An Introduction to a Biology 



resolves itself into a discussion of the relation of Mendel's 

 Law to Pearson's Law of Ancestral Inheritance. 



(b) MENDEL'S LAW TRUE OF UNITS : PEARSON'S, OF 

 MASSES 



Discussing a little while ago with a friend the relation 

 of Mendelism to biometry, I suggested as the briefest possible 

 statement of the difference between the two that Mendelism 

 treated of units, and biometry of masses of units. My 

 friend replied : " You speak of the animals and plants with 

 which you deal as breeding true to such and such a char- 

 acter. What do you mean by this statement ? Are the 

 offspring absolutely identical with their parents in respect 

 of the character under consideration ? If they are not, 

 how like are they ; and how is the degree of this similarity 

 to be measured, except by biometric methods ? " 



I saw that there was truth in what he said, but I could 

 not see the relation which the idea in his mind bore to my 

 original idea, to which I still adhered, that Mendelism dealt 

 with the units, while biometry was concerned with masses- 

 Now I see that my inability to do so was due to the fact 

 that biometry meant to my friend that the only way to 

 measure the resemblance between parents and children is 

 the method of the correlation table ; in which he was quite 

 right ; while biometry called up in my mind the Law of 

 Ancestral Inheritance, and especially the manner in which 

 data designed to establish that Law are collected and dealt 

 with ; such, for example, as in the case of one of the last 

 series of data from which such correlation coefficients have 

 been worked out that of greyhounds. I had in my mind 

 the collective treatment in one correlation table of such 

 different characters as Black, Brindled, Fawn, White, and 

 Red ; l while he was thinking of the only method of measuring 

 the intensity of inheritance within a single such character 

 say, Red. Now this, I believe, brings us to the heart 



1 Barrington, etc., :05, p. 264. 

 184 



