290 PROTOZOA 



in doing the same with sexual organisms. He has in fact already 

 done it and, on the whole, has done it just as satisfactorily in the 

 asexual Protozoa as in sexual Protozoa or higher organisms, with 

 the readily corrected exceptions pointed out above. 



The question of whether asexual equivalents of syngens exist 

 and can be recognized is more difficult, but not hopeless. The key 

 to progress in this direction is to recognize in the syngens of sex- 

 ual organisms a distinction between the means of ascertainment 

 and that which is ascertained. Breeding methods are the means 

 of ascertainment. The syngen which is thereby ascertained proves 

 to be a discrete group, the discontinuity of which with other 

 discrete groups is based upon complex genetic differences. The 

 discontinuity and the complexity of its genetic basis are the es- 

 sential features of the difference between closely related syngens. 

 This is what prevents the flow of genes between them and that 

 is why the test of gene flow is a means of ascertainment. The defi- 

 nition of a syngen on the basis of gene flow is an admirable opera- 

 tional definition. But conceptually the syngen could also be de- 

 fined on the basis of discontinuity and its complex genetic basis. 

 The same basis can be used for a concept of the asexual equiva- 

 lent of the syngen. Indeed, the same term, syngen, might be used 

 in both cases for it implies not only a common pool of genes but 

 also generation by a common group. Although another term might 

 be preferred for asexual organisms, the same one will tentatively 

 be employed here. 



The obvious difficult) with the concept of the asexual syngen 

 just set forth is the lack of a simple, neat operational definition 

 comparable to the one that serves for the sexual syngen. This 

 statement of the difficulty of course assumes that there is no gene 

 flow among different lines of descent in organisms not known to 

 have sexual processes. In the light of modern researches on trans- 

 formation and transduction in bacteria, this assumption might be 

 questioned. If such mechanisms or others come to be discovered 

 in other asexual organisms, so much the better; but this cannot 

 be counted upon and may not occur. For the present at least, 

 oilier approaches must be sought. The problem then is how to 

 recognize irreversible evolutionary divergence in the absence of 



