The author's general aim has been to survey as wide a field of evidence as possible and this had involved excursions into subjects of which he has little first hand knowledge. This width of range also has necessitated a somewhat arbitrary selection of evidence and has prevented full discussion of any indi- vidual problem. The author trusts that he has not misrepresented anyone's results or opinions, and if this has occurred, he can only plead in excuse the peculiar difficulty of giving a brief and yet accurate account of evidence of such a wide variety. The diagrams reproduced in the article have all been redrawn and in many cases the original figures or diagrams have been modified as, for instance, by recalculating dosage on the logarithmic scale. The original authors therefore have no direct responsibility for the diagrams in their present form. The author desires to thank Messrs Arnold and Co. for permitting the repro- duction of Figs. 9 and 23 from similar figures which appeared in his book "The Mode of Action of Drugs on Cells"; portions of other figures from this book also have been reproduced in modified form. The author also desires to thank Dr. J. M. ROBSON for help in correction of the proofs. Edinburgh, July, 1937. A. J. CLARK. Contents. I'age Int.roduction . . . . . . . . . . .