MORPHOLOGICAL STUDIES. 
67 
be inconvenient — very much so. So in PI. XX, fig. 4 we have a 
figure of an early stage of a higher chordate, in which the pig- 
ment has all disappeared ; and when in further development 
(PI. XX, figs. 7 and 8, Reptiles) we find it again, it is confined to 
its proper place in the retina, and the lens contains no pigment ; 
while if it had developed from the structures in figs, i, xi, and hi 
— if it could perform the physically impossible task of develop- 
ing — it would be loaded with pigment. Now, these diagrams are 
not untrue to nature, and all my criticism aims at proving is that 
Spencer’s arrangement of them is artificial and misleading. It 
would be a misfortune if these diagrams got into the text-books 
in the order in which Spencer gives them. Even if placed 
in a less artificial order they do not show the phylo- 
genetic development of the organ — that is unknown. 
All they show, if placed in a different order, is certain stages 
of the ontogeny and certain stages of the degeneration. 1 The 
ontogeny is shown in figs. 4, 5, 6, and 7, and the degeneration 
in figs. 6, 8, 10, 11, 12, and 9. These latter figures are a very 
heterogeneous assemblage, and only show the state of degenera- 
tion in a series of forms, and not the phylogenetic degeneration. 
Seeing that most of the epiblastic cells of Anura contain pig- 
ment, I do not see any advantage in placing figs. 2 and 3 in 
the series at all, while if fig. x has any place in the plate it 
ought to be last of all. 
The phylogeny of the parietal eye is a very difficult problem, 
and in spite of my former remarks (No. 3, p. 248) I do not think 
the question can be yet fully solved. Spencer (No. 14, p. 230) 
has compared its development with that of the paired eyes, 
which he believes originated as secondary differentiations from 
the brain — as secondary evaginations. This mode of regarding 
the problem is easily disposed of, for if the paired and unpaired 
eyes originated in that way, then in both cases the lens must be 
the same ; and indeed, on physical principles, it is easy to un- 
derstand that in the paired eyes the lens must of necessity be 
formed as it is from the lateral epiblast. The anterior wall of 
1 It is simply a fallacy to suppose that an organ in its degeneration passes 
through the stages, or even some of them, of its phylogeny. 
