[Illustration: Fig. 6.--PEDIGREE SHOWING INHERITANCE OF BLOOD GROUPS THROUGH FOUR GENERATIONS. THE GROUP OF EACH INDIVIDUAL IS INDICATED BY A NUMERAL. THOSE WHO WERE NOT AVAILABLE ARE REPRESENTED BY A O] The conclusions which emerge from this structure of theory and fact are obviously of very great clinical importance. It is now clearly demonstrated that a mother belonging, say, to Group I, may give birth to a child belonging to any one of Groups I, II, III, or IV; her blood may not be used for transfusing her child without a grave risk that the “maternal threat” may culminate in the death of the child. The same applies to the possible relations between a father and his child. Two brothers, again, may belong to Groups II and III respectively. Even the blood of twins may be mutually incompatible, except in the rare case of “identical twins,” who, it may be supposed on theoretical grounds, would certainly belong to the same group, though I am not aware of a case in which this has been put to the test. As much care, therefore, must be exercised in testing the blood groups of members of the same family before performing a transfusion as would be taken before using a donor who is not related to the patient. The medico-legal importance of the facts concerning the inheritance of blood groups is also evident, and, although this test has not yet been used as a test of legitimacy, there can be little doubt but that it will be so used in the near future. The information to be derived from it is of a negative rather than a positive character. Thus the occurrence of Group III blood in a child whose mother is of Group II and putative father of Group I cannot be taken as a proof either of legitimacy or the reverse. But if, as in Learmonth’s case, parents both of Group IV have a child of Group I, or if parents both of Group II have a child of Group I or III, then this may be taken as a proof of illegitimacy. There is not much experimental evidence concerning the effect of various pathological conditions on the agglutination reactions of the blood and serum. It has already been mentioned that there is no proof that the possession of any particular blood group confers upon its owner any special immunity from, or liability to, disease. The numbers, investigated by Alexander in the communication referred to on p. 81, are too small for the observation to be of much value; it is also necessary, as a preliminary to any such research, to demonstrate that there is no abnormal alteration in the reactions of the blood of these patients. It is probable, indeed, that evidence of this alteration in malignant disease already exists, for a reference to it is to be found in Kolmer’s work on serum-therapy,[7] but I have been unable to find a record of the investigation.