INTRODUCTION 9 



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 protozoology contributes toward the 

 advancement of our knowledge of the steps by which living 

 things in general evolved. 



Geneticists have undertaken studies on heredity and varia- 

 tion among Protozoa. Dimensional variation is encountered 

 commonly in various species, as has been shown by Jennings, 

 Jollos, Dobell, Hegner, and others. Whether the variation is 

 due to germinal or to environmental conditions cannot easily 

 be determined. The interesting case of the double form of 

 Uroleptus mohilis observed by Calkins apparently involved 

 germinal changes, since this form divided true for 367 genera- 

 tions through a period of 405 days. On the other hand, the 

 environmental changes or acquired characteristics are not in- 

 herited. Changes due to differences in temperature or composi- 

 tion of the medium last through numerous generations as long 

 as the differences are maintained, but thereafter the original 

 characteristics reappear. 



Parasitic Protozoa almost always are limited to one or more 

 specific hosts. By studies of the forms belonging to one and the 

 same genus or species, the phylogenetic relation among the host 

 animals maybe established or verified. The mosquitoes belong- 

 ing to the genera Culex and Anopheles, for instance, are known 

 to transmit Plasmodium praecox and human species of Plas- 

 modium, respectively. They are further infected by specific 

 microsporidian parasites. For instance, Thelohania legeri has 

 been found widely in the many species of anopheline mosquitoes 

 only; T. opacita has, on the other hand, been found in culicine 

 mosquitoes, although the larvae of species belonging to these 

 two genera live frequently in the same body of water. By 

 observing some intestinal Protozoa in certain monkeys, Hegner 

 has recently obtained evidence of the probable phylogenetic 

 relationship between them and other higher mammals. 



Study of a particular group of parasitic Protozoa and their 

 hosts may throw light on the geographic condition of the earth 



