SEXUAL PECULIARITIES AND DIMORPHISM.
Our _Tanais,_ which in nearly all the particulars of its structure is
an extremely remarkable animal, furnished me with a second fact worthy
of notice in connection with the theory of the origin of species by
natural selection.
When hand-like or cheliform structures occur in the Crustacea, these
are usually more strongly developed in the males than in the females,
often becoming enlarged in the former to quite a disproportionate size,
as we have already seen to be the case in _Melita._ A better known
example of such gigantic chelæ is presented by the males of the Calling
Crabs ( _Gelasimus_ ), which are said in running to carry these claws
“elevated, as if beckoning with them”—a statement which, however, is
not true of all the species, as a small and particularly large-clawed
one, which I have seen running about by thousands in the cassava-fields
at the mouth of the Cambriú, always holds them closely pressed against
its body.
A second peculiarity of the male Crustacea consists not unfrequently in
a more abundant development on the flagellum of the anterior antennæ of
delicate filaments which Spence Bate calls “auditory cilia,” and which
I have considered to be olfactory organs, as did Leydig before me,
although I was not aware of it. Thus they form long dense tufts in the
males of many Diastylidæ, as Van Beneden also states with regard to
_Bodotria,_ whilst the females only possess them more sparingly. In the
Copepoda, Claus called attention to the difference of the sexes in this
respect. It seems to me, as I may remark in passing, that this stronger
development in the males is greatly in favour of the opinion maintained
by Leydig and myself, as in other cases male animals are not
unfrequently guided by the scent in their pursuit of the ardent
females.
Now, in our _Tanais,_ the young males up to the last change of skin
preceding sexual maturity resemble the females, but then they undergo
an important metamorphosis. Amongst other things they lose the moveable
appendages of the mouth even to those which serve for the maintenance
of the respiratory current; their intestine is always found empty, and
they appear only to live for love. But what is most remarkable is, that
they now appear under two different forms. Some (Fig. 3) acquire
powerful, long-fingered, and very mobile chelæ, and, instead of the
single olfactory filament of the female, have from 12 to 17 of these
organs, which stand two or three together on each joint of the
flagellum. The others (Fig. 5) retain the short thick form of the chelæ
of the females; but, on the other hand, their antennæ (Fig. 6) are
equipped with a far greater number of olfactory filaments, which stand
in groups of from five to seven together.
Fig. 3. Head of the ordinary form of the male of Tanais dubius (?) Kr.
magnified. The terminal setæ of the second pair of antennæ project
between the cheliferous feet.
Fig. 4. Buccal region of the same from below; lambda, labrum.
Fig. 5. Head of the rarer form of the male, magnified.
Fig. 6. Flagellum of the same, with olfactory filaments, magnified.
In the first place, and before inquiring into its significance, I will
say a word upon this fact itself. It was natural to consider whether
two different species with very similar females and very different
males might not perhaps live together, or whether the males, instead of
occurring in two sharply defined forms, might not be only variable
within very wide limits. I can admit neither of these suppositions. Our
_Tanais_ lives among densely interwoven Confervæ, which form a coat of
about an inch in thickness upon stones in the neighbourhood of the
shore. If a handful of this green felt is put into a large glass with
clear sea-water, the walls of the glass are soon seen covered with
hundreds, nay with thousands, of these little, plump, whitish Isopods.
In this way I have examined thousands of them with the simple lens, and
I have also examined many hundreds with the microscope, without finding
any differences among the females, or any intermediate forms between
the two kinds of males.
To the old school this occurrence of two kinds of males will appear to
be merely a matter of curiosity. To those who regard the “plan of
creation” as the “free conception of an Almighty intellect, matured in
the thoughts of the latter before it is manifested in palpable,
external forms,” it will appear to be a mere _caprice_ of the Creator,
as it is inexplicable either from the point of view of practical
adaptation, or from the “typical plan of structure.” From the side of
Darwin’s theory, on the contrary, this fact acquires meaning and
significance, and it appears in return to be fitted to throw light upon
a question in which Bronn saw “the first and most material objection
against the new theory,” namely, how it is possible that from the
accumulation in various directions of the smallest variations running
out of one another, varieties and species are produced, which stand out
from the primary form clearly and sharply like the petiolated leaf of a
Dicotyledon, and are not amalgamated with the primary form and with
each other like the irregular curled lobes of a foliaceous Lichen.
