66 HEREDITY 



owing to causes yet undetermined. As Thomson 

 says, " There does not seem to be anything in this 

 conception which is at variance with more securely 

 established generalisations." The Mendelians have 

 taught us how characters may become latent or 

 " recessive," subsequently to find expression. 



There is, however, another way of looking at 

 reversion. It depends upon an important conception 

 which must here be briefly discussed. 



The " recapitulation theory " maintains that " onto- 

 geny " — the history of the individual — is a recapitu- 

 lation, with abbreviations, and gaps, and modifications, 

 of " phylogeny " — the history of the race. This is a 

 conception to which immense importance is attached 

 by Haeckel and Archdall Reid. For some years past, 

 however, it has been growing in disfavour amongst 

 many biologists, who are apt to declare that it is 

 scarcely more than a misleading metaphor. This 

 disfavour is due, I think, largely to a reaction from 

 the excessive dogmatism and rigidity with which the 

 recapitulation theory was formerly supported. But 

 even if we admit, to the full, the fact that the re- 

 capitulation of its racial history by the developing 

 individual is extremely blurred and often very partial 

 and imperfect, there yet remains sufficient positive 

 evidence to show that the theory is well based on 

 facts ; and there can be no question that it is most 

 fruitful in its applications to many and various pheno- 

 mena of embryology. 



Now Archdall Reid correlates the recapitulation 

 theory with the facts of reversion. He inclines to 

 regard reversion not as due to the sudden activity of 

 "latent ancestral units," but as simply due to the 



