females to be considered, namely, P. Theseus (Cramer), P. Molanides (De
Haan), P. Elyros (G. R. Gray), and P. Romulus (Linnaeus). The dark female
figured by Cramer as P. Theseus seems to be the common and perhaps the
only form in Sumatra, whereas in Java, Borneo, and Timor, along with
males quite identical with those of Sumatra, occur females of the
Polytes form, although a single specimen of the true P. Theseus taken at
Lombock would seem to show that the two forms do occur together. In the
allied species found in the Philippine Islands (P. Alphenor, Cramer = P.
Ledebouria, Eschscholtz, the female of which is P. Elyros, G. R. Gray,)
forms corresponding to these extremes occur, along with a number of
intermediate varieties, as shown by a fine series in the British Museum.
We have here an indication of how dimorphism may be produced; for let
the extreme Philippine forms be better suited to their conditions of
existence than the intermediate connecting links, and the latter will
gradually die out, leaving two distinct forms of the same insect, each
adapted to some special conditions. As these conditions are sure to vary
in different districts, it will often happen, as in Sumatra and Java,
that the one form will predominate in the one island, the other in the
adjacent one. In the island of Borneo there seems to be a third form;
for P. Melanides (De Haan) evidently belongs to this group, and has all
the chief characteristics of P. Theseus, with a modified colouration of
the hind wings. I now come to an insect which, if I am correct, offers
one of the most interesting cases of variation yet adduced. Papilio
Romulus, a butterfly found over a large part of India and Ceylon, and
not uncommon in collections, has always been considered a true and
independent species, and no suspicions have been expressed regarding it.
But a male of this form does not, I believe, exist. I have examined the
fine series in the British Museum, in the East India Company's Museum,
in the Hope Museum at Oxford, in Mr. Hewitson's and several other
private collections, and can find nothing but females; and for this
common butterfly no male partner can be found except the equally common
P. Pammon, a species already provided with two wives, and yet to whom we
shall be forced, I believe, to assign a third. On carefully examining P.
Romulus, I find that in all essential characters--the form and texture
of the wings, the length of the antennae, the spotting of the head and
thorax, and even the peculiar tints and shades with which it is
ornamented--it corresponds exactly with the other females of the Pammon
group; and though, from the peculiar marking of the fore wings, it has
at first sight a very different aspect, yet a closer examination shows
that every one of its markings could be produced by slight and almost
imperceptible modifications of the various allied forms. I fully
believe, therefore, that I shall be correct in placing P. Romulus as a
third Indian form of the female P. Pammon, corresponding to P.
Melanides, the third form of the Malayan P. Theseus. I may mention here
that the females of this group have a superficial resemblance to the
Polydorus group of Papilios, as shown by P. Theseus having been
considered to be the female of P. Antiphus, and by P. Romulus being
arranged next to P. Hector. There is no close affinity between these two
groups of Papilio, and I am disposed to believe that we have here a case
of mimicry, brought about by the same causes which Mr. Bates has so well
explained in his account of the Heliconidae, and which has led to the
singular exuberance of polymorphic forms in this and allied groups of
the genus Papilio. I shall have to devote a section of my essay to the
consideration of this subject.
The third example of polymorphism I have to bring forward is Papilio
Ormenus, which is closely allied to the well-known P. Erechtheus, of
Australia. The most common form of the female also resembles that of P.
Erechtheus; but a totally different-looking insect was found by myself
in the Aru Islands, and figured by Mr. Hewitson under the name of P.
Onesimus, which subsequent observation has convinced me is a second form
of the female of P. Ormenus. Comparison of this with Boisduval's
description of P. Amanga, a specimen of which from New Guinea is in the
Paris Museum, shows the latter to be a closely similar form; and two
other specimens were obtained by myself, one in the island of Goram and
the other in Waigiou, all evidently local modifications of the same
form. In each of these localities males and ordinary females of P.
