164 HEREDITY AND SELECTION IN SOCIOLOGY 



history do not contain the same id combinations, any more than 

 do the innumerable sperm-cells produced by the male. The 

 greater the number of chromosomes possessed by the species, 

 the greater the number of possible combinations resulting from 

 the perpetual rearrangement of the parental elements at each 

 successive reduction division.^ 



^ A phenomenon which remains to be noticed 4s that of " return to the 

 ancestral type," or atavism. This phenomenon is similar to that of the 

 transmission of latent characters. Return to the ancestral type is almost 

 always observable among young species. Among old and constant 

 species it occurs very seldom, if at all. The explanation is that a young 

 species may stiU possess a considerable minority of determinants of an 

 older specific type in its germ-plasm ; and the chances of reduction and 

 amphimixis may bring about, even after numerous generations, a return 

 to this tjrpe by the sudden " majorisation " of the minority of old de- 

 terminants. In proportion as a species increases in age it also increases 

 in stabihty, and returns to the ancestral type become less and less frequent. 

 Such atavisms are aberrations which, being unsuited to the actual con- 

 ditions of fife of the species, do not propagate their kind. The experi- 

 ments of breeders tend to show that six or eight generations, as a rule, 

 are required to fix a new character definitely, and to prevent the accident 

 of an atavistic return to the ancestral type. Atavism, it must be noted, 

 may occiu: not only in the direct line of descent, but also in the collateral 

 {vide Ribot, L'Heredite psychologique, pp. 197 ff. Paris, 7th edition, 

 1905). 



