156 PROTOZOA 



species concept. It has been maintained, not without some reason, 

 that there are more species of Protozoa than of all other animals 

 together, for each species of the higher animals is believed to 

 be sole host to at least one species of parasitic Protozoa. If half 

 or more of the Protozoa, and many other organisms as well, are 

 in principle outside the domain of the modern biological species 

 concept, it is applicable to only a minority of all organisms. 



Although this is perhaps the greatest objection to adopting 

 this species concept as the only valid one, there are also others. 

 In some organisms, what is now recognized as a single species 

 on morphological grounds would be broken up into fantastically 

 large numbers of "biological" species. Moreover, very commonly 

 these "biological" species would be to an appalling degree un- 

 recognizable and unidentifiable by routine taxonomic procedures 

 or by any other procedures that could reasonably be expected 

 of working biologists. 



The Protozoa illustrate on a lavish scale these difficulties and 

 limitations of the modern biological species concept. There is 

 need for a broader species concept which will embrace the whole 

 of biology. But it should not sacrifice unnecessarily the very real 

 values of the so-called modern biological species concept. The 

 present paper will review the situation in the Protozoa and will 

 inquire as to how there might be developed for obligatory in- 

 breeders and asexual organisms a concept which embraces for 

 them a level of biological organization comparable to the one 

 which is embraced for outbreeders by the modern biological 

 species concept. Both concepts would, it seems, have to be special 

 cases of a more general concept applicable to all organisms. How- 

 ever, as will appear, such conceptual ordering does not in itself 

 settle the question of whether the unit groups of individuals and 

 populations designated by these concepts should always be called 

 species and assigned specific names. This is a separate 1 matter and 

 is separately considered. 



Until relatively recently, much of what now appears to be 

 significant information about the Protozoa in relation to these 

 problems was unknown. This is particularly true of the Ciliates 

 which, because of the extensive literature now available on them, 



