Inheritance of Size 65 



that essentially the increase of size in the measureable 

 mass occurs all at once in each generation. 



Arcella. The genus which has been most studied 

 is Arcella. Pearl and Dunbar ('03) studied the vari- 

 ability occurring in a single wild culture of A. vul- 

 garis. The diameters proved to be unimodal in fre- 

 quency, but the variation was great (table 5), per- 

 haps due to lack of uniformity in the ultimate ancestry. 

 Nevertheless there was no essential change in the mean 

 size or in the variability throughout a period of three 

 weeks in this culture. 



Hegner ('20) compared the inheritances of size in 

 several races of the one species Arcella dentata. The 

 individuals of this species are characterized by having 

 two nuclei each. He found unimodal frequencies for 

 the diameters of shells, both in wild populations and 

 in pure lines or clones ; the variability among the latter 

 being considerably smaller than among the former. 

 Within a clone there was a high correlation (of +0.49) 

 between diameter of parent and diameter of offspring, 

 which is a fact of great importance. 



Occasionally a very small or a very large specimen 

 was produced; the progeny of the unusual individual 

 were then also unusually small or large, yet always 

 tended toward the mean. Four or five generations 

 were required to return to the mean. Meanwhile the 

 original small or large individual reproduced more 

 first-generation progeny, all of which were like the first 

 of its progeny in size. Similar regulations toward 

 mean size, extending through four or five generations 

 were observed by Hegner after a half or less of the 

 shell, protoplasm, and nuclei had been cut away (fig- 

 ure 28), and again after an individual had regained the 

 binucleate condition by means of the process of re- 

 producing an empty shell without fission of the living 

 material (figure 29). 