Let us suppose that the males of our _Tanais,_ hitherto identical in
structure, begin to vary, in all directions as Bronn thinks, for aught
I care. If the species was adapted to its conditions of existence, if
the _best_ in this respect had been attained and secured by natural
selection, fresh variations affecting the species as a species would be
retrogressions, and thus could have no prospect of prevailing. They
must rather have disappeared again as they arose, and the lists would
remain open to the males under variation, only in respect of their
sexual relations. In these they might acquire advantages over their
rivals by their being enabled either to seek or to seize the females
better. The best smellers would overcome all that were inferior to them
in this respect, unless the latter had other advantages, such as more
powerful chelæ, to oppose to them. The best claspers would overcome all
less strongly armed champions, unless these opposed to them some other
advantage, such as sharper senses. It will be easily understood how in
this manner all the intermediate steps less favoured in the development
of the olfactory filaments or of the chelæ would disappear from the
lists, and two sharply defined forms, the best smellers and the best
claspers, would remain as the sole adversaries. At the present day the
contest seems to have been decided in favour of the latter, as they
occur in greatly preponderating numbers, perhaps a hundred of them to
one smeller.
To return to Bronn’s objection. When he says that “for the support of
the Darwinian theory, and in order to explain why many species do not
coalesce by means of intermediate forms, he would gladly discover some
external or internal principle which should compel the variations of
each species to advance in _one_ direction, instead of merely
permitting them in all directions,” we may, in this as in many other
cases, find such a principle in the fact that actually only a few
directions stand open in which the variations are at the same time
improvements, and in which therefore they can accumulate and become
fixed; whilst in all others, being either indifferent or injurious,
they will go as lightly as they come.
Fig. 7. Orchestia Darwinii, n. sp. male. Fig. 7. Orchestia Darwinii, n.
sp. male.
The occurrence of two kinds of males in the same species may perhaps
not be a very rare phenomenon in animals in which the males differ
widely from the females in structure. But only in those which can be
procured in sufficient abundance, will it be possible to arrive at a
conviction that we have not before us either two different species, or
animals of different ages. From my own observation, although not very
extensive, I can give a second example. It relates to a shore-hopper (
_Orchestia_ ). The animal (Fig. 7) lives in marshy places in the
vicinity of the sea, under decaying leaves, in the loose earth which
the Marsh Crabs ( _Gelasimus, Sesarma, Cyclograpsus,_ etc.) throw up
around the entrance to their borrows, and even under dry cow-dung and
horse-dung. If this species removes to a greater distance from the
shore than the majority of its congeners (although some of them advance
very far into the land and even upon mountains of a thousand feet in
height, such as _O. tahitensis, telluris,_ and _sylvicola_ ), its male
differs still more from all known species by the powerful chelæ 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 chelæ—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 (Figs. 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 chelæ of the males occur in this species is so far
worthy of notice, because the formation of the chelæ, 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.
Figs. 8 and 9. The two forms of the chelæ of the male of Orchestia
Darwinii, magnified. Figs. 8 and 9. The two forms of the chelæ of the
male of _Orchestia Darwinii,_ magnified.
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.
Fig. 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. Fig. 11. Coxal
lamella of the same pair of feet of the female of M. insatiabilis. Fig.
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. Fig. 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 lamellæ 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 lamellæ 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,
or in the immediate vicinity of the two species of _Melita,_ live two
species of _Allorchestes,_ the pairs of which are met with almost more
numerously than the single animals, and yet their females show no trace
of the above-mentioned processes of the coxal lamellæ.
These cases, I think, must be brought to bear against the conception
supported with so much genius and knowledge by Agassiz, that species
are embodied thoughts of the Creator; and, with these, all similar
instances in which arrangements which would be equally beneficial to
all the species of a group are wanting in the majority and only
conferred upon a few special favourites, which do not seem to want them
any more than the rest.