Ormenus were also found. So far there is no evidence that these
light-coloured insects are not females of a distinct species, the males
of which have not been discovered. But two facts have convinced me this
is not the case. At Dorey, in New Guinea, where males and ordinary
females closely allied to P. Ormenus occur (but which seem to me worthy
of being separated as a distinct species), I found one of these
light-coloured females closely followed in her flight by three males,
exactly in the same manner as occurs (and, I believe, occurs only) with
the sexes of the same species. After watching them a considerable time,
I captured the whole of them, and became satisfied that I had discovered
the true relations of this anomalous form. The next year I had
corroborative proof of the correctness of this opinion by the discovery
in the island of Batchian of a new species allied to P. Ormenus, all the
females of which, either seen or captured by me, were of one form, and
much more closely resembling the abnormal light-coloured females of P.
Ormenus and P. Pandion than the ordinary specimens of that sex. Every
naturalist will, I think, agree that this is strongly confirmative of
the supposition that both forms of female are of one species; and when
we consider, further, that in four separate islands, in each of which I
resided for several months, the two forms of female were obtained and
only one form of male ever seen, and that about the same time, M.
Montrouzier in Woodlark Island, at the other extremity of New Guinea
(where he resided several years, and must have obtained all the large
Lepidoptera of the island), obtained females closely resembling mine,
which, in despair at finding no appropriate partners for them, he mates
with a widely different species--it becomes, I think, sufficiently
evident this is another case of polymorphism of the same nature as those
already pointed out in P. Pammon and P. Memnon. This species, however,
is not only dimorphic, but trimorphic; for, in the island of Waigiou, I
obtained a third female quite distinct from either of the others, and in
some degree intermediate between the ordinary female and the male. The
specimen is particularly interesting to those who believe, with Mr.
Darwin, that extreme difference of the sexes has been gradually produced
by what he terms sexual selection, since it may be supposed to exhibit
one of the intermediate steps in that process, which has been
accidentally preserved in company with its more favoured rivals, though
its extreme rarity (only one specimen having been seen to many hundreds
of the other form) would indicate that it may soon become extinct.
The only other case of polymorphism in the genus Papilio, at all equal
in interest to those I have now brought forward, occurs in America; and
we have, fortunately, accurate information about it. Papilio Turnus is
common over almost the whole of temperate North America; and the female
resembles the male very closely. A totally different-looking insect both
in form and colour, Papilio Glaucus, inhabits the same region; and
though, down to the time when Boisduval published his "Species General,"
no connexion was supposed to exist between the two species, it is now
well ascertained that P. Glaucus is a second female form of P. Turnus.
In the "Proceedings of the Entomological Society of Philadelphia," Jan.,
1863, Mr. Walsh gives a very interesting account of the distribution of
this species. He tells us that in the New England States and in New York
all the females are yellow, while in Illinois and further south all are
black; in the intermediate region both black and yellow females occur in
varying proportions. Lat. 37 deg. is approximately the southern limit of the
yellow form, and 42 deg. the northern limit of the black form; and, to
render the proof complete, both black and yellow insects have been bred
from a single batch of eggs. He further states that, out of thousands
of specimens, he has never seen or heard of intermediate varieties
between these forms. In this interesting example we see the effects of
latitude in determining the proportions in which the individuals of each
form should exist. The conditions are _here_ favourable to the one form,
_there_ to the other; but we are by no means to suppose that these
conditions consist in climate alone. It is highly probable that the
existence of enemies, and of competing forms of life, may be the main
determining influences; and it is much to be wished that such a
competent observer as Mr. Walsh would endeavour to ascertain what are
the adverse causes which are most efficient in keeping down the numbers
of each of these contrasted forms.
Dimorphism of this kind in the animal kingdom does not seem to have any
direct relations to the reproductive powers, as Mr. Darwin has shown to
be the case in plants, nor does it appear to be very general. One other
case only is known to me in another family of my eastern Lepidoptera,
the Pieridae; and but few occur in the Lepidoptera of other countries.
