PARTHENOGENESIS AND PUEE LINES 211 



The change from wingless to winged aphids is far greater 

 than most mutational changes that we know, yet must 

 involve a different kind of change because the result is 

 reversible, while a mutation, having once taken place, is 

 relatively irreversible. 



Summing up, it may be said that the evidence shows 

 that whenever the same chromosomal complex containing 

 the same genes is found, the measurements of any charac- 

 ter in successive generations show the same frequency 

 distributions of the measurements, and the form may be 

 said in a general sense to belong to a pure line. The 

 evidence shows that whether the chromosomal complex is 

 heterozygous or homozygous, the results are the same, 

 so far as the pure line is concerned ; but it is also obvious 

 that in most animals and plants, where redistribution 

 (reduction) of the chromosomes takes place in each gen- 

 eration, only forms already homozygous will give pure 

 lines. This was the special feature of the material that 

 Johannsen worked with, but aside from its practical value 

 in studying the selection problem, the limitation of the 

 definition of pure lines to such an exceptional situation 

 leaves out of sight the wider bearing of the evidence. 



