174 
ANIMAL PEDIGREES. 
Aug., 1891 . 
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 
position of the madreporite in Elasipodous Holothurians, or 
the suckerless condition of the ambulacral feet in many 
Echinoderms. 
A more important consideration is that if the develop¬ 
mental 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 com¬ 
plex 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 in the struggle for 
existence. It is not enough that the ultimate stage should 
be more advantageous than the initial or earlier condition, 
but each intermediate stage must also be a distinct advance. 
If, then, the development of an organ is strictly recapitulatory, 
it should present to us a series of stages, each of which is not 
merely functional, but a distinct advance on the stage imme¬ 
diately preceding it. Intermediate stages, e.g., the solid 
oesophagus of the tadpole, which are not and could not be 
functional, can form no part of an ancestral series ; a con¬ 
sideration well expressed by Sedgwick thus :—“ Any phylo¬ 
genetic hypothesis which presents difficulties from a physiolo¬ 
gical standpoint must be regarded as very provisional indeed.” 
