(25) 



final result only to a very slight aegree. And if selec- 

 tion withir a clone is really effective, macs selection on 

 a large scale, in a favorable race should show such an 

 effect. ( By a favorable i-ace, I mean, of course, one in 

 which the external appearance of ah Individual is a fairly 

 accurate index of its genetic constitution ). 



Selection experiment 4. Decided effect of mass selec- 

 tion in a favorable race. On June 29, 19iG, an indivi- 

 dual of Centropyxis was isolated from material collected 

 from a freshv/ater pond on Cuttyhunk Island near Woods Hole, 

 Mass., and was allowed to multiply. The course of this 

 experiment is summarized in Table VIIT. On August 1, when 

 the liiie comprised 133 iridividuals , it was divided into two 

 groups. This race was of large size, with a medium number 

 of spines. On August 1 the average spine number was al- 

 most exactly 2.5 spines. All ir.dividuals with 0, 1 or 2 

 spines, 63 in number, were s:a asiu- ar; pi'ogenitors of low 

 lines, and all individuals with 3or 4 spines ( there were 

 70 of these ) became progenitors of high lines. As soon 

 as any of these individuals divided, the new one was ex- 

 amined. In the low lines, if the now individual had 3 or 

 4 spines, it was destroyed; but if it had 0, 1 or 2 spines, 

 it was retained and its parent destroyed. In the high 

 lines, of course, the reverse procedure was followed. Such 

 selection was continued until August 23, only a few days 

 before I had tc leave iVoods Hole. From August 26 to Sep- 

 tember 3, inclusive, no selection was practiced, and all 

 the remaining individuals of both groups were allowed to 



