60 The Recapitulation Theory and Human Infancy 



mediate ancestors. They will on the whole decrease with the 

 widening of the interval between the ancestors and descendants 

 compared. 



7. The millions of years occupied by any line of descent has 

 given opportunity for every degree of reconstruction of the life- 

 history to the most radical and it is surprising that a later 

 ontogeny should preserve any traces whatever of the life-history 

 of remote ancestors. 



8. In many groups there are very few, if any, such traces, 

 the life-history showing from an early period only the features 

 of its present adult condition. 



9.' In other groups the life-history is not direct but circui- 

 tous, and presents forms which require an explanation other 

 than that of adaptation to the environmental conditions of the 

 adult. These forms often have some kind of ancestral refer- 

 ence, and have been retained probably because of some utility 

 either to the larvae in their independent struggle for existence, 

 or as structural bases for higher differentiations in case they 

 are found in embryos. Some are rudiments without utility 

 of any discernible sort. 



10. In the majority of cases the ancestral reference of embry- 

 onic or larval stages is to embryonic and larval stages of ances- 

 tors. These may have originated either as larval or embryonic 

 conditions as such, or as adult structures which have been re- 

 tained in some fashion in the earlier ontogeny of descendants. 



11. In the smaller number of cases where the ancestral refer- 

 ence of early ontogenetic stages is conclusively to adult forms 

 the explanation is as follows : A life-history originally direct may 

 have been modified at a not early age and thus left the earlier 

 part to resemble the adult ancestor. If this should have happen- 

 ed successively so that modifications each time came later than 

 the already acquired modifications, the later stages in the devel- 

 opment of descendants would indicate something of the sequence 

 in which the modifications were successively added in the phy- 

 logeny. These stages would then " recapitulate " that part of the 

 phylogeny during which the modifications took place in this 

 fashion. If the modifications had been made uniformly in this 

 way, then the whole ontogeny would present a "recapitulation" 

 or chronological record of phylogeny, except where suppression, 

 consolidation, unequal acceleration, and retardation had inci- 

 dentally prevented. 



