The fine-spun, elaborate theories about sex, so current twenty years ago, have fallen into almost complete desuetude among scientists. With the discovery of the place of the chromosomes in inheritance, biologists began to give their almost undivided attention to a rigid laboratory examination of the cell. This has included sex phenomena since McClung and Sutton pointed out the function of the sex chromosome in 1902 and 1903. Present-day "theories" are little more than working hypotheses, developed, not in a library or study, but with one eye glued to a high-power microscope. Besides its faulty foundation as to facts, the old gynæcocentric theory involved a method of treatment by historical analogy which biologists have almost entirely discarded. Anyone interested in the relative value of different kinds of biological data for social problems would do well to read the opening chapter of Prof. Morgan's "Critique of the Theory of Evolution"[9], for even a summary of which space is lacking here. College reference shelves are still stocked with books on sex sociology which are totally oblivious of present-day biology. For example, Mrs Gilman (Man-Made World), Mrs Hartley (Truth About Woman) and the Nearings (Woman and Social Progress) adhere to Ward's theory in substantially its primitive form, and not even sociologists like Professor Thomas (Sex and Society) have been able to entirely break away from it. The old question of male and female predominance in inheritance has been to a considerable extent cleared up, to the discomfiture of both sides to the controversy. Most exhaustive experiments failed to trace any characters to any other part of either sperm or egg than the nucleus. Transmission of characteristics seemed to be absolutely equal by the two parents. The male nucleus enters the egg practically naked. Hence if the characters are transmitted equally, there is certainly ground for supposing that only the nucleus of the egg has such functions, and that the remainder merely provides material for early development. Yet this does not seem to be strictly true. Parthenogenesis (development of eggs without agency of male sperm) proves that in many simple forms the female nucleus alone possesses all the essential determiners for a new individual. Boveri's classic experiment[10] proved the same thing for the male nucleus. He removed the nuclei from sea-urchin eggs and replaced them with male nuclei. Normal individuals developed. To make things still more certain, he replaced the female nucleus with a male one from a different variety of sea-urchin. The resulting individual exhibited the characteristics of the _male nucleus_ only--none of those of the species represented by the egg. Here, then, was inheritance definitely traced to the nucleus. If this nucleus is a male the characters are those of the male line; if a female those of the female line, and in sexual reproduction where the two are fused, half and half.