INTRODUCTION 7 



covering the nature and mechanism of various phenomena, the 

 sum-total of which is known collectively as life. Though the in- 

 vestigators generally have been disappointed in the results, in- 

 asmuch as the assumed simplicity of unicellular organisms has 

 proved to be offset by the complexity of their cell-structure, never- 

 theless any discussion of biological principles today must take into 

 account the information obtained from studies of Protozoa. It is now 

 commonly recognized that adequate information on various types 

 of Protozoa is a prerequisite to a thorough comprehension of biology 

 and to proper application of biological principles. 



Practically all students agree in assuming that the higher types of 

 animals have been derived from organisms which existed in the 

 remote past and which probably were somewhat similar to the 

 Protozoa of the present day. Since there is no sharp distinction 

 between the Protozoa and the Protophyta or between the Protozoa 

 and the Metazoa, and since there are intermediate forms between 

 the major classes of the Protozoa themselves, progress in proto- 

 zoology contributes toward the advancement of our knowledge on 

 the probable steps by which living things in general evolved. 



Geneticists have undertaken studies on heredity and variation 

 among Protozoa. "Unicellular animals," wrote Jennings (1909), 

 "present all the problems of heredity and variation in miniature. 

 The struggle for existence in a fauna of untold thousands showing 

 as much variety of form and function as any higher group, works 

 itself out, with ultimate survival of the fittest, in a few days under 

 our eyes, in a finger bowl. For studying heredity and variation we 

 get a generation a day, and we may keep unlimited numbers of 

 pedigreed stock in a watch glass that can be placed under the micro- 

 scope." Morphological variations are encountered commonly in all 

 forms. Whether variation is due to germinal or environmental condi- 

 tions, is often difficult to determine. The recent discovery of the 

 mating types in Paramecium aurelia (Sonneborn; Kimbell) and in 

 P. bursaria (Jennings) will probably assist in bringing to light many 

 genetic problems of Protozoa which have remained obscure in the 

 past. 



Parasitic Protozoa are limited to one or more specific hosts. 

 Through studies of the forms belonging to one and the same genus 

 or species, the phylogenetic relation among the host animals may 

 be established or verified. The mosquitoes belonging to the genera 

 Culex and Anopheles, for instance, are known to transmit avian and 

 human Plasmodium respectively. They are further infected by 

 specific microsporidian parasites. For instance, Thelohania 



