20 



HEREDITY 



several classes fall off on either side of tliis central one, 

 and on the whole very symmetrically on the two sides. 

 This falhng off of numbers is moderately fast at first, 

 becomes very rapid in the middle portion of each side, 

 and less rapid again towards the base. This curve 

 expresses mathematically the ordinarily observed facfc 

 that a large proportion of men are near the average 



35 



30 



25 



20 





e J5 



fO 



2 3 4 5 6 7 8 9 10 II 12 13 14. 

 Fig. 4. — Number of Heads. 



height, those differing largely from the average being 

 relatively few. 



Variability with regard to many of the attributes 

 of hving things gives a curve of the same type as that 

 shown in Fig. 3. 



But this form of curve expresses also the variability 

 in results that are, as we say, due to " sheer chance," 

 such as the variation in the number of " heads " or 

 " tails " when a number of coins are tossed several 

 successive times. In an actual experiment ^ where 

 fourteen coins were tossed 150 times, the results were 

 obtained which are shown in Fig. 4. 



1 Taken from Elderton, A Primer of Statistics, p. 11. 



