EFFECTS OF AMPllIiMIXIS ON ONTOGENY 259 



portion as the ascendancy of the ancestor concerned decreases. 

 Any more exact calculation of the share taken by a certain 

 remote ancestor in the composition of the germ-plasm of its 

 descendant would be erroneous. Hitherto the customary 

 method of making such a calculation has been to assume that 

 the following shares are taken by the various ancestors in the 

 predisposition of the offspring: — parents, 2 x '. ; grandparents, 

 4x\, and so on ; that of the sixth generation of ancestors being 

 32x^1^. Thus, in the last-mentioned generation, <^w^ idant out 

 of the thirty-two assumed to be present in Man would, according 

 to the theory of the germ-plasm, still remain. This does not by 

 any means imply that each of the thirty-two ancestors of the 

 sixth generation is still represented by one idant in the germ- 

 plasm of the descendant ; it is quite as probable that thirty or even 

 twenty of these ancestors take part in its constitution, and the 

 number may possibly, though improbably, be still less than this. 

 In treating of the phenomena of reversion, I shall have occasion 

 to refer to this subject again. 



It is at any rate certain that m no case can 7nore than the half 

 of the idant s of one parent be present in the gerjn-piasm of the 

 fertilised egg-cell. 



This statement is, how^ever, apparently contradicted by certain 

 facts. 



Plant-hybrids frequently keep to the mean between the two 

 ancestral species : that is to say, they contain all the characters 

 of these two species in equal proportions. Thus all the primary 

 constituents of each parent would be contained in the fertilised 

 egg-cell, although, according to our theory, only half of the 

 parental idants are concerned in its constitution. This contra- 

 diction is easily accounted for, if it be borne in mind that we are 

 here concerned with the mingling of the characters of two species, 

 and not of those of two individuals of the same species. The 

 characters of the species must be contained in the majority of ids 

 in each idant., if not in every id, and half the idants may in this 

 case produce the sanie effect as would result if all the idants were 

 present: that is to say, they contain e^/ery specific character. 

 In cross-breeding, specific characters are opposed to specific 

 characters, and in comparison with the greater differences 

 between these, the lesser individual ones disappear. 



The reverse is true in the case of reproduction in Man. espe- 

 cially within one and the same race. The specific characters 



