member that there are certain fundamental 

 differences between "pure lines" and "clones," 

 which render it very important to maintain 

 the distinction between them. I will mention 

 but two of these differences by way of ex- 

 ample: (1) In the "clone" it is possible to 

 retain as a permanent feature of the group 

 any purely heterozygous character, as for in- 

 stance the vigorous constitution dependent 

 upon the stimulation of heterozygosis ; such a 

 phenomenon is impossible in the " pure line." 

 (2) When clonal individuals reproduce sex- 

 ually, either by self-fertilization or by crossing 

 with other individuals, they need not, and 

 usually do not, produce genotypically equal 

 offspring, because the individuals of the 

 " clone " are not necessarily homozygous, as 

 the individuals of the "pure line" generally 

 are. The " clones " of horticultural plants are 

 notorious for the heterogeneity of their seed- 

 ling offspring. The investigator of inter- 

 mittently parthenogenetic organisms like 

 aphids, rotifers and Hieracium, and of inter- 

 mittently vegetatively produced organisms 

 like paramecium and many plants, can not 

 properly assume that their races are geno- 

 typically pure in the sense that they are 

 homozygous, while the worker with "pure 

 lines" can make such assumption with small 

 probability of error, in case his self-fertiliza- 

 tions have been controlled with adequate care 

 during a sufficiently large number of gen- 

 erations. 



There is another prevalent misconception 

 regarding "pure lines," to which attention 

 needs to be called. The word "pure" in this 

 connection does not refer to the genotypic 

 equality of the individuals, but only to the 

 exclusion of all crossing as a source of geno- 

 typic differentiation. 



In Dr. Harris's criticism 10 of Boomer's 

 work with peas, he points out with very evi- 



10 Amer. Nat., XLV., 686-700, November, 1911. 