The spring and autumn broods of some European species differ very
remarkably; and this must be considered as a phenomenon of an analogous
though not of an identical nature, while the Araschnia prorsa, of
Central Europe, is a striking example of this alternate or seasonal
dimorphism. Among our nocturnal Lepidoptera, I am informed, many
analogous cases occur; and as the whole history of many of these has
been investigated by breeding successive generations from the egg, it is
to be hoped that some of our British Lepidopterists will give us a
connected account of all the abnormal phenomena which they present.
Among the Coleoptera Mr. Pascoe has pointed out the existence of two
forms of the male sex in seven species of the two genera Xenocerus and
Mecocerus belonging to the family Anthribidae, (Proc. Ent. Soc. Lond.,
1862); and no less than six European Water-beetles, of the genus
Dytiscus, have females of two forms, the most common having the elytra
deeply sulcate, the rarer smooth as in the males. The three, and
sometimes four or more, forms under which many Hymenopterous insects
(especially Ants) occur, must be considered as a related phenomenon,
though here each form is specialized to a distinct function in the
economy of the species. Among the higher animals, albinoism and melanism
may, as I have already stated, be considered as analogous facts; and I
met with one case of a bird, a species of Lory (Eos fuscata), clearly
existing under two differently coloured forms, since I obtained both
sexes of each from a single flock, while no intermediate specimens have
yet been found.
The fact of the two sexes of one species differing very considerably is
so common, that it attracted but little attention till Mr. Darwin showed
how it could in many cases be explained by the principle of sexual
selection. For instance, in most polygamous animals the males fight for
the possession of the females, and the victors, always becoming the
progenitors of the succeeding generation, impress upon their male
offspring their own superior size, strength, or unusually developed
offensive weapons. It is thus that we can account for the spurs and the
superior strength and size of the males in Gallinaceous birds, and also
for the large canine tusks in the males of fruit-eating Apes. So the
superior beauty of plumage and special adornments of the males of so
many birds can be explained by supposing (what there are many facts to
prove) that the females prefer the most beautiful and perfect-plumaged
males, and that thus, slight accidental variations of form and colour
have been accumulated, till they have produced the wonderful train of
the Peacock and the gorgeous plumage of the Bird of Paradise. Both these
causes have no doubt acted partially in insects, so many species
possessing horns and powerful jaws in the male sex only, and still more
frequently the males alone rejoicing in rich colours or sparkling
lustre. But there is here another cause which has led to sexual
differences, viz., a special adaptation of the sexes to diverse habits
or modes of life. This is well seen in female Butterflies (which are
generally weaker and of slower flight), often having colours better
adapted to concealment; and in certain South American species (Papilio
torquatus) the females, which inhabit the forests, resemble the AEneas
group of Papilios which abound in similar localities, while the males,
which frequent the sunny open river-banks, have a totally different
colouration. In these cases, therefore, natural selection seems to have
acted independently of sexual selection; and all such cases may be
considered as examples of the simplest dimorphism, since the offspring
never offer intermediate varieties between the parent forms.
The phenomena of dimorphism and polymorphism may be well illustrated by
supposing that a blue-eyed, flaxen-haired Saxon man had two wives, one a
black-haired, red-skinned Indian squaw, the other a woolly-headed,
sooty-skinned negress--and that instead of the children being mulattoes
of brown or dusky tints, mingling the separate characteristics of their
parents in varying degrees, all the boys should be pure Saxon boys like
their father, while the girls should altogether resemble their mothers.
This would be thought a sufficiently wonderful fact; yet the phenomena
here brought forward as existing in the insect-world are still more
extraordinary; for each mother is capable not only of producing male
offspring like the father, and female like herself, but also of
producing other females exactly like her fellow-wife, and altogether
differing from herself. If an island could be stocked with a colony of
human beings having similar physiological idiosyncrasies with Papilio
Pammon or Papilio Ormenus, we should see white men living with yellow,
red, and black women, and their offspring always reproducing the same
types; so that at the end of many generations the men would remain pure
white, and the women of the same well-marked races as at the
commencement.
