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Rh error of from three to five per cent. At low temperatures the production of females was expected, but always about the above proportion of males appeared; so also at the higher temperatures a similar proportion of females appeared. It is probable that this error was due to sexually intermediate stages, arrhenoplasmic females at the high temperature, thelyplastic males at the low temperature. Where the problem is more complicated, as in the case of cattle, to say nothing of human beings, the process of investigation will yield still less harmonious results, and the correction of the interpretation which will have to be made by allowing for the disturbance due to the existence of sexually intermediate forms will be much more difficult.

The study of comparative pathology of the sexual types is as necessary as their morphology, physiology and development. In this region of inquiry as elsewhere, statistics would yield certain results. Diseases manifestly much more abundant in one sex might be described as peculiar to or idiopathic of thelyplasm or arrhenoplasm. Myxoedema, for instance, is idiopathic of the female, hydrocele of the male. But no statistics, however numerous and accurate, can be regarded as avoiding a source of theoretical error until it has been shown from the nature of any particular affection dealt with that it is in indissoluble, functional relation with maleness or femaleness. The theory of such associated diseases must supply a reason why they occur almost exclusively in the one sex, that is to say, in the phrase of this treatise, why they are thelyplasmic or arrhenoplasmic.