194 



CAUSES OF VARIATION 



Galton has endeavored to assess mathematically the fraction 

 of direct inheritance, or, more accurately, the similarity between 

 the child and its various ancestors. From his studies he con- 

 cludes that the visible or dominant characters of the child are, 

 on the average, inherited (that is, correspond with those of the 

 various ancestors), roughly, as follows 1 : 



From the immediate parents 50 per cent 



From the grandparents ....... 25 per cent 



From the great-grandparents 12.5 per cent 



From the great, great-grandparents . . . 6.25 per cent 

 Earlier ancestors in proportion 



Pearson, working with larger numbers and diverse characters, 

 concludes that Galton's fraction of direct inheritance (0.50) is 

 too high, and is inclined to believe it not above 0.40 for blended 

 characters. The subject will be pursued farther under " The 

 Law of Ancestral Heredity," but this glimpse into the nature of 

 inheritance is the best method known to the author to dissolve 

 the almost supernatural mystery that has been thrown around 

 reversion and its natural corollary, latent characters. The study 

 can be pursued no farther at this point, but what has been said 

 will serve to show that reversion (regression) is a fertile cause 

 of variation as calculated from the type of the parent. It will 

 serve also as an introduction, preparing the student for the more 

 serious study of inheritance later on, when we shall learn that 

 the real type, from which all departures should be reckoned, is 

 the type of the race, and not the special type of the parent, or 

 even of the mid-parent. 



SECTION X INDIVIDUAL CHARACTERS DEPENDENT 



UPON SEX 



That both sexes possess and transmit all the characters of the 

 race is a well-established fact in evolution. It is also true that 

 the particular characters to undergo development,. and the extent 



1 Vernon, Variation in Animals and Plants, p. 123; Proceedings of the Royal 

 Society, LXI, 401, 1897; Galton, Natural Inheritance, p. 191. This does not 

 mean that every individual will inherit in this proportion, but that the fractions 

 express averages. 



