The sexual protagonists are no longer at the surface but within the most secret recesses of the organism, and they appear to science under the name of Hormones or Internal Secretions, always at work within and never themselves condescending to appear at all.

Those products of the sexual glands which in both sexes are cast out of the body, and at an immature stage of knowledge appeared to be excretions, are of primary reproductive importance, but, as regards the sexual constitution of the individual, they are of far less importance than the internal secretions of these very same glands.

It is, however, by no means only the specifically sexual glands which thus exert a sexual influence within the organism.

Other glands in the brain, the throat, and the abdomen,--such as the thyroid and the adrenals,--are also elaborating fermentative secretions to throw into the system.

Their mutual play is so elaborate that it is only beginning to be understood.