THE RECAPITULATION THEORY 345 



history. It may at this point reasonably be asked 

 whether there is ~any way of distinguishing the 

 palingenetic history from the later cenogenetic 

 modifications grafted on to it ; any test by which 

 we can determine whether a given larval character 

 is or is not ancestral. Most assuredly there is no 

 one rule, no single test that will apply in all cases ; 

 but there are certain considerations which will help 

 us, and which should be kept in view. 



A character that is of general occurrence among 

 the members of a group, both high and low, may 

 reasonably be regarded as having strong claims to 

 ancestral rank ; claims that are greatly strengthened 

 if it occurs at corresponding developmental periods 

 in all cases ; and still more if it occurs equally in 

 forms that hatch early as free larvae, and in forms 

 with large eggs, which develop directly into the 

 adult. As examples of such characters may be cited 

 the mode of formation and relations of the notochord, 

 and of the gill clefts of vertebrates, which satisfy 

 all the conditions mentioned. Characters that are 

 transitory in certain groups, but retained throughout 

 life in allied groups, may with tolerable certainty 

 be regarded as ancestral for the former ; for instance 

 the symmetrical position of the eyes in young flat 

 fish, the spiral shell of the young limpet, the 

 superficial positions of the madreporite in Elasi- 

 podous Holothurians, or the suckerless condition of 

 the ambulacral feet in many Echinoderms. 



i_&_jmore important consideration is that if the 

 developmental changes are to be interpreted as a 

 correct record of ancestral history, then the several 



