An Introduction to a Biology 



The Law of Contribution, on the other hand, deals with 

 that which is below as well as with that which is above 

 the horizontal line, and it is true of the individuals. 



Now, although Galton's Law is true of the mass but 

 not of the component individuals, the Law of Contribution 

 is true of the individuals and of the mass as well, because 

 according to it all the individuals are contributed to, in 

 the same degree. 



The widespread belief in the existence of a Law which 

 is true both of mass and individual is the result of the inter- 

 action of two outstanding characters of the human mind : 

 (i.) an inability to distinguish between truths about masses 

 and truths about individuals ; and (ii.) a passion for ex- 

 plaining things ; for possessing a formula for familiar pheno- 

 mena ; in the case of heredity, for seeing below the line in 

 the diagram on p. 182. 



Man has observed that on the whole Galton's Law is 

 true of masses — though he has not expressed himself in these 

 words. Feature No. (i.) had led him to think that the truth 

 is of individuals, and has thus satisfied No. (ii.) and sup- 

 plied him with a brief formula, a simple picture of the way 

 in which characters are inherited — a picture of what is 

 below the line AB in the diagram. 



Even if this is not a true history of the origin of the 

 Law of Contribution, the point about that Law which I 

 wish to enforce is that it is a physiological theory of heredity. 

 It is an attempt to account for the phenomena of heredity 

 by picturing the way in which the characters of an organism 

 are represented in the germ-cell which produces it. And 

 being a physiological Law, it is profoundly different from a 

 statistical one like Galton's, which does not pretend to 

 account for anything, but is a generalisation about the 

 relation between the aggregates of adults of successive 

 generations. 



In a word, a physiological Law deals with the individuals 

 of each generation on both sides of the line AB in the 



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