The tendency in recent years in biological work has been far too much to lay stress upon the curiously mathematical laws Mendel discovered, and consequently to concentrate attention upon the physical chromosomes as containing the factors which carry hereditary qualities. Physiologists are now making an attempt to bring back into the treatment of life a more rational outlook, and nothing has contributed more to the scientific basis of this than the recent following up of the suggestions made so long ago as 1869 by Brown-Séquard. Since Starling named the internal secretions Hormones (see the Croonian Lecture, 1905) they have been much discussed by physiologists and some medical men (see for instance the recent work of Blair Bell, _The Sex Complex_, 1916 already quoted). To form a rough mental picture of what is happening one must combine the physiological and the mechanical outlooks. One then obtains the idea that the mother is, through her mental states, affecting and to some extent controlling the production of the various internal secretions, and other more subtle and still undetected influences from various organs upon other organs, and that, in so doing she is making the environment for the various hereditary factors, in which their potentialities find it possible to develop or to be suppressed according to the circumstances which she thus creates. As is now beginning to be realized, we all have an immense number of latent potentialities, which may lie dormant and develop only under suitable circumstances. Thus in my view the mother may actually and in every sense fundamentally influence and control the character of her child, working through the remote effects of internal secretions which play on the complex material factors of hereditary qualities which form the material basis of the child’s potentialities. Thus both heredity and environment have a vital part to play in building character, _but greater than either is the subtler environment within the prospective mother created by her during the nine antenatal months_. Sometimes people who would otherwise like to believe that a mother has this power, are deterred by their own experience or that of others, who have, under conditions of distress and unfavourable circumstances, had children whose dispositions seem not to have suffered, but appear as sunny and happy as a child apparently conceived under more favourable circumstances. Here, however, one is immediately faced by the difficulties of accurate observation entailing a large number of data which tend to cancel out; for the mother who may personally have been below her usual standard of health and spirits while bearing the child may, nevertheless, actually be in such a good physical condition, or be a member of such a sound, healthy stock that the child’s heredity was better than that of the average human being, and consequently that the child itself was provided with a healthy well-run body.