In saying as much, it is not implied that the reader may not on his travels meet with medical men sceptical or hostile to this theory, but this is because the training required to appreciate the cogency of the facts adduced in proof is that, not of a medical man, but of a naturalist, and though the profession of medicine numbers in its ranks many distinguished naturalists, it is quite possible to gain the highest qualifications without acquiring any knowledge of zoology sufficient to render the student capable of really forming an opinion on such a point; for, as a matter of fact, the five years of medical training are so overburdened with absolutely necessary medical subjects, that any critical knowledge of the associated subjects of chemistry, physics, and biology must needs be left to the after years of those to whom good fortune affords sufficient leisure to admit of their attacking the fringe of these great subjects when they are no longer _in statu pupilaris_. Hence, especially among the older hands, there are numbers of medical men who would regard the above statements as premature, but the reader will find it difficult to find any naturalist who entertains any doubts on the subject. Many details undoubtedly remain to be worked out, but the broad data may be taken as absolute facts, of a character that future investigations can only amplify. These facts may be shortly stated as follows:-- (1) Malarial fevers are caused by the presence in the blood of minute animal parasites. There are several species of these, corresponding to the various types of fever; but the life-history of all is broadly the same. (2) These animalcules multiply in the blood, and when they have become sufficiently numerous, determine an attack of fever, but while in this stage cannot pass from the blood of one human being to another, except by the somewhat difficult vivisectional experiment of injecting the living blood of an infected person into the vessels of a healthy subject, a process which cannot occur in Nature. (3) Large numbers of the malaria animalcules are destroyed by what may be called the vital powers of the patient’s blood, and the question whether an untreated case of malaria dies or recovers, depends on the outcome of the struggle between the parasites and the vital forces of their host.