THE EVOLUTIONARY SIGNIFICANCE OF THE PROTO- 

 ZOAN PARASITES OF MONKEYS AND MAN 



By ROBERT HEGNER 

 Department of Protozoology, Johns Hopkins University School of Hygiene and Public Health 



INTRODUCTION 



PARASITISM is one of the many 

 types of association that exist 

 between different species of 

 living organisms. When used 

 in a narrow sense the term implies injury 

 to one member of the association, the host, 

 and benefit to the other, the parasite; but 

 as generally employed the term parasite 

 is given to any animal or plant that lives 

 on or within and at the expense of another 

 animal or plant. Such associations are, 

 at least in some instances, actually bene- 

 ficial to the host, as Cleveland (1916) has 

 shown in the case of termites (white ants) 

 and their intestinal flagellates. The latter 

 digest the cellulose in the wood eaten by 

 the termites, thus making it available for 

 the nourishment of these insects; other- 

 wise the termites would starve to death in 

 the midst of an abundance of food. Many 

 groups in the animal kingdom contain 

 parasitic species, and the evidence is 

 overwhelmingly in favor of the view that 

 the parasitic habit has evolved from the 

 free-living mode of existence and has 

 arisen many times within each group. 

 Parasites are particularly abundant among 

 the Protozoa, flat worms (Platyhelmin- 

 thes), round worm (Nemathelminthes), 

 and Arthropoda (insects and arachnids). 



Protozoa and Primates belong to the 

 first and last phyla in the animal kingdom 

 when these are arranged in an evolutionary 



series. Both groups have been very 

 successful in the struggle for existence, 

 since one of the primates, Man, is now the 

 dominant species on the surface of the 

 earth, and the Protozoa probably excel 

 in number of species and in number of 

 individuals every other group of animals. 

 These two types of animals are not only 

 able to live and reproduce under the 

 complex conditions that now exist in the 

 world, but many of them live together as 

 parasite and host. The results of this 

 type of parasitism are often beneficial to 

 the protozoa and may or may not be 

 harmful to the primates. 



The monkeys and man are included in 

 the order Primates in the Class Mammalia. 

 One difficulty in any discussion of the 

 primates is that of nomenclature; so many 

 common and scientific names have been 

 proposed for the various species that much 

 confusion exists and it is often impossible 

 to determine which species is being 

 referred to. The large Review of the 

 Primates by Elliot (1913) is not very 

 satisfactory from the standpoint of classi- 

 fication but is used as a basis by many 

 writers. Recently Stiles and Orleman 

 (192.7) have prepared a bulletin on The 

 Nomenclature for Man, the Chimpanzee, the 

 Orang-utan, and the Barbary Ape, and the 

 names of the other primates that are 

 known to be parasitized by Protozoa are 

 soon to be presented according to modern 

 methods of nomenclature in another 



1x5 



