144 DARWINISM AND HUMAN LIFE 



Wliatever the inheritance may be in its expression — 

 whether it show a blend or takes after one side of the 

 house — it is made up, to begin with, of equal con- 

 tributions from the two parents. Obviously, how- 

 ever, if the concept of the continuity of the germ- 

 plasm be correct, the contribution from the father 

 is made up of contributions from his two parents, 

 and the contribution from the mother is made up 

 of contributions from her two parents. And so 

 on backwards. Thus we reach the idea, to be 

 corrected in cases where Mendelian inheritance has 

 been proved for particular characters, that an 

 individual inheritance is a mosaic of ancestral 

 contributions. Incidental corroborations of this 

 fruitful idea are familiar to all — e.g. in the re- 

 expression of peculiarities which were characteristic 

 of, say, a grandfather or a great-grandmother. 



But we owe to Galton's careful statistical work, 

 as to stature and other qualities in man, and as 

 to coat-colour ia Basset hounds, a generahsation 

 which formulates the share which the various 

 ancestors have, on an average, in the inheritance 

 of any individual organism. This Law of Ancestral 

 Inheritance is as follows : " The two parents 

 between them contribute, on the average, one-half 

 of each inherited faculty, each of them contributing 

 one-quarter of it. The four grandparents con- 

 tribute between them one-quarter, or each of them 

 one-sixteenth ; and so on, the sum of the series 

 2 + i + I + i~6 + • • • > being equal to 1, as 

 it should be. It is a property of this infinite 

 series that each term is equal to the sum of all 

 those that follow : thus J = J + i + re + ■ • • , 

 J = ^ + iJg -I- . . . , and so on. The prepo- 

 tencies or subpotencies of particular ancestors, in 



