EVOLUTION 



495 



I prefer, therefore, to keep the two laws quite distinct and 

 term the first the law of ancestral heredity, it applies to 

 blended inheritance; the second I term the laiv of reversioji, 

 it applies to exclusive inheritance. It will be now clearer 

 to the reader why I have asserted above that we must seek 

 for reversion in exclusive inheritance. If the above law were 

 correct, we should have before us the whole of the quanti- 

 tative theory of reversion. We must not seek for reversion 

 at haphazard, we must first ascertain whether the character 

 under consideration follows the laws of blended or ex- 

 clusive inheritance. In the former case every ancestor 

 contributes, it may be, a very small share of his character 

 to each offspring ; in the latter case each ancestor con- 

 tributes the full intensity of his character to his share, and 

 it may be an indefinitely small share of the offspring. 

 These two conceptions, summed up in the terms regression 

 and reversion, ought to be kept apart.^ 



Now it can be shown that the coefficients of correlation 

 for exclusive inheritance, when there is pangamy, are, by 

 Mr. Galton's law, precisely the same as those in the case 

 of blended inheritance with pangamy. Thus it is fairly 

 clear that the eye-colour values will neither fit in with 

 blended inheritance, nor with exclusive inheritance and 

 reversion distributed according to the above law. 



In the direct line we should have for the correlation co- 

 efficients expressing the intensity of exclusive inheritance: — 



1 Mr. Francis Gallon, in his paper on Basset Hounds, states the law of 

 ancestral heredity as I have given it on p. 477, but I venture to think he is 

 really using the law of reversion, unless we consider all the ofifspring of one 

 set of ancestry as one individual or representing one heritage. 



