330 STATISTICAL STUDY OF INHERITANCE 



different, how a 2 gives rise to a 3 , which again may be more or 

 less different. 



To bring out the contrast between statistical and physiological 

 conclusions, Darbishire refers to the familiar riddle, " Why do 

 white sheep eat more than black ones ? " with its answer " Because 

 there are more of them." " When you ask the riddle you do not say 

 that you are not referring to individual white and black sheep, 

 but the man of whom the riddle is asked invariably thinks you 

 are " with interesting consequences. " If he is a biologist he may 

 be trying to think of some physiological explanation of the fact, 



FIG. 30. Diagram to illustrate the difference between statistical and 

 physiological formulation. (After Darbishire.) 



in connection possibly with the well-established relation between 

 pigmentation and the getting rid of waste products." " In the 

 answer he is told that the amount eaten by the sum-total of white 

 sheep as compared with that eaten by the sum-total of black sheep 

 is the subject under discussion." 



" If the antithesis between truths about masses, and truths about 

 individuals, which constitutes the point in this riddle, were more 

 widely and more clearly perceived than it is to-day, there would 

 no longer be that confusion in the minds of most biologists which 

 prevents them seeing the profound difference that exists between 

 a physiological law like Mendel's, which is true of units, and a 

 statistical one like the Law of Ancestral Inheritance, which is true 

 of masses. All intending students of heredity should be asked this 

 riddle ; and if they cannot detect the fallacy in it they should be 

 declared unfit for their intended task/' 



Some Saving Clauses. (a) The laws of ancestral inheritance 

 and of filial regression are average statements as regards the 



