REVIEWS 191 



subject, and discusses some of them as completely as possible within its narrow 

 limits. It begins with a short historical and geographical chapter, continues with 

 a study of the parasites and a discussion of the carrying agents, the Anopheline 

 mosquitoes, and describes the recent cultivation of the parasites by Bass and 

 Johns, recently confirmed by D. Thomson and Ziemann and others. It also 

 gives a good resume of the various theories regarding relapses. In this last 

 connection, I think that the hypothesis of intracorpuscular conjugation has been 

 much over-rated. As long ago as 1898, I supported Mannaberg's ideas from my 

 own observations in connection with the so-called Proteosojna of birds, in which 

 many corpuscles contained five or six small parasites which appear to coalesce 

 in order to produce the sexual forms. But it is very difficult to give a genuine 

 proof of their really doing so, while the supposed process appears to be very 

 contrary to what we should expect from our general biological knowledge. But 

 even if we admit that this conjugation does occur, it is still impossible to under- 

 stand how it can have any effect in continuing the species of parasites in the host. 

 The conjugation is supposed to produce the sexual forms, and these, for reasons 

 which I have frequently given, cannot be proved to be concerned in such con- 

 tinuation of the species — though of course they continue the species in the 

 mosquito. Rather an unnecessary fuss is made upon this question, because, 

 after all, the long continuance of the malaria parasites in the host is precisely 

 similar to the continuance of other organisms, and probably depends upon 

 immunity questions. The author gives also a short review of my analytical study 

 of the factors concerned in the spread of malaria in a community, though he 

 does not deal with my actual general epidemiological equations (given in the 

 second edition of my book on The Prevention of Malaria and also in Nature, 

 October 5, 191 1, and February 8, 1912). It is singular that though these 

 equations are of vital importance in the whole of epidemiology, they have received 

 scarcely any attention from any of the very numerous epidemiologists who instruct 

 us on these matters. Perhaps medical men are too little given to exact thought 

 to make very good epidemiologists. Their processes of reasoning are generally 

 more qualitative than quantitative, and their mistakes are, still more frequently, 

 correspondingly astonishing. Nevertheless, Dr. Henson shows more exactness 

 than usual, and his book does not admit of a detailed study of this subject. Much 

 recent work on malaria is open to a logical fallacy connected with microscopical 

 work which I have also ventured to point out. An observer examines a large 

 number of objects one after another. What he sees is often described correctly 

 enough ; but when he attempts to connect the different objects by means of a 

 mental thread which he calls a life-history, he is apt to forget that this thread 

 is hypothetical, and is finally tempted to imagine that it is as real as the objects 

 which he sees. This is an explanation of the extremely inconclusive work which 

 is done on the life-history of many microscopical organisms. For instance, scores 

 of observers have come to different conclusions regarding the life-history of various 

 trypanosomes, at a great expense of money and time ; and yet our knowledge 

 is very defective. What is wanted is new methods, especially those of cultiva- 

 tion and correct enumeration. As regards malaria, these new methods are already 

 yielding fruit, and will probably revolutionise our study of the subject very soon. 

 In the meantime Dr. Henson's book ought to be in the hands of all those who 

 require a condensed knowledge of the subject. 



R. ROSS. 



