No. 627] SHORTER ARTK LhS J YD Vl^,CUS^J()^ 



375 



That selection by one means or another is an oflcctivc a.nciicy 

 in producing racial changes IS not i|urst lulled to da v. as it \vas ten 

 years ago.^ The only question now ai issur is wlicilicr ilio single 

 gene is changeable. I am nicliticd to i li mk. wit li M ii i tcvaiu . t liat 

 while single genes do occasionallv cliaimv ])rodiiciiiu' nuilti])!*- al- 

 lelomorphs, a much more common orcnnviico is diaiitiv in visible 

 characters through modifymg taciois. Wliotlin' ilic diroi-tion 

 of genetic variation is controllable otlicr tliaii bv the iiianii)nla- 

 tion of modifying genes or the .bsrowcv ol nuilliplc allelo- 

 morphs remains to be determnied. Ilie cvh l.nc' at present is 

 largely negative. It is undeniable that lial)ilii\- lo wnetic varia- 

 tion is much greater in some or^'atiisnis than oihers. innch greater 

 as regards some kinds of character than as regards others, but 

 whether we can produce variabilit\ ot a tien.tie character is 

 quite a different question. AVc (•(M'lainlv at present have to 

 follow nature's lead rather than to lead nature, as regards the 

 course of evolutionary change. 



W. E. CASTI.E 



