T. M. SONNEBORN 283 



host changes during the course of evolution have obviously been 

 coped with adequately by the symbionts without recourse to 

 sexual reproduction (Kirby, 1949). Thus, under conditions of 

 relaxed selection for sexual reproduction, it seems that it de- 

 generated step by step from normal meiosis and crossing, to 

 one-step meiosis, to autogamy, and finally to asexual reproduc- 

 tion. 



The Problems. In the series of transitional stages from non- 

 obligatory but preferred inbreeding to exclusively asexual repro- 

 duction, changes take place in the species problem and in the 

 problem of adjusting the features of life to assure maintenance of 

 an unbroken line of descent through evolutionary time. I am not 

 sufficiently familiar with the intimate details of the lives of the 

 Flagellates to attempt to work out for them, as I did for the 

 Ciliates, a full picture of how the major life features are adapted 

 to the mode of reproduction. A few of the more important and 

 obvious features will be mentioned below. 



Those who have considered the species problem in relation to 

 types of reproduction often make a sharp contrast between spe- 

 cies in outbreeding organisms and in obligatorily inbreeding and 

 asexual organisms. For example, Dobzhansky (1937, 1941) draws 

 a sharp line between the two, maintaining that there is little in 

 common save the word "species," which has very different mean- 

 ings in the two cases. He further states, "in asexual groups either 

 every biotype is to be called a species or else species do not exist 

 there at all. ' These comments are of course based primarily on 

 the concept of a species as a common gene pool, which Dob- 

 zhansky has strongly urged for 20 years. The nature of species 

 in this special sense does not suddenly change, but undergoes 

 continuous, progressive change in a single direction as one passes 

 through the various degrees of outbreeding and inbreeding to 

 asexual reproduction. One might therefore expect to find equiva- 

 lent levels of biological organization in different systems of 

 breeding and reproduction. I shall explore that possibility after 

 taking up first the problem of asexual species in the routine 

 taxonomic sense. 



Hoare ( 1952 ) maintains that only morphological differences 



