286 PROTOZOA 



paramecia in the same clone by reason of loss of kappa from some 

 individuals. And it is no more sensible to use the one or the 

 other as a species differential. These cases are of additional in- 

 terest iu showing that visible hereditary differences need not 

 necessarily be preceded by invisible physiological genetic differ- 

 ences, as many have maintained. 



Rhizopods and the Results of Selection. Jennings (1916) and 

 his students were able to select strikingly different morphological 

 types within a clone in various Rhizopods. The different types 

 thus obtained tended to perpetuate themselves in the absence 

 of further selection. For example, Jennings obtained the most 

 varied shell (test) types in Difflugia: spineless and many spined, 

 with few teeth around the mouth and with many, and so on. Con- 

 trariwise, it was possible to start with two diverse forms and 

 select the same type from both. Here again there is no indication 

 that physiologic genetic difference precedes morphologic differ- 

 ence in the divergence of two forms. Even persistence of the dif- 

 ference among unselected progeny is no assurance that the two 

 types belong to different clones, and similarity of type is no 

 guarantee of belonging to the same clone. 



Criteria of Species in Asexual Organisms. From the forego- 

 ing examples and discussion a few useful general principles seem 

 to emerge. Species differences in sexual organisms are based 

 upon complex, not simple, genetic differences. To reduce species 

 differences to a single trait dependent upon a single gene differ- 

 ence is to equate species differences in asexual organisms to the 

 level of individual differences in sexual organisms. From this 

 point of view, a single, genetically simple visible or morpholog- 

 ical trait is in no better case than a single, genetically simple 

 physiological trait. If asexual species are to represent a level of 

 evolutionary divergence comparable to the one used in taxonomy 

 of sexual species, the distinctions would have to be not only 

 morphological but genetically complex. To employ differentials 

 which <an be wiped out by one or even a few mutational steps 

 is iu in\ opinion indefensible. 



Variation and Speciation in Asexual Organisms. The condi- 

 tions of life for an asexual organism are likely to be less varied 



