314 PROTOZOA 



organisms. This requires reducing species to one of its current 

 meanings. By restricting it to designate a readily identifiable 

 group which shows the minimal irreversible discontinuity with 

 other groups, species can be set up just as well in asexual as in 

 sexual organisms. It is primarily designed for the convenience of 

 biologists and is not intended to represent a biological reality. 

 The latter is reserved for the term syngen. This designates the 

 group which is potentially able to contribute to the further evo- 

 lution of their descendants, i.e., a group within which irreversible 

 evolutionary divergence has not yet occurred. When this cannot 

 be defined by breeding methods as in asexual organisms, it is in 

 principle possible to do so by intensive comparison of every as- 

 pect of many strains and judging by every means available the 

 complexity of their genetic differences and the limits of untrans- 

 gressible discontinuities. This is precisely what is discovered by 

 breeding methods in sexual organisms. In both cases the syngen 

 is independent of readily recognized differences and depends only 

 upon complex genetic deviation associated with untransgressible 

 discontinuity. 



Conclusions. The conclusions on the three major subjects 

 dealt with in this paper are given in full in the section and will 

 not be repeated here. 



Addenda 



1. In the six-month interim between writing and proofreading. 

 new discoveries about P. aurelia make necessary the following 

 changes. 



2. The P. aurelia-multirnicronucleatum complex is not divisible 

 into well-marked species on the basis of currently used criteria: 

 body size and micronnclear number; but at least three distinct 

 types are distinguishable by size of micronuclei and structure of 

 their central chromatic area. The mieronuclear diameters are 2.6 /-, 

 3 /j. and more than 4 /a; their chromatic areas are solid, doughnut 

 shaped, and spongy, respectively. The first combination is typical 

 multimicronucleatum (varieties 13, 15, and new 16, see addendum 

 3) and it is usually associated with more than two micronuclei; 



