846 REPORT— 1890. 
We haye now considered the more important of the influences which are 
recognised as affecting developmental history in such a way as to render the recapi- 
tulation of ancestral stages less complete than it might otherwise be, which tend to 
prevent ontogeny from correctly repeating the phylogenetic 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 he 
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 develop- 
mental periods in all cases; and still more if it occurs equally in forms that 
hatch early as free larva, 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 
Elasipodous Holothurians, or the suckerless condition 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, 
z.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 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 ad- 
vance on the stage immediately preceding it. Intermediate stages, eg., the solid 
cesophagus of the tadpole, which are not and could not be functional, can form 
no part of an ancestral series ; a consideration well expressed by Sedgwick? thus: 
‘Any phylogenetic hypothesis which presents difficulties from a physiological 
standpoint must be regarded as very provisional indeed.’ 
A. good example of an embryological series fulfilling these conditions is afforded 
by the development of the eye in the higher Cephalopoda. The earliest stage 
consists in the depression of a slightly modified patch of skin; round the edge 
of the patch the epidermis becomes raised up as a rim; this gradually grows 
inwards from all sides, so that the depressed patch now forms a pit, com- 
municating with the exterior through a small hole or mouth. By further 
growth the mouth of the pit becomes still more narrowed, and ultimately 
completely closed, so that the pit becomes converted into a closed sac or 
vesicle ; at the point at which final closure occurs formation of cuticle takes 
place, which projects as a small transparent drop into the cavity of the sac; by 
formation of concentric layers of cuticle this drop becomes enlarged into the 
spherical transparent lens of the eye, and the development is completed by 
histological changes in the inner wall of the vesicle, which conyert it into the 
? Sedgwick, ‘On the Early Development of the Anterior Part of the Wolffian 
Duct and Body in the Chick,’ Quarterly Journal of Microscopical Science, yol. xxi. 
1881, p. 456. 
