288 PROTOZOA 



ants can be rapidly multiplied and spread by asexual reproduc- 

 tion. The individual wastage of recombination and the swamping 

 of variations by crosses are avoided. As in any system which has 

 proved itself in nature, what is lost by one feature of the system 

 is compensated by others. But a short-term advantage is of course 

 no guarantee of long-term persistence. Very often temporarily 

 successful asexual organisms may have been wiped out on occa- 

 sions of stress. Those which have adapted to constant or but 

 slowly changing environments may, however, succeed even on 

 the geological time scale, as studies on the Flagellates of roaches 

 and termites have shown ( Kirby, 1949 ) . 



The nonexistence of species in asexual organisms is asserted 

 only by those who define species as syngens. For reasons given 

 earlier, I have rejected this definition and have accepted the 

 necessity for readily recognizable distinctive features in a species. 

 In the present section, emphasis has been placed upon the com- 

 plex genetic basis of species differences in sexual organisms, and 

 genetic differences of the same kind and magnitude were de- 

 manded for species differentials in asexual organisms. On the 

 whole, the taxonomy of the Protozoa does in fact utilize such 

 species differentials. The chief infractions of the rule are found 

 among the parasitic and medically important Protozoa which 

 have come to be known in great detail. This intimate knowledge 

 has led the "splitters" to use such genetically simple criteria for 

 species differentials that their species become the equivalent 

 of individuals or families in sexual organisms. "Lumping" is now 

 needed until the discontinuities between groups become geneti- 

 cally complex as well as readily discernible. 



Before proceeding to a consideration of the more important 

 question of the possible existence in asexual organisms ol the 

 equivalent of the syngen in sexual organisms, some further com- 

 ments on the usage of the term species in genera] and its paral- 

 lelism in sexual and asexual organisms are in order. Difficulties in 

 the application of the term species arise from the attempt to 

 make it do double duty in serving both as designating an evolu- 

 tionary unit, the one which shows minimal irreversible discon- 

 tinuity, and as designating a readily recognizable group. For 



