100 The Recapitulation Theory and Human Infancy 



existing ontogenies; more especially particular structures were 

 often retained through long periods of time with considerable 

 faithfulness. Because of the slowness with which modifications 

 occurred, any ontogeny repeats most of the features of its proxi- 

 mate ancestral ontogeny, and in general the record grows less 

 and less true of phyletic ontogenies with remoteness in descent. 

 Because any ontogeny thus throws some light upon earlier ones, 

 and particularly upon proximate ones, the facts of ontogeny 

 gathered together help to reconstruct the whole line of descent. 

 Probably this is what is often meant by the statement that on- 

 togeny is a record of phylogeny. The statement should read 

 that genetically related ontogenies when compared supply some 

 ground for reconstructing phylogenies. 



Although the effect of mutation and selection was in general 

 productive of alteration of the life-cycle, evidence points to a 

 considerable amount of extension of the life-cycle coincidentally 

 with alteration. This happened at times with the form of the 

 organism as a whole, but more often with particular organs or 

 structures. 



These considerations apply to the period of infancy with 

 qualification. Infancy was evolved from an ancient life-cycle 

 which did not possess such a period, presumably during and 

 since the Age of Mammals. We can only guess as to the nature 

 of its origin. It may represent a modified later part of the 

 ancient embryogeny set over into the post-natal period, or it 

 may be a remodelled part of the ancient period of precocious 

 maturity, or it may be both. In any case it is probably not due 

 to the successive extension of the mature end of old ontogenies, 

 and there is no need presupposing evidence within it of such 

 extensions. 



It is, rather, probably the ancient structure of maturity, 

 either in process of growth or in completion, shorn of and supple- 

 mented by those things which plasticity presupposes. In its 

 most ancient features, such as the emotional phases and involun- 

 tary rudiments of instinct, it probably retains the characteristics 

 of maturity. We have seen no reason for assigning differences 

 between human infancy and adulthood in these respects. As 

 for the rest, we may tentatively bring over the classification of 

 features employed by Duckworth in connection with human 

 embryogeny, as follows: 



