2l6 



INVOLUTION, GENETICS, AND EUGENICS 



darker specimens predominating in one series and lighter specimens in 

 the other, he obtained a final generation of individuals showing practi- 

 ce I iy the same range of variability in both series, and in neither series 



was there any consistent 

 change from the condition 

 present at the beginning 

 of the experiment. 



More recently Sewall 

 Wright, working in connec- 

 tion with the Bureau of 

 Animal Industry, has pro- 

 duced a considerable num- 

 ber of pure lines by long 

 continued brother-and-sis- 

 ter mating in guinea pigs. 

 Sixteen families have been 

 established by over thirty 

 generations of the closest 

 possible inbreeding. No 

 selection was practiced. 

 The result was that most 

 of the pure lines showed 

 marked reduction in fer- 

 tility and in vigor, as 

 compared with the control 

 cross-bred stock. Each 

 line, however, differed 

 from all the others in these 



Fig. 51. — Diagram to il- 

 lustrate the results of selection 

 in pure lines. Ineffectiveness 

 of selection through twelve 

 generations within a homozy- 

 gous strain in the case of the 

 Colorado potato-beetle (Lepti- 

 notarsa). In each generation 

 extreme dark specimens were 

 selected as the parents of the 

 succeeding generation but the 

 progeny always swung back to 

 the type. (After Tower.) 



