PARTHENOGENESIS AND PURE LINES 209 



ants, the broken line the fluctuations of the shortest 

 variants. It was found that much of the fluctuation 

 observed was connected with temperature. The tempera- 

 ture was therefore kept constant at about 65° F. for the 

 next twenty generations, and as shown in Fig. 97, the 

 fluctuation in the fraternal line was cut down. No in- 

 fluence of the selection is observable in the chart. This 

 evidence, in conjunction with that for other characters, 

 shows that no change takes place in the characters of 



63 C* 65 66 67 6i 69 70 7J 71 73 7'/ 75 76 77 78 79 20 ?I n 



IM 



—Curve showing the effect of selection for the second score of generations. 



(See Fig. 96.) 



the insect so long as the same group of chromosomes 

 remains. It would be difficult to find a better example 

 than these parthenogenetic insects to test the claim that 

 selection can change the germ-plasm, for here the con- 

 ditions are even simpler than in unisexual forms unless 

 they have first been made homozygous. 



The aphids also furnish favorable material to illus- 

 trate how the environment may cause very great changes, 

 even when the genetic complex remains the same. The 

 parthenogenetic aphids appear often as winged individ- 

 uals (Fig. 95, 6). There is an entire change in structure 

 involving practically every part of the body. The winged 



14 