The distinctive character therefore of dimorphism is this, that the
union of these distinct forms does not produce intermediate varieties,
but reproduces the distinct forms unchanged. In simple varieties, on the
other hand, as well as when distinct local forms or distinct species are
crossed, the offspring never resembles either parent exactly, but is
more or less intermediate between them. Dimorphism is thus seen to be a
specialized result of variation, by which new physiological phenomena
have been developed; the two should therefore, whenever possible, be
kept separate.
3. _Local form, or variety._--This is the first step in the transition
from variety to species. It occurs in species of wide range, when groups
of individuals have become partially isolated in several points of its
area of distribution, in each of which a characteristic form has become
more or less completely segregated. Such forms are very common in all
parts of the world, and have often been classed by one author as
varieties, by another as species. I restrict the term to those cases
where the difference of the forms is very slight, or where the
segregation is more or less imperfect. The best example in the present
group is Papilio Agamemnon, a species which ranges over the greater part
of tropical Asia, the whole of the Malay archipelago, and a portion of
the Australian and Pacific regions. The modifications are principally of
size and form, and, though slight, are tolerably constant in each
locality. The steps, however, are so numerous and gradual that it would
be impossible to define many of them, though the extreme forms are
sufficiently distinct. Papilio Sarpedon presents somewhat similar but
less numerous variations.
4. _Co-existing Variety._--This is a somewhat doubtful case. It is when
a slight but permanent and hereditary modification of form exists in
company with the parent or typical form, without presenting those
intermediate gradations which would constitute it a case of simple
variability. It is evidently only by direct evidence of the two forms
breeding separately that this can be distinguished from dimorphism. The
difficulty occurs in Papilio Jason, and P. Evemon, which inhabit the
same localities, and are almost exactly alike in form, size, and
colouration, except that the latter always wants a very conspicuous red
spot on the under surface, which is found not only in P. Jason, but in
all the allied species. It is only by breeding the two insects that it
can be determined whether this is a case of a co-existing variety or of
dimorphism. In the former case, however, the difference being constant
and so very conspicuous and easily defined, I see not how we could
escape considering it as a distinct species. A true case of co-existing
forms would, I consider, be produced, if a slight variety had become
fixed as a local form, and afterwards been brought into contact with the
parent species, with little or no intermixture of the two; and such
instances do very probably occur.
5. _Race or subspecies._--These are local forms completely fixed and
isolated; and there is no possible test but individual opinion to
determine which of them shall be considered as species and which
varieties. If stability of form and "_the constant transmission of some
characteristic peculiarity of organization_" is the test of a species
(and I can find no other test that is more certain than individual
opinion) then every one of these fixed races, confined as they almost
always are to distinct and limited areas, must be regarded as a species;
and as such I have in most cases treated them. The various modifications
of Papilio Ulysses, P. Peranthus, P. Codrus, P. Eurypilus, P. Helenus,
&c., are excellent examples; for while some present great and
well-marked, others offer slight and inconspicuous differences, yet in
all cases these differences seem equally fixed and permanent. If,
therefore, we call some of these forms species, and others varieties, we
introduce a purely arbitrary distinction, and shall never be able to
decide where to draw the line. The races of Papilio Ulysses, for
example, vary in amount of modification from the scarcely differing New
Guinea form to those of Woodlark Island and New Caledonia, but all seem
equally constant; and as most of these had already been named and
described as species, I have added the New Guinea form under the name of
P. Autolycus. We thus get a little group of Ulyssine Papilios, the whole
comprised within a very limited area, each one confined to a separate
portion of that area, and, though differing in various amounts, each
apparently constant. Few naturalists will doubt that all these may and
probably have been derived from a common stock, and therefore it seems
desirable that there should be a unity in our method of treating them;
either call them all _varieties_ or all _species_. Varieties, however,
continually get overlooked; in lists of species they are often
altogether unrecorded; and thus we are in danger of neglecting the
interesting phenomena of variation and distribution which they present.
