158 PROTOZOA 



( 1888, 1889 ) discovered that a declining food supply was usually 

 a necessary, but not sufficient, condition to bring about conjuga- 

 tion. He further maintained that outbreeding was the rule, partly 

 because he obtained fruitful conjugation only when he had put 

 into his aquaria collections from different natural sources. When 

 conjugation took the form of inbreeding, it led only to death of 

 the exconjugants. 



These observations and conclusions of Maupas were contro- 

 verted by the studies of Jennings (1910) on Paramecium cauda- 

 tum and P. aurelia. He cultivated in the same container strains 

 of the same species that could be distinguished by the size of the 

 animals. When conjugation occurred in these cultures, each 

 strain appeared to conjugate only among its own members, even 

 when both strains were conjugating at the same time. Moreover, 

 working with isolated single individuals of a single strain, he 

 found that conjugation could occur among individuals produced 

 in the course of a few fissions from a single ancestor and that 

 such exconjugants produced normal viable clones. A number of 

 successive closely inbred generations were followed with the 

 same result. On the basis of these observations, he denied the 

 validity of Maupas' generalization and held that conjugation in 

 Paramecium could occur only between individuals of the same 

 strain and that within a strain any individual could probably 

 mate and mate fruitfully with any other. 



Early workers on breeding systems thus came to diametrically 

 opposed observations and conclusions. According to Maupas, the 

 Ciliates are obligatory outbreeders; according to Jennings, they 

 are obligatory inbreeders. As will appear, the observations of 

 both investigators were entirely correct. The differences are in 

 the Ciliates themselves. Some are outbreeders; some are inbreed- 

 ers. On the question of who can and does mate with whom, and 

 with what results, we shall have much to say below. 



A few later workers (De Garis, 1935a,b; Midler, 1932) claimed 

 to have succeeded in obtaining crosses between different strains 

 and even between different species of Paramecium. Although 

 the methods employed by De Garis were not unobjectionable, 

 the fact that diverse strains of these species can be crossed has 



