204 Mr. W. Bateson. Heredity, Differentiation, 



sensibly with each other. The attempt to treat or study them as 

 similar is leading to utter confusion in the study of evolution.* 



If normality thus imagined can be shown to be a real phenomenon 

 it is conceivable that we might then profitably attempt to determine 

 in specified cases the average value of homotypic correlation for each 

 case, but the average value for a miscellaneous collection of cases would 

 still have no natural significance. 



(Note, added November., 1901.) 



On p. 287 Professor Pearson has added a note in which he seeks 

 to meet a part of my objections. He says : "A diversity due to 

 differentiation and a variability due to chance are quite distinct 

 things. The one is the result of dominating factors which can be 

 isolated and described ; the other of a great number of small factors, 

 varying from organ to organ, and incapable of being defined or 

 specified. Indeed, upon each dominating factor of differentiation is 

 superposed such a chance variability. Of course all things which 

 differ even by chance variation are in a certain sense differentiated." 

 This welcome passage outlines the conception that must form the 

 point of departure in any attempt to understand variation in its 

 relation to Evolution. The same conception I have myself often 

 laboured to express. On former occasions to these two kinds of 

 diversity of which Professor Pearson speaks I have applied the 

 terms "Discontinuous" and "Continuous." Though useful in prac- 

 tice, those terms are open to misconstruction and perversion. In 

 the present paper I have suggested the nearly equivalent terms 

 " Specific " and " Normal." Similarly, to variations occurring among 

 repeated parts or homotypes we might apply the terms " Differen- 

 tiant" and "Normal." Throughout nature the variations between 

 the members of fraternities may be discontinuous and specific, and 

 in like manner may the variations between repeated parts be specific 

 and differentiant, though in both classes normal or continuous varia- 

 tions are always superposed on them. 



In most cases the naturalist is seldom in much doubt with which 

 he is dealing. But though these two great classes of variation can 

 broadly be recognised and treated as distinct, the distinction may be 

 evasive, and when the differentiation is irregular that distinction 

 must often be obscured and not " statistically discoverable." Pro- 

 fessor Pearson is mistaken in supposing that such differentiation 

 must show itself in his seriations. It may appear only as a lowering 



* For example, in his criticisms (p. 360 and elsewhere) of the view that sexually 

 produced off spring are more variable than offspring not sexually produced, Professor 

 Pearson is merely confusing different kinds of variations and applying to certain 

 kinds conclusions derived from a study of another kind. 



