REVETJSTON 67 



premature arrest of the process of recapitulation. 

 The reverting individual is thus really an example of 

 arrested development. It should have showed, let us 

 say, the stages offish, amphibian, mammal, in turn; 

 but it declined to undertake the complete recapitu- 

 lation of its racial history, and thus appears, when 

 adult, as semi-amphibian instead of manunalian. 

 In criticising supposed cases of reversion, Thomson 

 says, " Often there is not the slightest attempt to 

 eliminate the phenomena of arrested development." 

 Reid maintains that reversion is essentially a pheno- 

 menon of arrested development ; that is to say, of 

 " incomplete recapitulation." 



Here, I make no attempt to adjudicate between 

 these rival theories, but present them both, each 

 being plausible and each of interest. I will merely 

 note that Reid, accepting the very recent view that, 

 despite Weismann, bi-parental reproduction is not a 

 cause of progressive variations, maintains it to be a 

 cause of regressive variations, that is, of reversion to 

 type; the said reversion being otherwise expressed 

 as incomplete recapitulation of the racial history by 

 the developing individual. 



Here the biometricians offer us results of import- 

 ance. Rejecting Weismann's theory, they maintain 

 — as does Reid — that one of the most important 

 results of amphimixis is the steady tendency to 

 maintain — or, if necessary, to revert or regress to — 

 the type. " In the tenth generation a man has 1024 

 tenth grandparents, and is thus the product of an 

 enormous population, the mean of which can hardly 

 differ from that of the general population. Houco 

 this heavy weight of mediocrity produces regression 



