20 



HEREDITY 



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

 and on the whole very symmetrically on the two sides. 

 This falling 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 fact 

 that a large proportion of men are near the average 



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

 FIQ. 4. Number of Heads. 



height, those differing largely from the average being 

 relatively few. 



Variability with regard to many of the attributes 

 of living 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 l 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. 



