ns (allied to Penoeus) first appear under the simple
nauplius-form, and after passing through two or more zoea-stages, and
then through the mysis-stage, finally acquire their mature structure:
now in the whole great malacostracan order, to which these crustaceans
belong, no other member is as yet known to be first developed under the
nauplius-form, though many appear as zoeas; nevertheless Muller assigns
reasons for his belief, that if there had been no suppression of
development, all these crustaceans would have appeared as nauplii.

How, then, can we explain these several facts in embryology--namely, the
very general, though not universal, difference in structure between the
embryo and the adult; the various parts in the same individual embryo,
which ultimately become very unlike, and serve for diverse purposes,
being at an early period of growth alike; the common, but not
invariable, resemblance between the embryos or larvae of the most
distinct species in the same class; the embryo often retaining, while
within the egg or womb, structures which are of no service to it, either
at that or at a later period of life; on the other hand, larvae which
have to provide for their own wants, being perfectly adapted to the
surrounding conditions; and lastly, the fact of certain larvae standing
higher in the scale of organisation than the mature animal into which
they are developed? I believe that all these facts can be explained as
follows.

It is commonly assumed, perhaps from monstrosities affecting the embryo
at a very early period, that slight variations or individual differences
necessarily appear at an equally early period. We have little evidence
on this head, but what we have certainly points the other way; for it
is notorious that breeders of cattle, horses and various fancy animals,
cannot positively tell, until some time after birth, what will be the
merits and demerits of their young animals. We see this plainly in our
own children; we cannot tell whether a child will be tall or short, or
what its precise features will be. The question is not, at what period
of life any variation may have been caused, but at what period the
effects are displayed. The cause may have acted, and I believe often has
acted, on one or both parents before the act of generation. It deserves
notice that it is of no importance to a very young animal, as long as it
is nourished and protected by its parent, whether most of its characters
are a