PROTOZOA 



ence in method imposed by the difference in reproduction, cor- 

 responding levels of biological organization are thereby defined. 

 In both cases, the level of organization includes the group of 

 individuals that can potentially contribute to the further evolu- 

 tion of the group. Differences in the sharpness of delimitation 

 of the asexual syngen are to be expected. Similar differences are 

 found in sexual organisms, as will now appear. 



The Relation of Breeding Systems and Methods of Reproduc- 

 tion to the Species Problem. The preceding attempt to general- 

 ize the biological species or syngen runs counter to the view of 

 proponents of the biological species concept (e.g., Dobzhansky, 

 1937, 1941 ) that biological species do not exist among obligatory 

 inbreeders or asexual organisms. This denial, as indicated above, 

 is based upon an operational definition of biological species. Since 

 the operation, testing gene flow, is impossible in asexual or- 

 ganisms, they deny the existence in them of the thing this opera- 

 tion discovers in sexual organisms, i.e., the biological species or 

 syngen. Their statement of the situation thus implies an abrupt 

 change in the organization of nature and in the units of evolu- 

 tionary divergence correlated with an abrupt change from out- 

 breeding to obligatory inbreeding and asexual reproduction. By 

 subordinating the method of ascertainment to the thing ascer- 

 tained and by seeking methods of ascertainment in asexual repro- 

 duction, the concept of biological species or syngens was 

 generalized. This implies the absence of an abrupt change in the 

 organization of nature and in the units of evolutionary divergence 

 with changes in breeding system or method of reproduction. 



No such abrupt change is in fact found in the present review of 

 conditions in the Protozoa. On the contrary, this review shows a 

 progressive series of changes in the organization of nature and in 

 the units of evolution running parallel to the sequence of breed- 

 ing systems from extreme outbreeding through various degrees 

 of out- and inbreeding to extreme inbreeding. The passage from 

 inbreeding to asexual reproduction is accompanied by the final 

 gradual change in the series. Nowhere does an abrupt, marked 

 change occur. The continuity of the whole series stresses the 

 unity of the problems of species and syngens in both sorts of 



