62 GENETICS AND EUGENICS 



coins are thrown several times and a count made of the 

 number of heads following each throw and these results are 

 then combined and plotted we shall get a frequency of error 

 curve about the number five which will be the most frequent. 



Fig. 11. " Frequency of error " or " normal " curve M, mode. Q, Q', qiiartile; one-half 

 the area of the figure lies between Q and Q'. After Lock. 



i. e., the modal result, heads being of the same frequency as 

 tails. See Fig. 12 and Table 2. 



Biometry has established the fact that biological variation, 

 when measurable, is commonly of the frequency of error 



TABLE 2 

 Probable Results of Tossing Ten Coins Simultaneously. (After Lock) 



Tails Relative Probability Heads 



1 4 and 



1 10 3 « 



2 45 2 " 



3 120 1 " 



4 210 " 



5 252 



type, which means that it must be the result of several inde- 

 pendent contingencies or causes. Some of these causes are 

 doubtless environmental, others are due to heredity. Their 

 combined action is to produce variation of the frequency of 

 error type. 



