s 



CHAPTER IV 

 INHERITANCE OF SIZE 



ize has always been recognized as sufficiently charac- 

 teristic of unicellular organisms that approximate 

 dimensions were quite properly included in taxonomic 

 descriptions of species. But with the demonstration 

 of variability within a species it became necessary not 

 only to define the probable limits of variability, but 

 also to find out whether or not all members of a species 

 inherited size alike. 



1. Paramecium 



The close similarity of size among numerous adult 

 individuals would indicate that in general the individ- 

 uals of one generation are of the same size as the in- 

 dividuals of the preceding generation. In Paramecium 

 there is no direct proof that this is so; it is perfectly 

 conceivable that there is some sort of alternation of 

 generations; that large and small generations are 

 mixed. This similarity has, however, proved actually 

 to exist in Colpoda and in several species of shelled 

 rhizopods. Over periods of at least two or three gen- 

 erations which are living in equilibrium with a con- 

 stant environment, adults of successive generations 

 are alike in size. But there are obvious exceptions in 

 those cases where some sort of differentiation of gen- 

 erations occurs, as for example in the formation of 

 cysts, spores, or gametes. 



Racial constancy. The problem of inheritance of 

 size was studied in great detail in Paramecium by 

 Jennings ('08b). He noticed that length or breadth 

 was often much more variable among the individuals 

 of wild cultures than among the individuals belonging 



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