In the transfusion of blood from one individual to another, that is sometimes necessary, it is essential that the blood corpuscles of the donor are not agglutinated by the serum of the recipient. Thus it is a matter of great importance to select a donor that does not bring about such a catastrophe. The simple rules are that individuals belonging to the same blood group (I, II, III, or IV) do not agglutinate each other’s blood, but the blood corpuscles of an individual represented by AA or Aa will be precipitated if the donor contains the agglutinin represented by aa, and conversely the blood corpuscles of an individual represented by BB or Bb will be precipitated if the donor contains the agglutinin represented by bb. Inspection of the diagram will show that group II (with serum bb) precipitates III and IV, and group III (with serum aa) precipitates II and IV. Further the serum of group I (aa bb) precipitates all of the other groups; while the serum of group IV precipitates none of the others. My fourth illustration has probably in some cases a glandular basis, and in this sense has probably also a quantitative chemical background. Height or stature in man is, in part, an hereditary trait. It is sometimes said that short is dominant to tall, because short parents may have both tall and short children, but tall parents produce only tall children. This is probably an overstatement, or at least a rather loose generalization. Height may be due to long legs, or to a long body, or to a long neck or to time of reaching maturity or to any combination of these; and these differences may themselves be due to independent factors in inheritance. The best that we can do with height at present is to refer it to a multiple factor basis, the actual factors being little understood. In addition to these differences in stature, all of which we call normal differences, there are certain extreme conditions superimposed on these as a background, in which the endocrine glands probably play an important rôle. While it may well be that many of these cases are caused by tumors of one of the glands, more especially of the pituitary, thyroid, or testis, it is quite possible that there may be actual inherited differences in the size and activity of these glands. So far as I know there are no thoroughly worked out cases of the inheritance of such differences in man or in mammals, but in the case of certain races of birds I have been able to show both by breeding tests and by castration experiments that glandular differences are inherited according to the Mendelian scheme. [Illustration: FIG. 10. Above (A) normal adult hen-feathered Campine cock. Below (B) castrated cock about one year after operation. The castrated bird has developed the secondary sexual characters of cock-feathering.]