8 GENERAL CONSIDERATIONS 



Knowledge of the life histories of endoparasites, particularly those of 

 man, has grown amazingly, and prophylaxis has grown with it (see 

 Becker, infra, Chapter XVII; and Kirby, infra, Chapter XIX-XX; and 

 for immunity, see Taliaferro, infra. Chapter XVIII). 



Some Historical Facts 



Each of the myriads of species represented in this diversity of habitats 

 has its own specific fundamental organization which carries the possibility 

 of indefinite life in the future. We cannot visualize the conditions under 

 which life came into being in times past, but we can observe, study, 

 describe, and in part measure the manifestations — vitality — which have 

 kept it going and enabled it to withstand all of the vicissitudes of nature, 

 through upheavals, floods, droughts, and other symptoms of the might 

 of nature which have played so prominent a part in the history of the 

 earth's activity since the dawn of life. 



We have as a basis for such study the vast mass of knowledge which 

 has accumulated since Protozoa were introduced to science by the Dutch 

 naturalist Leeuwenhoek in the latter part of the seventeenth century. 

 Many different kinds were soon recognized, and this recognition led to 

 classifications and logical groupings in general which facilitated the dis- 

 covery and descriptions of new species, modes of life, adaptations to 

 changing conditions, and the like. 



Almost from the time of their discovery, the Protozoa have played 

 an important part in problems of general biological interest. Spontaneous 

 generation, for example, or origin of living things from nonliving mat- 

 ter, which was popularly and generally believed up to the sixteenth 

 century, had received hard knocks from the experiments of Redi, Spal- 

 lanzani, Harvey, and other scientific men. All life from life, all living 

 things from eggs [omne vivum ex ovo) supplanting the common belief 

 regarding generation. 



The use of the microscope, revealing a novel and marvelous world 

 of living things, gave a new lease of life to the theory of spontaneous 

 generation. Appearing in containers of pure water, it was asked "How 

 could such water become animated with living things if these had not 

 arisen there by spontaneous generation.''" The problem, thus reopened 

 with the discovery of Leeuwenhoek's animalcula, was not solved until 

 near the end of the nineteenth century by the careful work of Pasteur, 



