T. M. SONNEBORN 311 



tion to outbreeding, not inbreeding. Other indications of out- 

 breeding are: the system of multiple (probably many) mating 

 types, a regular immature period, and high mortality after in- 

 breeding. On the other hand, some setting may occur, as this was 

 observed even within a clone. Its possible relation to age is un- 

 known. Eventually work on E. patella was abandoned because 

 the animals became difficult to culture. Whether this was due 

 partly to aging and inbreeding is unknown. 



Other Ciliates. In Paramecium trichium three interbreeding 

 pure mating types are known. Selfers occur, but their relation 

 to the former is unknown. P. calkinsi has at least two varieties 

 with two mating types in each. Colpidium truncation has multi- 

 ple mating types and much selfmg in the one variety observed. 

 Stylonychia putrina includes at least two varieties with at least 

 11 and 15 mating types, respectively, and selfmg is very rare. 



One variety with nine mating types has been found in Oxy- 

 tricha bifaria. Some were collected in nature, others arose among 

 their sexual progeny. Selfmg is very rare, no hormones can be 

 demonstrated, and conjugation is regularly between unlike mat- 

 ing types. This variety is an outbreeder. In addition to multiple 

 mating types and the rarity of selfmg, other indices of out- 

 breeding are a considerable immature period and a long mature 

 period. The one culture which selfed a little usually failed to 

 survive selfmg, but usually survived when crossed to one of the 

 two other types reported upon. The system of survival and 

 death in relation to inbreeding and outbreeding is still unknown. 



Obligatory Inbreeding and Asexual Protozoa. The Flagellates 

 which are symbiotic in wood roaches and termites seem to show 

 an evolutionary series from sexual to asexual reproduction. The 

 sexual species are found in wood roaches; the related asexual 

 species occur in termites. The coincidence in time between sexual 

 processes in the Flagellates and transmission to new roach hosts 

 seems like a mechanism for providing genetic variability in the 

 symbiont at the time it might be needed for adaptation to varied 

 hosts. But the biology of the roaches indicates little variation 

 among the individuals of a colony, and the sexual processes in 

 the svmbionts are such as to vield little or no variability in them. 



