A second example.--We are already acquainted with four species of Melita
(M. valida, setipes, anisochir, and Fresnelii), and I can add a fifth
(Figure 1), in which the second pair of feet bears upon one side a small
hand of the usual structure, and on the other an enormous clasp-forceps.
This want of symmetry is something so unusual among the Amphipoda, and
the structure of the clasp-forceps differs so much from what is seen
elsewhere in this order, and agrees so closely in the five species, that
one must unhesitatingly regard them as having sprung from common
ancestors belonging to them alone among known species. But one of these
species, M. Fresnelii, discovered by Savigny, in Egypt, is said to want
the secondary flagellum of the anterior antennae, which occurs in the
others. From the trustworthiness of all Savigny's works there can
scarcely be a doubt as to the correctness of this statement. Now, if the
presence or absence of the secondary flagellum possessed the
significance of a distinctive generic character, which is usually
ascribed to it, or if there were other important differences between
Melita Fresnelii and the other species above-mentioned, which would make
it seem natural to separate M. Fresnelii as a distinct genus, and to
leave the others united with the rest of the species of Melita--that is
to say, in the sense of the Darwinian theory, if we assume that all the
other Melitae possessed common ancestors, which were not at the same
time the ancestors of M. Fresnelii--this would stand in contradiction to
the conclusion, derived from the structure of the clasp-forceps, that M.
Fresnelii and the four other species above-mentioned possessed common
ancestors, which were not also the ancestors of the remaining species of
Melita. It would follow:--

1. From the structure of the clasp-forceps: that M. exilii, etc. and M.
Fresnelii would branch off together from a stem which branches off from