Steinach’s experiments with rats are important, because by grafting ovarian tissue into the castrated male, the male was caused to assume certain characteristics peculiar to the female. The mammary glands that are rudimentary in the male became much enlarged--not only the glandular tissue increased in amount, but the mammæ themselves were greatly developed. The hair of the male is coarser than that of the female. In the feminized male the hair was soft like that of the female. The size was smaller than that of the male. The skeleton also was affected, and Steinach thinks that it changed in the direction of a female skeleton. Even more striking was the sexual behavior of the feminized rat. The individual no longer reacted as male, but showed some of the reflexes peculiar to the female. These results, that stand almost alone, appear to show that several of the secondary sexual characters of the female rat are due directly to the presence of the ovary. One of the most striking and definite results shown by castrated rats (Steinach), guinea-pigs (Pirsche, Steinach), rabbits (Pauncet), hedgehog (Marshall), and man is to be seen in the effect on the accessory glands connected with the male ducts as well as on the penis. These remain small and infantile. Some substances produced by the testes are essential for the development of these parts. Natural selection rather than sexual selection would be the agency that here comes into play. In man the effects of castration have been often described. Eunuchs have had a commercial value in some countries, as in Turkey and China, and castration has been deliberately practiced on young children. Certain religious sects, such as the Skops of Russia, have advocated and carried out the operation. Disease has also at times necessitated the removal of the testis, more often in adults than in the young. The full effects are shown only when the operation has been carried out before the secondary sexual characters have developed. The more striking difference between the sexes involve the beard, and the hair on other parts of the body, the voice, the shape of the pelvis, and the mammary glands. For a detailed account of the results, the publications of Tandler and Grosz and Marshall’s book on the “Physiology of Reproduction” should be consulted.