240 ANIMAL PEDIGREES 



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 tran- 

 sitory 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 position of the madreporite in 

 Elasipodous Holothurians, or the suckerless con- 

 dition of the ambulacral feet in many Echinoderms. 

 A more important consideration is that if the 

 developmental changes are to be interpreted as a 

 correct record of ancestral history, then the several 

 stages must be possible ones, the history must be 

 one that could actually have occurred i.e., the 

 several steps of the history as reconstructed must 

 form a series, all the stages of which are practicable 

 ones. Natural selection explains the actual 

 structure of a complex organ as having been 

 acquired by the preservation of a series of stages, 

 each a distinct, if slight, advance on the stage 

 immediately preceding it, an advance so distinct as 

 to confer on its possessor an appreciable advantage 



