246 Heredity and Eui^aiies 



to maintain the race. In so far as color characters are concerned, 

 by artificial selection we can easily produce and maintain a race, but 

 cannot establish it as an independent one; it is possible to create 

 isolated races from extreme variations, and by selection keep them 

 isolated, but it seems very difficult to permanently establish them. 

 On the whole, selection would appear as a relatively impotent factor 

 in evolution. Two points should be noted, in passing, namely, the 

 numljer of highly divergent variations beyond the normal range of 

 fluctuating \-ariation produced in this last series of e.x[)eriments, and 

 the increased percentage of individuals which show ^'ariations capable 

 of being transmitted to the progeny. 



It seems to be a well-established fact that by quanti- 

 tati^'e accumulation, pigment or any other character in the 

 organism can be made to diverge rapidly up to a certain 

 limit, beyond which it is practically impossible to go. More 

 striking, however, is the rapidity with which such races 

 rcA'ert to the modal condition of the parent stock when 

 the selective process has ceased . This phenomenon has 

 been characterized as regression toward the mean, and 

 it ma}' represent either one of two processes — an inherent 

 tendenc}- in the organism to revert to the standard, or it 

 may represent a selective process which goes on within the 

 organism. I am of the opinion that the latter is the correct 

 explanation of this, for the following reason; modification 

 bv the quantitative accumulation of minute fluctuating 

 variations is always working against what may be termed 

 the natural selective tendency, that is, against those tend- 

 encies which surround every organism and which eliminate 

 extremes in each generation through one cause or another, 

 allowing the modal tj-pe to persist. Any effort, therefore, 

 to preser\'c the extreme ti-pe comes in conflict with that 

 which serves to eliminate extremes and preserve the modal 

 type. There are two conflicting forces, one which tends 



