THE GROWTH OF GROUPS 51 



If this representation is a true picture of the present 

 state of the house rats as a whole, it indicates the method 

 of the origin of a race. For example, it is evident that a 

 race of black rats may arise among a multitude of the 

 brown kind without the destruction of a large number of 

 brownish black intermediates. 



Before going further we must define certain terms. 

 The words normal and abnormal are used here to mean 

 that which is in the majority and that which is in the 

 minority. It seems that the Mutation Theory invites 

 us to reconstruct our ideas concerning normality and 

 abnormality. If we regard an organism as a com- 

 plex of character units we can no longer regard the 

 normal rat or the normal man as a reality in the sense 

 of Quetelet. 



The terms normal and abnormal have a definite 

 meaning in relation to a single measurable character 

 of an individual, but no meaning in relation to the 

 individual as a whole. An organism possessing a character 

 A, in quantity equal to the mean value of A, as deter- 

 mined in the species collectively, is normal as regards 

 that character. All other members of the species, 

 which possess A in greater or less measure, are abnormal 

 in that respect. But if in addition to determining A, 

 we ascertain the mean values of other characters B, C, D, 

 in the same species, and with them compare those particular 

 characters as they are to be seen in a single individual, 

 we shall find that whereas that individual may be normal 

 as regards A it is more or less abnormal in B, C, D. 

 In other words we shall not be able to find a normal 

 individual. 



Not long ago, I wrote as follows : " We can only 



