MODERN THEORIES OF HEREDITY 



375 



I propose to deal first with the so-called biometrical theory 

 based primarily on statistical methods dealing with large numbers 

 of observations. This theory is associated with the names of 

 Sir Francis Galton, the founder of the Eugenics Laboratory in 

 London University, and Professor Karl Pearson. The main 

 part of this theory is often called the " theory of ancestral 

 contributions," and it endeavours to define the exact degree 

 in which the characters of an organism may be correlated with 

 those of various relations, particularlv the parents, grand- 

 parents, and great-grand-parents. 



Since the statistical use of the facts may be most easily 

 demonstrated by graphical methods, I shall first give a very simple 

 example, quite unconnected with the subject of heredity. If we 



Fig. 2. 



take two pennies and toss them several times, w^e shall find that 

 they will come down in the proportion of : two heads and no 

 tails, t; one head and one tail- 2; no heads and two tails, i. 

 If, however, we toss ten pennies instead of two, we find that to 

 get all of them coming down heads is a very rare occurrence, 

 only happening about once in a thousand times. The propor- 

 tions are : — 



TO heads o tails i time 



9 ,. I tail 10 times 



8 ,. 2 tails 45 ,, ■ 



7 .. 3 ,. 120 „ 



6 ,, 4 210 



5 .. 5 252 „ etc. 



We can express this as a frequency " curve," as shown in Fig. 2. 



