334 STATISTICAL STUDY OF INHERITANCE 



rapid and permanent changes of type, when selection is 

 stringent." 



Prof. Pearson has worked out the following case. Suppose 

 the mean height of a population be 5' 8", that a start is 

 made with individuals 6' 2", and that for successive genera- 

 tions individuals of this height are selected as parents. It is 

 calculated that in the first generation the offspring would show 

 0-62 of the particular quality selected (h), viz. 6" of deviation 

 above the general mean height. It is calculated that after two 

 generations the offspring will show 0-82/2, after three generations 

 0-89^, and so on up to 0-92/2. Thus by persistent selection 

 an array of individuals would result, almost all of whom were 

 over six feet in height. 



But if at a given generation the artificial selection of tall 

 parents stops, and the tall array is left to inbreed, there will be 

 a gradual sinking back towards the mean height of the population. 



The importance of definite conclusions of this kind can hardly 

 be overestimated. 



" Looked at from the social standpoint, we see how exceptional 

 families, by careful marriages, can within even a few generations 

 obtain an exceptional stock, and how directly this suggests 

 assortative mating as a moral duty for the highly endowed. On 

 the other hand, the exceptionally degenerate isolated in the 

 slums of our modern cities can easily produce permanent stock 

 also : a stock which no change of environment will permanently 

 elevate, and which nothing but mixture with better blood will 

 improve. But this is an improvement of the bad by a social 

 waste of the better. We do not want to eliminate bad stock 

 by watering it with good, but by placing it under conditions 

 where it is relatively or absolutely infertile " (Pearson, Grammar 

 of Science, p. 486). 



By statistical methods Pearson has reached the interesting 

 conclusion that while blended inheritance illustrates regression, 

 it is to cases of exclusive inheritance that we should look for 



