An Introduction to a Biology 



similar) character or characters in the preceding generation. 

 Galton's Law states the amount which a given generation 

 contributes I to the generation which it produces. It defi- 

 nitely states that on the average a half of the filial genera- 

 tion are like the parental, a quarter like the grandparental, 

 and an eighth like the great-grandparental, and so on. From 

 the knowledge that the parents of a given generation of cats 

 are tabbies, and that half of its grandparents are tabbies, 

 a quarter whites, and a quarter blacks, you are enabled to 

 predict by Galton's Law the proportion in which these three 

 kinds of cats will occur in that generation. Pearson's Law 

 does not enable you to do this : it is of an entirely different 

 kind. For Pearson's Law to be true it is not necessary that 

 any of the children should be like any of the parents ; all 

 that is necessary is that a particular kind of parent should 

 be associated with (i.e. should produce) as often as not a 

 particular kind of child. On the next page is an imagi- 

 nary Correlation Table, in which Pearson's Law is borne 

 out, yet in which none of the children are like any of the 

 parents. 



The fact that the relation between a given generation 

 and those that precede it is described by a series of figures 

 which in the case of Galton's Law is -5, -25, -125, -0625, etc., 

 and which in the case of Pearson's, for eye colour in man, 

 for example, is 4947, -3166, -1879, 2 has led some to believe 

 that the figures mean the same thing (which, of course, they 

 do not) and has thus constituted a trap for the unwary. 

 Castle has done good service to progress in the study of 

 heredity by falling into it. 3 



I hope I have made clear what the difference be- 

 tween the meanings of the two series is ; for to under- 

 stand this is to understand the difference between the two 

 Laws. 



This difference is sometimes expressed in the statement 

 that Pearson's Law is more comprehensive and less bio- 



1 See Appendix B, p. 197. 2 Pearson, :03a, p. 221. 3 Castle, :03, p. 224. 



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