Testicular and ovarian extracts were injected.  The more feminine birds were often killed by the testicular extract, the more masculine by the ovarian extract.  Finally, to make assurance doubly sure, some females which should theoretically have been the most feminine were dissected and shown to be so.  That is, out-crosses which produced a predominance of females in the fall were mated with females which had been overworked at egg production until they threw nearly all females.  Dissecting the females thus produced, they were shown to have right ovaries, which means double femaleness, since normally the pigeon is functional only in the left ovary, like other birds.  The right one usually degenerates before or at hatching and is wholly absent in the week-old squab.

In pigeons, Riddle thinks the “developmental energy” of the eggs is in an inverse ratio to their size.  The last and largest eggs of the season develop least and produce most females.  The second egg of a clutch is larger than the first, but develops less and the bird produced is shorter-lived.  Overworking and other conditions tending to produce large eggs and females also throw white mutants and show other signs of weakness.  Old females lay larger eggs than do young ones.  These eggs produce more females.  They store more material, have a lower metabolism and less oxidizing capacity than do the earlier male-producing eggs.

It would be unsafe to draw specific conclusions about mammals from these bird and insect experiments.  Both the secretory action and the chromosome mechanisms are different.  The quantitative nature of sex, and also the existence of intersexual types, between males and females, would seem to be general phenomena, requiring rather slight corroboration from the mammals themselves.  We have such mammalian cases as the Free-Martin cattle, and some convincing evidence of intersexuality in the human species itself, which will be reviewed presently.

The notion of more “developmental energy” or a higher metabolism in males is borne out in the human species.  Benedict and Emmes[7] have shown by very careful measurements that the basal metabolism of men is about 6% higher than that of women.  Riddle cites the work of Thury and Russell on cattle to show that a higher water value (as he found in the pigeon eggs), associated with increased metabolism, helps to produce males.

In males, the secretion of the sex glands alone seems to be of particular importance, again suggesting this idea of “strength” which comes up over and over again.  Removal of these glands modifies the male body much more profoundly than it does the female.[8] It is quite generally supposed that the action of this one secretion may have much to do with the superior size and vigour of males.  For example, Paton says[9]:  “The evidence thus seems conclusive that in the male the gonad, by producing an internal secretion, exercises a direct and specific