T. M. SONNEBORN 157 



receive greatest attention. The pertinent modern literature is in 

 great need of survey from a broad unitary point of view. That 

 is attempted here in considerable detail. The basic facts about 

 the six genera of Ciliates that have been investigated along 

 modern lines are given species by species. This sort of work is 

 being pursued more extensively now than ever before. The pres- 

 ent review therefore will doubtless soon be outdated by new dis- 

 coveries. 1 Nevertheless, it may serve a useful purpose not only 

 in summarizing the situation as it now stands, but also in point- 

 ing up the gaps in knowledge and in suggesting a central idea to 

 guide further investigation. 



This central idea is that the system of breeding — inbreeding 

 or outbreeding to various degrees — which an organism normally 

 follows is closely correlated with a considerable number of its 

 major biological features; that these features have significance 

 primarily with respect to their consequences for the breeding 

 system; and that herein lies the key to the species and evolution- 

 ary problems in the Protozoa. Attempts are therefore made to 

 recognize, for each organism dealt with, those biological features 

 that bear upon the breeding system and species problems and to 

 assess their consequences. As will abundantly appear, although 

 enough is known to validate this point of view in a general way, 

 the attempt to apply it in detail to the organisms that have been 

 studied reveals a great dearth of basic facts. One of the chief 

 purposes of this paper is to call attention to the kinds of facts 

 that are needed, in the hope that investigators will be stimu- 

 lated to supply them. As a stimulus to such efforts, the paper 

 attempts to portray the depth of understanding of fundamental 

 problems and the unification of a field to which such efforts 

 surely would lead. 



Before proceeding to the modern work on particular species 

 of Ciliates, which began in 1937, a few comments on some earlier 

 observations concerning the breeding systems of Ciliates will 

 serve to introduce it. Breeding in Ciliates has long been known 

 to take the form of conjugation. The animals unite in pairs, un- 

 dergo meiosis, fertilize each other, and separate; then both ex- 

 con jugants of each pair multiply by repeated fissions. Maupas 



