334 STATISTICAL STUDY OF INHERITANCE 



" 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 

 assortive 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 

 reversion {i.e. the reappearance of a character which occurred 

 in a definite ancestor). In exclusive inheritance, in which the 

 offspring inherits the full character of either parent, and does 

 not blend the two, the law of ancestral inheritance in the strict 

 sense ceases to hold, for it presupposes a blend. Thus eye- 

 colour in man rarely, if ever, blends, and it is in regard to such 

 characters that we should look for reversion. 



By statistical methods Pearson has sought to ascertain how 

 far the inheritance of the duration of life extends, and has reached 

 the important conclusion that in a large percentage of cases 

 there is evidence in the death-rate that discriminate selection is 

 at work. It is no longer possible to say of natural selection, 

 as Lord Salisbury did in 1894, that " no man, so far as we know, 

 has seen it at work." " It is at work, and at work among civilised 

 men, where intra-group struggle — i.e. autogeneric selection — 

 is largely suspended, with an intensity of a most substantial 

 kind. Of the existence of natural selection there can be no 

 doubt ; we require careful experiments and observation to indicate 



