126 RETROGRESSION 



progression, others have undergone retrogression or have re- 

 mained stationary, and since variations have been interpolated, 

 no generation of ancestors is ever reproduced exactly by descen- 

 dants. Therefore the statement that ancestors are represented in 

 orderly succession (in the order in which they came into existence) 

 is true in a general and vague sense only. 



207. Apparently, however, many biologists believe, not that 

 the germ-plasm undergoes such a change during evolution that 

 development is in a general sense a recapitulation of the life- 

 history, not that the successive generations of ancestors are vaguely 

 mimicked in turn during development, but that each ancestor 

 adds to the germ-plasm a * unit ' or ' contribution ' which may 

 control, or may assist in controlling, development from start to 

 finish, from germ-cell to adult. 



208. The best known of the hypotheses which are founded on the 

 notion of ancestral contributions is Sir Francis Galton's ' Law of 

 Ancestral Inheritance.' This hypothesis, or modifications of it, which 

 differ in detail but are similar in principle, have been so widely 

 accepted that it is necessary to discuss it somewhat at length. 

 Galton analysed statistical data concerning stature, eye-colour, 

 artistic faculty, and health, collected from about one hundred and 

 fifty families of human beings, and extending over three or more 

 generations. Next, he analysed statistics concerning the colour 

 of Bassett hounds, a race of dwarf blood-hounds originated by 

 Sir Everett Millais some twenty years previously. He concluded, 

 " The two parents contribute between them on the average one-half 

 (or 0.5) of the total heritage of the offspring ; the four grandparents 

 one-fourth (or O-5) 2 ; the eight great-grandparents one-eighth 

 (or o.5) 3 , and so on. Thus the sum of the ancestral contributions 

 is expressed by the series (0.5) + (o.5) 2 -f (o.5) 3 , etc., which, being 

 equal to unity, accounts for the whole heritage." l 



209. Now obviously Galton's statistics do not furnish evidence 

 that ancestors contribute units to the germ-plasm. That deduction, 

 if it is in his mind, is drawn from the observation that offspring 

 may, in this or that character, resemble ancestors more than they 

 do parents. Probably it was reached before the statistics were 

 collected. Galton merely noted certain resemblances between the 

 individuals of three or four generations, and calculated that on the 

 average offspring resemble each parent to the extent of one- 

 quarter of their total characters, each grandparent to the extent 



1 The Average Contribution of each Several Ancestor to the Total Heritage of the 

 Offspring, Proc. Roy. Soc. London, vol. Ixi. pp. 401-13. 



