Facts and Arguments for Darwin | Page 9

of the second pair of
feet. Orchestia gryphus, from the sandy coast of Monchgut, alone
presents a somewhat similar structure, but in a far less degree;
elsewhere the form of the hand usual in the Amphipoda occurs. Now
there is a considerable difference between the males of this species,
especially in the structure of these chelae--a different so great that we
can scarcely find a parallel to it elsewhere between two species of the
genus--and yet, as in Tanais, we do not meet with a long series of

structures running into one another, but only two forms united by no
intermediate terms (Figures 8 and 9). The males would be
unhesitatingly regarded as belonging to two well-marked species if
they did not live on the same spot, with undistinguishable females. That
the two forms of the chelae of the males occur in this species is so far
worthy of notice, because the formation of the chelae, which differs
widely from the ordinary structure in the other species, indicates that it
has quite recently undergone considerable changes, and therefore such
a phenomenon was to be expected in it rather than in other species.
(FIGURES 8 AND 9. The two forms of the chelae of the male of
Orchestia Darwinii, magnified 45 times.)
I cannot refrain from taking this opportunity of remarking that (so far
as appears from Spence Bate's catalogue), for two different kinds of
males (Orchestia telluris and sylvicola) which live together in the
forests of New Zealand, only one form of female is known, and
hazarding the supposition that we have here a similar case. It does not
seem to me to be probable that two nearly allied species of these social
Amphipoda should occur mixed together under the same conditions of
life.
(FIGURE 10. Coxal lamella of the penultimate pair of feet of the male
(a), and coxal lamella, with the three following joints of the same pair
of feet of the female (b) of Melita Messalina, magnified 45 diam.
FIGURE 11. Coxal lamella of the same pair of feet of the female of M.
insatiabilis.)
As the males of several species of Melita are distinguished by the
powerful unpaired clasp-forceps, the females of some other species of
the same genus are equally distinguished from all other Amphipoda by
the circumstance that in them a peculiar apparatus is developed which
facilitates their being held by the male. The coxal lamellae of the
penultimate pair of feet are produced into hook-like processes, of which
the male lays hold with the hands of the first pair of feet. The two
species in which I am acquainted with this structure are amongst the
most salacious animals of their order, even females which are laden

with eggs in all stages of development, not unfrequently have their
males upon their backs. The two species are nearly allied to Melita
palmata Leach (Gammarus Dugesii, Edw.), which is widely distributed
on the European coasts, and has been frequently investigated;
unfortunately, however, I can find no information as to whether the
females of this or any other European species possess a similar
contrivance. In M. exilii all the coxal lamellae are of the ordinary
formation. Nevertheless, be this as it will, whether they exist in two or
in twenty species, the occurrence of these peculiar hook-like processes
is certainly very limited.
Now our two species live sheltered beneath slightly tilted stones in the
neighbourhood of the shore: one of them, Melita Messalina, so high
that it is but rarely covered by the water; the other, Melita insatiabilis, a
little lower; both species live together in numerous swarms. We cannot
therefore suppose that the loving couples are threatened with
disturbance more frequently than those of other species, nor would it be
more difficult for the male, than for those of other species, in case of
his losing his female, to find a new one. Nor is it any more easy to see
how the contrivance on the body of the female for insuring the act of
copulation could be injurious to other species. But so long as it is not
demonstrated that our species are particularly in want of this
contrivance, or that the latter would rather be injurious than beneficial
to other species, its presence only in these few Amphipoda will have to
be regarded not as the work of far-seeing wisdom, but as that of a
favourable chance made use of by Natural Selection. Under the latter
supposition its isolated occurrence is intelligible, whilst we cannot
perceive why the Creator blessed just these few species with an
apparatus which he found to be quite compatible with the "general plan
of structure" of the Amphipoda, and yet denied it to others which live
under the same external conditions, and equal them even in their
extraordinary salacity. Associated with,