THE SPECIES OF MELITA.
A false supposition, when the consequences proceeding from it are
followed further and further, will sooner or later lead to absurdities
and palpable contradictions. During the period of tormenting doubt—and
this was by no means a short one—when the pointer of the scales
oscillated before me in perfect uncertainty between the _pro_ and the
_con_, and when any fact leading to a quick decision would have been
most welcome to me, I took no small pains to detect some such
contradictions among the inferences as to the class of Crustacea
furnished by the Darwinian theory. But I found none, either then, or
subsequently. Those which I thought I had found were dispelled on
closer consideration, or actually became converted into supports for
Darwin’s theory.
Nor, so far as I am aware, have any of the _necessary_ consequences of
Darwin’s hypotheses been proved by any one else, to stand in clear and
irreconcilable contradiction. And yet, as the most profound students of
the animal kingdom are amongst Darwin’s opponents, it would seem that
it ought to have been an easy matter for them to crush him long since
beneath a mass of absurd and contradictory inferences, if any such were
to be drawn from his theory. To this want of demonstrated
contradictions I think we may ascribe just the same importance in
Darwin’s favour, that his opponents have attributed to the absence of
demonstrated intermediate forms between the species of the various
strata of the earth. Independently of the reasons which Darwin gives
for the preservation of such intermediate forms being only exceptional,
this last mentioned circumstance will not be regarded as of very great
significance by any one who has traced the development of an animal
upon larvae fished from the sea, and had to seek in vain for months,
and even years, for those transitional forms, which he nevertheless
knew to be swarming around him in thousands.
A few examples may show how contradictions might come forth as
necessary results of the Darwinian hypotheses.
It seems to be a necessity for all crabs which remain for a long time
out of the water (but why is of no consequence to us here), that air
shall penetrate from behind into the branchial cavity. Now these crabs,
which have become more or less estranged from the water, belong to the
most different families—the Raninidæ (_Ranina_), Eriphinæ (_Eriphia
gonagra_), Grapsoidæ (_Aratus, Sesarma,_ etc.), Ocypodidæ (_Gelasimus,
Ocypoda_), etc., and the separation of these families must doubtless be
referred to a much earlier period than the habit of leaving the water
displayed by some of their members. The arrangements connected with
aerial respiration, therefore, could not be inherited from a common
ancestor, and could scarcely be accordant in their construction. If
there were any such accordance not referable to accidental resemblance
among them, it would have to be laid in the scale as evidence against
the correctness of Darwin’s views. I shall show hereafter how in this
case the result, far from presenting such contradictions, was rather in
the most complete harmony with what might be predicted from Darwin’s
theory.
Fig. 1. Melita exilii n. sp., male, enlarged. The large branchial
lamellæ are seen projecting between the legs. Fig. 1. _Melita exilii_
n. sp., male, enlarged. The large branchial lamellæ are seen projecting
between the legs.
A second example.—We are already acquainted with four species of
_Melita_ (_M. valida, setipes, anisochir,_ and _Fresnelii_), and I can
add a fifth (Fig. 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 _Melitæ_ 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—
From the structure of the clasp-forceps: From the presence or
absence of the
secondary flagellum.
_M. palmata, etc., M. exilii, etc., M. Fresnelli._ _M. palmata,
etc., M. exilii, etc., M. Fresnelii._
As, in the first case, among the Crabs, a typical agreement of
arrangements produced independently of each other would have been a
very suspicious circumstance for Darwin’s theory, so also, in the
second, would any difference more profound than that of very nearly
allied species. Now it seems to me that the secondary flagellum can by
no means furnish a reason for doubting the close relationship of _M.
Fresnelii_ to _M. exilii,_ etc., which is indicated by the peculiar
structure of the unpaired clasp-forceps. In the first place we must
consider the possibility that the secondary flagellum, which is not
always easy to detect, may only have been overlooked by Savigny, as
indeed Spence Bate supposes to have been the case. If it is really
deficient it must be remarked that I have found it in species of the
genera _ Leucothoë, Cyrtophium_ and _Amphilochus,_ in which genera it
was missed by Savigny, Dana and Spence Bate—that a species proved by
the form of the Epimera (_coxæ_ Sp. B.) of the caudal feet (_uropoda_
Westw.), etc., to be a true _Amphithoë_[1] possesses it—that in many
species of _Cerapus_ it is reduced to a scarcely perceptible
rudiment—nay, that it is sometimes present in youth and disappears
(although perhaps not without leaving some trace) at maturity, as was
found by Spence Bate to be the case in _ Acanthonotus Owenii_ and
_Atylus carinatus,_ and I can affirm with regard to an _Atylus_ of
these seas, remarkable for its plumose branchiæ—and that from all this,
at the present day when the increasing number of known Amphipoda and
the splitting of them into numerous genera thereby induced, compels us
to descend to very minute distinctive characters, we must nevertheless
hesitate before employing the secondary flagellum as a generic
character. The case of _Melita Fresnelii_ therefore cannot excite any
doubts as to Darwin’s theory.
[1] I accept this and all the other genera of Amphipoda here
mentioned, with the limits given to them by Spence Bate (‘Catalogue of
Amphipodous Crustacea’).