vi PREFACE 



great light on some of the fundamental mysteries of living matter- 

 as, for example, sex and a special chapter dealing with the 

 physiology of the Protozoa has been added. 



In so wide a field it is almost necessary to exercise some favour- 

 itism in the choice of objects, and greater stress has been laid in 

 this work upon the parasitic forms, both on account of the many 

 interesting biological problems which they present, and also because 

 they come into closer relationship with the practical needs of human 

 life than the non-parasitic species. The author wishes, however, 

 to point out clearly that he is not a medical man, but one who 

 approaches the study of the parasitic Protozoa solely from the 

 standpoint of a naturalist who is more concerned, so to speak, with 

 the interests of the parasite than with those of the host. Conse- 

 quently, purely medical problems such as, for example, the 

 symptoms and treatment of diseases caused by trypanosomes and 

 other Protozoa are not dealt with in this book, since the author 

 deems it no part of his task to attempt to instruct medical men 

 concerning matters with which they are better acquainted by their 

 training and experience than himself. The needs of medical men 

 have, however, been specially kept in view, and the author hopes 

 that the book will succeed in supplying them with useful informa- 

 tion, at least from a general zoological or biological standpoint. 



In a science, such as protozoology, which is growing actively and 

 receiving continually new additions, and in which njost of the data 

 are based upon an elaborate and delicate technique, there are 

 necessarily many controversial matters to be dealt with. In such 

 cases the points at issue have been reviewed critically, and the 

 author has, wherever possible, attempted to give a lead by indicating 

 more or less decisively what is, in his opinion, the most probable 

 solution of the problem under discussion. Such judgments, how- 

 ever, are not intended to be put forward in a dogmatic or polemical 

 spirit, since the author recognizes fully that any conclusion now 

 reached may be upset entirely by fresh evidence to the contrary. 



The vast literature of the Protozoa would, if cited in full, easily 

 fill by itself a volume of the size of the present one. It has been 

 necessary, therefore, to restrict the limits of the bibliography as 

 much as possible, both by selecting carefully the memoirs to be 

 cited and by abbreviating their titles. The works selected for 

 reference comprise, first, comprehensive treatises which deal with 

 the subject, or with some part of it in a general way, and in which 

 full references to older works will be found ; secondly, classical 

 memoirs on particular subjects, also containing, as a rule, full 

 bibliographies ; and, thirdly, such memoirs of recent date as have 



