greater, the further we go back in the development, and this not only in
those cases in which one of two nearly allied species is directly
developed, and the other passes through several larval stages, such as
the common Crayfish and the Prawns which are produced from
Nauplius-brood. The same may be said, for example, of the Isopoda and
Amphipoda. In the adult animals the number of limbs is the same; at the
first sight of a Cyrtophium or a Dulichia, and even after the careful
examination of a Tanais, we may be in doubt whether we have an Isopod or
an Amphipod before us; in the newly-hatched young the number of limbs is
different, and if we go back to their existence in the egg, the most
passing glance to see whether the curvature is upwards or downwards
suffices to distinguish even the youngest embryos of the two orders.

In other instances, the courses which lead from a similar starting-point
to a similar goal, separate widely in the middle of the development, as
in the Prawns with Nauplius-brood already described.

Finally, so that even the last possibility may be exhausted, it
sometimes happens that the greatest similarity occurs in the middle of
the development. The most striking example of this is furnished by the
Cirripedia and Rhizocephala, whether we compare the two orders or the
members of each with one another; from a segmentation quite different in
its course (see Figures 61 to 64) proceed different forms of Nauplius,
these become converted into exceedingly similar pupae, and from the
pupae again proceed sexually mature animals, differing from each other
toto coelo.

"If the formation of the organs occurs in the order corresponding to
their importance, this sequence must of itself be a criterion of their
comparative value in classification." THAT IS TO SAY, SUPPOSING THE