The Method of Evolution 



59 



established: one of narrow-striped animals, minus series; 

 the other of wide striped, plus series. In each generation 

 the most extreme individuals were selected as parents; in 

 the narrow series, those with narrowest stripe; in the wide 

 series, those with widest stripe. 



+3 



-1 



Generation 12345678 



FIG. 32. Chart showing effects of selection in eight successive generations 

 upon the color pattern of hooded rats. A, A', average condition of selected 

 parents; B, B', of their offspring. 



The result of the selection is shown graphically in Fig. 

 32 (compare Table I). The offspring in the narrow series 

 became with each generation narrower; those in the wide 

 series became with each generation wider, with a single 

 exception. In the second generation the wide stock was 

 enlarged by the addition of a new strain of animals. This 

 caused a temporary falling off in the average grade of the 

 young, the two series overlapping for that generation. No 



