*Overestimation of the Internal Genitals.*--That sexual excitement can be independent to a considerable extent of the production of sexual substance seems to be shown by observations on castrated males, in whom the libido sometimes escapes the injury caused by the operation, although the opposite behavior, which is really the motive for the operation, is usually the rule. It is therefore not at all surprising, as C. Rieger puts it, that the loss of the male germ glands in maturer age should exert no new influence on the psychic life of the individual. The germ glands are really not the sexuality, and the experience with castrated males only verifies what we had long before learned from the removal of the ovaries, namely that it is impossible to do away with the sexual character by removing the germ glands. To be sure, castration performed at a tender age, before puberty, comes nearer to this aim, but it would seem in this case that besides the loss of the sexual glands we must also consider the inhibition of development and other factors which are connected with that loss. *Chemical Theories.*--The truth remains, however, that we are unable to give any information about the nature of the sexual excitement for the reason that we do not know with what organ or organs sexuality is connected, since we have seen that the sexual glands have been overestimated in this significance. Since surprising discoveries have taught us the important rôle of the thyroid gland in sexuality, we may assume that the knowledge of the essential factors of sexuality are still withheld from us. One who feels the need of filling up the large gap in our knowledge with a preliminary assumption may formulate for himself the following theory based on the active substances found in the thyroid. Through the appropriate excitement of erogenous zones, as well as through other conditions under which sexual excitement originates, a material which is universally distributed in the organism becomes disintegrated, the decomposing products of which supply a specific stimulus to the organs of reproduction or to the spinal center connected with them. Such a transformation of a toxic stimulus in a particular organic stimulus we are already familiar with from other toxic products introduced into the body from without. To treat, if only hypothetically, the complexities of the pure toxic and the physiologic stimulations which result in the sexual processes is not now our appropriate task. To be sure, I attach no value to this special assumption and I shall be quite ready to give it up in favor of another, provided its original character, the emphasis on the sexual chemism, were preserved. For this apparently arbitrary statement is supported by a fact which, though little heeded, is most noteworthy. The neuroses which can be traced only to disturbances of the sexual life show the greatest clinical resemblance to the phenomena of intoxication and abstinence which result