In the case of diphtheria, the antitoxin appears to be more efficacious than an opsonin. A horse is inoculated with diphtheria, and when he has recovered, his blood is collected. This blood is then rich in antitoxin, the natural antidote that has enabled the horse to get well again. The blood is allowed to clot, and the clot is removed; the fluid residue is called serum, and the serum contains the antidote. If now another horse has diphtheria, and you want to cure him quickly, what more natural than inject the serum of the horse who has just recovered? it will save the second horse the trouble and the time of making the antitoxin for himself, and it has been proved over and over again that the second horse does recover with amazing celerity. The pathologists then advanced a step, and asked, Why should this antidote be used solely for animals when they have diphtheria? Why should not the horse's serum be beneficial to human beings when they are attacked with the same disease? The diphtheria poison is much more harmful to a man, and kills him more quickly than it does a horse; it is therefore imperative to use the antidote early. The crucial experiment was made; entire success followed it, and now, as Professor Richet says, it is the only treatment employed, and any medical man who refuses to use it is little short of a criminal. I have entered into this brief and, I trust, simple explanation of serum treatment, because so many people want to understand it and are unable to comprehend the technical terms which scientific men, writing for scientific readers, almost exclusively employ. I am even hopeful that some of the more reasonable opponents of animal experimentation may be convinced that by carrying out the new methods of serum therapy, we are not going against nature but helping her. It is just these 'messy things' that nature uses for curing infectious diseases, and the introduction of an opsonin or an antitoxin is not putting matter in its wrong place, but in its right place; and therefore the use of the terms filth and dirt in this relationship should be confined either to the foul-mouthed or to the ignorant. W. D. HALLIBURTON. _July 1908._ P.S.--The proof sheets of Professor Richet's book have passed through my hands during their issue from the press. Beyond a few verbal amendments, and a footnote here and there which I have added and initialled, no alterations have been made in the original. I am also responsible for the insertion of Appendix C, regarding the aims and objects of the Research Defence Society. These additions and minor alterations have all met with Professor Richet's approval.