PROTOZOA 



reality, but in the method of ascertainment which is the inevita- 

 ble result of the different methods of reproduction. 



General Comments and Conclusions 



Before passing to a general statement of the major conclusions, 

 two outstanding limitations of the present paper should be 

 pointed out. First, the restriction of the treatment of Ciliates to 

 those in which mating types have been found may well introduce 

 important biases. Such a discovery is likely to be made only in 

 organisms that are not too specialized in their conditions of life, 

 for the investigator has to be able to cultivate them in the labora- 

 tory. They also must conjugate under the conditions provided. 

 This excludes entirely asexual Ciliates, if such exist, and also 

 those with undiscovered fastidious requirements for mating. This 

 might also exclude organisms with the longest immature periods 

 or shortest mature periods. Further, even if conjugation occurs, 

 the demonstration of mating types is difficult or impossible if 

 there is excessive selfing. This would exclude some extreme in- 

 breeders. In fact, most work on conjugation and life cycles in 

 Ciliates prior to studies on mating types were performed on 

 selfers. In this category belong the classic studies on Uroleptus 

 mobilis by Calkins (1919, 1920) and on Spathidium spathula by 

 Woodruff and Spencer ( 1924 ) . These and other studies are rich 

 in information on life cycles. There is much need to bring them 

 into relation to the modern work and particularly to make in- 

 vestigations on breeding systems and speciation in these organ- 

 isms. 



The second general limitation of the present paper is due to 

 the twin vices of ignorance and haste. As a newcomer to the fields 

 of taxonomy, speciation and evolution, I have only the most 

 superficial acquaintance witli the large and impressive literature 

 in these fields. As a result, I have doubtless failed in my obliga- 

 tions to make appropriate references to this literature. My onl\ 

 defense is that strong pressure from editor and publisher allowed 

 me no time to correct my ignorance. 



The major conclusions to be drawn from the present study con- 

 cern three general topics: (1) the problem of taxonomic species 