I think it advisable, therefore, to name all such forms; and those who
will not accept them as species may consider them as subspecies or
races.
6. _Species._--Species are merely those strongly marked races or local
forms which when in contact do not intermix, and when inhabiting
distinct areas are generally believed to have had a separate origin, and
to be incapable of producing a fertile hybrid offspring. But as the test
of hybridity cannot be applied in one case in ten thousand, and even if
it could be applied would prove nothing, since it is founded on an
assumption of the very question to be decided--and as the test of
separate origin is in every case inapplicable--and as, further, the test
of non-intermixture is useless, except in those rare cases where the
most closely allied species are found inhabiting the same area, it will
be evident that we have no means whatever of distinguishing so-called
"true species" from the several modes of variation here pointed out, and
into which they so often pass by an insensible gradation. It is quite
true that, in the great majority of cases, what we term "species" are
so well marked and definite that there is no difference of opinion about
them; but as the test of a true theory is, that it accounts for, or at
the very least is not inconsistent with, the whole of the phenomena and
apparent anomalies of the problem to be solved, it is reasonable to ask
that those who deny the origin of species by variation and selection
should grapple with the facts in detail, and show how the doctrine of
the distinct origin and permanence of species will explain and harmonize
them. It has been recently asserted by Dr. J. E. Gray (in the
Proceedings of the Zoological Society for 1863, page 134), that the
difficulty of limiting species is in proportion to our ignorance, and
that just as groups or countries are more accurately known and studied
in greater detail the limits of species become settled. This statement
has, like many other general assertions, its portion of both truth and
error. There is no doubt that many uncertain species, founded on few or
isolated specimens, have had their true nature determined by the study
of a good series of examples: they have been thereby established as
species or as varieties; and the number of times this has occurred is
doubtless very great. But there are other, and equally trustworthy
cases, in which, not single species, but whole groups have, by the study
of a vast accumulation of materials, been proved to have no definite
specific limits. A few of these must be adduced. In Dr. Carpenter's
"Introduction to the Study of the Foraminifera," he states that "_there
is not a single specimen of plant or animal of which the range of
variation has been studied by the collocation and comparison of so large
a number of specimens as have passed under the review of Messrs.
Williamson, Parker, Rupert Jones, and myself, in our studies of the
types of this group_;" and the result of this extended comparison of
specimens is stated to be, "_The range of variation is so great among
the Foraminifera as to include not merely those differential characters
which have been usually accounted_ SPECIFIC, _but also those upon which
the greater part of the_ GENERA _of this group have been founded, and
even in some instances those of its_ ORDERS" (Foraminifera, Preface, x).
Yet this same group had been divided by D'Orbigny and other authors into
a number of clearly defined _families_, _genera_, and _species_, which
these careful and conscientious researches have shown to have been
almost all founded on incomplete knowledge.
Professor DeCandolle has recently given the results of an extensive
review of the species of Cupuliferae. He finds that the best-known
species of oaks are those which produce most varieties and subvarieties;
that they are often surrounded by provisional species; and, with the
fullest materials at his command, two-thirds of the species he considers
more or less doubtful. His general conclusion is, that "_in botany the
lowest series of groups,_ SUBVARIETIES, VARIETIES, _and_ RACES _are very
badly limited; these can be grouped into_ SPECIES _a little less vaguely
limited, which again can be formed into sufficiently precise_ GENERA."
This general conclusion is entirely objected to by the writer of the
article in the "Natural History Review," who, however, does not deny its
applicability to the particular order under discussion, while this very
difference of opinion is another proof that difficulties in the
determination of species do not, any more than in the higher groups,
vanish with increasing materials and more accurate research.