72 



siu lists are generally unwilling to give in, these disputes fre- 

 quently end in a deadlock. 



(3) The critic, assuming the judicial attitude of the unbiassed 

 mind, sets out the evidence on the one hand and the evidence 

 on the other hand; he generally concludes that more research 

 is needed before the matter can be settled. This method, also, 

 is excellent. It is acceptable and helpful to rival controver- 

 sialists, because the critic presents each side fairly; and the 

 critic has the satisfaction of feeling that he has not committed 

 himself to any opinion which may ultimately turn out to be 

 wrong. 



(4) Hypotheses may be neither wholly right nor wholly 

 wrong ; the elements of truth which each contain may be pieced 

 together and reconstructed. The critic who attempts recon- 

 struction may be performing a useful task, but he is taking risks. 

 The controversialists are not likely to be pleased, because nobody 

 cares to be told that his theory is only good " in parts " ; and 

 if the critic's efforts at reconstruction happen to fail, the re- 

 sponsibility is entirely his own. 



In dealing with the questions raised in this report, I think 

 that method (4), though beset with pitfalls, ought not to be 

 neglected. 



General Principles. 



In the endeavour to form a mental picture of chemical 

 specificity as applied to bacterial antigens, I imagine that this 

 specificity manifests itself in a variety of ways. 



(1) To take the simplest case first, with some bacterial species 

 there is a certain antigenic structure (A) which is common to 

 each member of the species and is always demonstrable as the 

 specific antigen. 



(2) With other species, each member may possess a common 

 nucleus (N) of antigenic structure, but to this nucleus there is 

 always attached one or other of a variety of chemical groups, 

 a, 6, or c, &c. ; and this attachment is of such a nature that 

 antigenic properties are not manifested by N per se but only by 

 the complex Na or N6, &c. ; hence Na antigen reacts only with 

 Na serum, not with N6 serum. 



(3) With other species, again, the conditions are more complex. 

 There is not the uniformity of (1), due to the invariable dominance 

 of A, nor the sharp diversity of (2), due to the dominance of 

 different antigenic groups, a, b, &c. attached to a nucleus winch 

 is antigenically inert. Within the species there are different 

 antigenic structures, e.g. , A, B or C, and to these may be attached 

 a variety of chemical groups, a, b, c, &c. ; but in this case the 

 attachment is of such a nature as not to mask the antigenic 

 value of the A, B or C. Hence, in comparing strains of such 

 a species, if the term " species " be admissible for such an 

 irregular group of organisms, resemblances will be found between 

 any two possessing in common any one antigenic feature, A or a 



