350 BIOLOGY OF THE PROTOZOA 



be argued that here is an example of the inheritance of an acquired 

 characteristic, whereas it is merely a matter of general vitality. 



B. Biparental Inheritance.— Through amphimixis there is a possi- 

 bility of introducing changes in the organization of a species from 

 within. The new amphinucleus is a new creation and its interac- 

 tion with the cytoplasm must differ from previous interactions. The 

 cytoplasm is also different in cases of merogamy and in cases of 

 conjugation. In merogamy there is a fusion of cell bodies as well 

 as of nuclei; in conjugation the old macronucleus, a product of the 

 old amphinucleus, is distributed throughout the cytoplasm and 

 absorbed. As a result of the interactions of new nucleus and new 

 cytoplasm, new structures and new activities or changed activities 

 may ensue. 



While a priori such origination of variations in Protozoa is a 

 logical consequence, as a matter of fact it has been rarely observed 

 in Protozoa. Here genotypes as well as fixed and congenital varia- 

 tions usually vary little from the fluctuating variations of a species. 

 The remarkable fixity of the genotype is indicated by the world-wide 

 distribution of the common species, and is clearly demonstrated by 

 long-continued cultures of any given species. Vitality also is remark- 

 ably constant as illustrated by Woodruff's long culture of Para- 

 mecium aurelia, or by cultures of Uroleptus mobilis in which the 

 average relative vitality of the first 12 series representing the F to 

 F 4 generations by conjugations was 83 per cent and the relative 

 vitality of a recent set of series representing the F i8 to the F 2 2 

 generation was 85.6 per cent. Here, although there w T as an interval 

 of six years between the two sets compared, the vitality remained 

 practically the same. 



Despite this constancy there is some unmistakable evidence of 

 variations in the Protozoa. There is also considerable evidence 

 that has been misinterpreted as mutations. Among the latter, 

 abnormalities in reorganization may be responsible for apparent 

 mutations. Thus a bi-micro nucleated, short race of Paramecium 

 caudatum w T as obtained as a result of conjugation of two normal 

 individuals (Calkins, 1906). Its two micronuclei, shortened body 

 and rounded posterior end were characteristic of Paramecium 

 aurelia and the latter was erroneously interpreted as a mutation 

 of Paramecium caudatum. The aurelia characters persisted for 45 

 generations by division when they were lost, and reversion to the 

 caudatum type occurred, presumably during a period of endomixis. 

 In like manner we may account for the amicronucleate races of 

 many ciliates (Hance, Moody, Dawson, Woodruff, etc.), the amicro- 

 nucleate condition persisting for many generations, but ultimately 

 ending in death, since failure to conjugate is characteristic of such 

 races. These are evidently not cases of mutation but temporary 

 abnormalities resulting from imperfect reorganization. 



