only one type of sperm—­two of eggs—­as to the sex chromosome.  For instance, by overworking females at egg-production, the same birds which had been producing more males than females were made to reverse that relation.

One of the interesting results of the experiments was the production of a number of intersexual types of various grades.  This was easily verifiable by colour and other characteristics.  To make sure that the instincts were correspondingly modified, behaviour was registered on moving-picture films.  Where the first egg of a clutch (the one with a small, normally male-producing yolk) produces a female, she is usually found to be more masculine than her sister from the second egg with the larger yolk.  This is true both as to appearance and as to behaviour.  Some of these were quite nearly males in appearance and behaviour, though they laid eggs.