I HA VE mentioned various changes in the way in which we regard man, which have come about during the course of the present century. You will have noticed that the new work in anatomy and physiology is a logical outcome of the old. There have heen thrilling discoveries, hut it cannot be said that we look at man in a fundamentally different way as a result of those which we have discussed so far. That does not apply at all to what I shall now relate, for in the realms of inheritance and sex determination we now know much, where practically nothing was known before. The beginning of this century was the time when the new knowledge suddenly began to spring into existence.