PEEFACE. 



It is my purpose, in the present volume, to discuss some of the old 

 and some of the new problems in biology as illustrated by the lowest 

 forms of animal life, the protozoa, the subject matter being founded 

 on a course of Lowell Institute lectures given in the fall and winter of 

 1907. Interest in these organisms, of late, has centred mainly in the 

 practical side until, to many biologists and to most medical men, 

 protozoology implies the science dealing with pathogenic protozoa. 

 Protozoology has a broader scope than this, and it is one purpose of 

 this book to check, if possible, the limiting tendency and to point out 

 again the important part that the protozoa play in the problems of 

 modern biology. This is the more necessary because, in my opinion, 

 in the application of biological principles which underlie the vital 

 phenomena of free and parasitic forms alike, will be found the most 

 valuable data for the more practical sides of protozoology. Here in 

 these mere specks of animated jelly, which rarely measure more than 

 the hundredth part of an inch, we find, in their simplest forms, the 

 manifold processes of the living organism. Digestion and assimilation; 

 respiration, with its dual action of oxidation and renewal; excretion 

 and secretion; irritability and fatigue; reproduction, together with the 

 unfathomed mystery of fertilization and inheritance, all find expres- 

 sion in these simple animals and raise the lowest protozoon immeasur- 

 ably above the most complex of non-living substances. With such 

 vital processes reduced to their lowest terms in these protozoa, we 

 should expect to find a wealth of material for the study of life phenom- 

 ena which in the higher animals are masked under a cloak of differ- 

 entiated structures, and the study of these more general functions 

 should form the basis for explanations or interpretations of the more 

 specialized adaptations which are characteristic of pathogenic forms. 

 This more comprehensive field, as I understand it, is the scope of 

 modern protozoology. 



The researches of Louis Pasteur, in connection with fermentation, 

 souring of wine, and the silkworm disease, led him to many reflections 

 and conclusions as to the nature of various contagious and hereditary 

 diseases. Perhaps more than any other single research, his investi- 

 gations, begun in 1865, on the cause and prevention of silkworm 

 epidemics (to which De Quatrefages had given the name of pebrine, 



