*
(* I must not omit remarking that what has been said as to the
development of the Crabs applies essentially only to the groups
Cyclometopa, Catometopa and Oxyrhyncha, placed together by Alph.
Milne-Edwards as "Eustomes." Among the Oxystomata, as also among the
"Anomura apterura," Edw., which approach so nearly to the Crabs, I am
unacquainted with the earliest young states of any of the species.)
As we have already seen, Mysis and the Isopoda depart from all other
Crustacea very remarkably by the fact that their embryos are curved
upwards, instead of, as elsewhere, downwards. Does not so isolated a
phenomenon as this, it might be asked, in the sense of Darwin's theory,
indicate a common inheritance? Does it not necessitate that we should
unite as the descendants of the same primitive ancestors, Mysis with the
Isopoda on the one hand, and on the other the rest of the Podophthalma
with the Amphipoda? I think not. Such a necessity exists only for those
who estimate a peculiarity at a higher value because it makes its
appearance at an earlier period of the egg-life. Whoever regards species
as not created independently and unchangeably, but as having gradually
become what they are, will say to himself that, when the ancestors of
our Mysides came (probably much later than those of the Amphipoda and
Isopoda) to develop numerous body-segments and limbs whilst still
embryos, as they could no longer find room in the egg when extended
straight out, and were therefore compelled to bend themselves, this
could only take place either upwards or downwards, and whatever
conditions may have decided the direction actually adopted, any near
relationship to either of the two orders of Edriophthalma could hardly
have taken part in it.
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