190 THE PHILOSOPHY OF BIOLOGY 



lines above and below the mean one indicate the 

 proportion (relative to the value or frequency of the 

 mean) of people of each grade of stature. The latter 

 is proportional to the distance from the mean measured 

 along the vertical line, distances below this line indicat- 

 ing statures below the mean, and vice versa. 



If, on the other hand, the men and women with 

 statures considerably below the mean marry, their 

 children will ultimately exhibit statures which are 

 greater than that of their parents, but which are less 

 than that of the whole population. Regression again 

 occurs, but in the opposite direction, and such a case 

 would be represented by the above diagram reversed. 

 Continued selection of this kind would lead to an 

 immediate increase in the mean stature (or the opposite, 

 if the " sign " of the selection were reversed) in one or 

 two generations, but after that the amount of change 

 would be very small, while if the selection were to 

 cease the race produced would slowly revert to the 

 mean, which is characteristic of the whole population 

 from which it arose. It is very important to grasp 

 this result of the practical and theoretical study of 

 heredity — the selection of the ordinary variations 

 shown by a general population leads at once to a small 

 change in the mean value of the character which is 

 selected, but continued selection thereafter makes 

 very little difference to this result, while the race 

 slowly reverts to the value of that from which it arose 

 on the cessation of the selection. 



Races which " breed true " do, of course, exist ; 

 thus the mean height of the Galloway peasant is 

 greater than that of the Welsh. In the cases of 

 "pure races" — that is, races which breed true with 

 respect to one or more characters, we have to deal 

 with another kind of variation, one which shows no 



