No. 600] PIEBALD RATS AND MULTIPLE FACTORS 



739 



Discussion 1 



A great difficulty has been placed on the discussions of 

 this subject by the different terminology used by those 

 holding different opinions. Calling the visible character 

 the Mendelian unit is a striking example of this difficulty. 

 There is a vital difference between a unit character and 

 a factor, which must be constantly recognized if this dis- 

 cussion is to progress. 



It is unfortunate that the word selection has come to 

 have the significance of a slogan. For the nature of the 

 actual power of selection itself is not in question. What 

 selection is, can be easily defined and agreed upon. If the 

 nature of the changes in the germ plasm could be de- 

 termined, there would be little disagreement as to what 

 selection could accomplish. Even those who are not con- 

 sidered to be selectionists believe that natural selection is 

 very important in evolution. So the epithets, selectionist 

 and pure-lineist, fail to indicate the difference between 

 the two groups to which they have been applied. It 

 would be quite impossible to divide biologists into two 

 distinct schools on the basis of a subject upon which there 

 are many different shades of opinion. Any such classi- 

 fication would be inaccurate, even if the most precise 

 definitions of the classes were generally accepted. When 

 there are no accepted definitions, and those most clearly 

 cut are offered by individuals in the opposite group (each 

 one realizing the diverse ideas within his own group and 

 wishing to crystallize an opposing view in order to attack 

 it) such classification of opinion is far from scientific. In 

 the present instance, the classification into selectionists 

 and pure-lineists has tended to magnify the differences 

 between investigators. With a desire to try to overcome 



1 Since the -writing of this paper, there have appeared papers by Pearl 

 ("Fecundity in The Domestic Fowl and The Selection Problem," Amer. 

 Nat., 1916, p. 89) and Castle ("Can Selection cause Genetic Change?" 



discussion. It has been considered wiser to leave this paper as written, 

 than to enter the controversy by including discussions of the two papers 



