146 THE FIRST PRINCIPLES OF HEREDITY 



mathematics the " Normal Curve of Frequency of Error," 

 or, shortly, the " Normal Frequency Curve/' 



reoo lnd'i\/iduiL!s 



62 63 64 65 66 



Stature In Inches, 



67 68 69 70 



71 



72 



73 74 76 



76 



Fig. 6^. — Curve of Stature. 



{From R. H. Lock, " Recent Progress in the Study of Variation, 



Heredity, and Evolution.") 



In order to comprehend what the Normal Frequency 

 Curve stands for, we must enter somewhat into the field 

 of the mathematical theory of probability.* If we toss 

 up two similar coins simultaneously there are three possi- 

 bilities : We may get head-head (H-H), head-tail (H-T), 

 or tail-tail (T-T). When tossing up the coins a very great 

 number of times, we find that, according to the Law of 

 Probability, we get head-tail twice as often as either 

 head-head or tail- tail. We get, in fact : 



I H-H + 2H-T+1 T-T. 



(This is due to the fact that in tossing up the 

 two coins many times — e.g., fifty times — we throw up 



* We can give only the very briefest account of this subject, 

 which is mathematical and very abstract, and shall also in the 

 following pages state only the most essential facts of Biometrics, 

 seeing that this science involves the application of advanced 

 mathematics. 



