PREFACE 



The subject of Protozoology has, in recent years, shown a tendency to 

 become divided into two sections. In the one the student's attention 

 is directed chiefly to the study of free-living Protozoa, in the other to 

 parasitic forms, more especially those which give rise to disease in man 

 and domestic animals. Such a division, if it becomes absolute, cannot 

 lead to a clear understanding of the group as a whole, for it is evident 

 that without some knowledge of free-living Protozoa, from which they 

 have been undoubtedly evolved, a wrong conception of parasitic forms 

 will be obtained. As in other branches of science, specialization appears 

 to be inevitable if any advance is to be made, but however specialized 

 a student becomes, it is his duty to keep himself informed of any progress 

 made outside his particular field. Anyone who wishes to make an intel- 

 ligent study of parasitic Protozoa must be acquainted with the funda- 

 mental principles of general Protozoology, and, indeed, with those of 

 general Zoology, Physiology, and even other sciences. This is merely 

 another way of stating the well-recognized fact that all sciences are inter- 

 dependent. On this account the student of the Protozoa which are 

 pathogenic to man and domestic animals should have a sound knowledge 

 of other parasitic Protozoa, and at least a good working knowledge of 

 non-parasitic forms as well. Conversely, those who study free-living 

 Protozoa should have a clear conception of the parasitic forms, for the 

 extensive investigations of recent years have contributed so much to our 

 knowledge that in many respects they are better known than their free- 

 living relations, particularly as regards the completeness of their life- 

 histories and the probable course of their evolution. 



In this manual the writer has attempted to present the subject of 

 Protozoology in such a light that it will be of use to the zoologist who 

 wishes to obtain information regarding the general principles of the subject 

 and detailed knowledge of parasitic forms, and to medical men and 

 veterinarians who are chiefly concerned with those Protozoa with which 

 they have professionally to deal. 



The investigations of Smith and Kilborne on the parasite of Texas 

 fever of cattle and its transmission by ticks; those of Laveran, Grolgi, 

 Ross, and Grassi on malarial parasites of man and birds, and their 

 carriage by mosquitoes ; and the researches of Bruce, who demonstrated the 



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