disappears without leaving a trace soon after hatching; in Cyclops the
setae of the tail would be more important than all the natatory feet; in
the Cirripedia the posterior antennae, as to which we do not know what
becomes of them, would be more important than the cirri, and so forth.
The most unimportant of all organs would be the sexual organs, and the
most essential peculiarity would consist in colour, which is to be
referred back to the ovarian egg.

"The embryos, or young states of different animals, resemble each other
the more, the younger they are," or, as Johannes Muller expresses it,
"they approach the more closely to the common type." Different as may be
the ideas connected with the word "type," no one will dispute that the
typical form of the penultimate pair of feet in the Amphipoda is that of
a simple ambulatory foot, and not that of a chela, for the latter occurs
in no single adult Amphipod; we know it only in the young of the genus
Brachyscelus, which therefore in this respect undoubtedly depart more
widely than the adults from the type of their order. This applies also
to the young males of the Shore-hoppers (Orchestia) with regard to the
second pair of anterior feet (gnathopoda). In like manner no one will
hesitate to accept the possession of seven pairs of feet as a "typical"
peculiarity of the Edriophthalma, which Agassiz, on this account, names
Tetradecapoda; the young Isopoda, which are Dodecapoda, are also in this
respect further from the "type" than the adults.

It is certainly a rule, and this Darwin's theory would lead us to
expect, that in the progress of development those forms which are at
first similar gradually depart further from each other; but here, as in
other classes, the exceptions, for which the Old School has no
explanation, are numerous. Not unfrequently we might indeed directly
reverse the proposition and assert that the difference becomes the