vii.] DISCUSSION OF THE DATA OF STATURE. 121 



call this value P, and write each stature under the 

 form of P + x (in which x includes its sign), and if the 

 number of times with which any value P + x occurs, 

 compared to the number of times in which P occurs, 

 be called y, then x and y are connected by the law 

 of Frequency of Error. 



Though the impediments to flight are less unfavour- 

 able, on the average, to the stature P than to any other, 

 they will differ in different experiences. The course of 

 one animal may chance to pass through denser foliage 

 than usual, or the obstacles in his way may be higher. 

 In that case an animal whose stature exceeded P would 

 have an advantage over mediocrities. Conversely, the 

 circumstances might be more favourable to a small 

 animal. 



Each particular line of escape would be most favour- 

 able to some particular stature, and whatever the value 

 of x might be, it is possible that the stature P + x 

 might in some cases be more favoured than any other. 

 But the accidents of foliage and soil in a country are 

 characteristic and persistent, and may fairly be con- 

 sidered as approximately of a typical kind. Therefore 

 those that most favour the animals whose stature is 

 P will be more frequently met with than those that 

 favour any other stature P + x, and the frequency 

 of the latter occurrence will diminish rapidly as x 

 increases. If the number of times with which any 

 particular value of P + x is most favoured, as compared 

 with the number of times in which P is most favoured, 

 be called y f , we may fairly assume that tf and x are 



