HEREDITY AND VARIATIONS IN PROTOZOA 571 



culture. The double Uroleptns at the age of five months was more 

 ^•io:orous than at the outset, but like all other series of Uroleptns it 

 ultimately died. It lived and reproduced however for more than 

 fourteen months (see p. 465). 



Similarly with mutilations. The mutilated portions are passi^'ely 

 handed down to progeny by division, but the organization is not 

 affected and in the course of a few divisions the normal type is 

 regenerated. This was demonstrated by Jennings (1908) and con- 

 firmed by Calkins and by Peebles (1911, 1912) in cutting oft" the 

 anterior or posterior end of Paramecium leaving a truncated indi- 

 vidual which did not regenerate but divided to form a perfect 

 individual from the posterior end and a truncated individual from 

 the anterior end (Fig. 103, p. 219); after a few divisions both ante- 

 rior and posterior indi\'iduals were perfectly normal. Abnormal 

 projections such as spines or clefts in the cortex, etc., are likewise 

 passively transmitted to descendants by division for a limited time, 

 but no permanent change in organization is brought about. 



In general the upshot of all experiments with poisons, heat, ab- 

 normalities, etc., is failure to modify the organization of Protozoa 

 in any permanent manner. The experiment of Jollos of treating 

 Paramecium at the time of reorganization is, however, a possible 

 exception. 



Modifications of the organization which arise from within the 

 organism itself, on the other hand, may be permanent. Such 

 modifications are possible through the sifting out of germinal 

 characteristics in the course of continued metabolic activity and 

 division. Some are manifested by morphological characters which 

 aftord a basis for selection on the part of the investigator. Experi- 

 ments to this end have been carried out mainly by Jennings and his 

 associates. The underlying principle in such selection work is that 

 a single individual from a "wild" population is the result of a great 

 number of hereditary characteristics stored up in the past through 

 amphimixis and united now in the organization of the single indi- 

 vidual. Such an individual, if cultivated under uniform conditions, 

 gives rise to progeny showing di\ersities in structure or function 

 which are probably ancestral characters. The extreme indi\'iduals 

 showing such diversity are selected and bred independently. 



Jennings has clearly shown that such dift'erences are characteristic 

 of all the pure lines he has studied and his findings have been con- 

 firmed by Root (191S) for Centropyxis acideata; by Hegner (1919) 

 for ArceJla denfata; and by Reynolds (1923) for ArceUa poli/pora. 

 While the fundamental character (genotx-pe) of a race is maintained 

 there are minor dift'erences in organization which may or may not 

 be manifested by structural peculiarities. This is strikingly shown 

 in Jennings studies on Difflugia corona (191(3) a favorable form since 

 the characteristics of the shell can be measured or counted and the 



