Part 3

Professor DeCandolle has recently given the results of an extensive review of the species of _Cupuliferæ_. 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.

Another striking example of the same kind is seen in the genera _Rubus_ and _Rosa_, adduced by Mr. Darwin himself; for though the amplest materials exist for a knowledge of these groups, and the most careful research has been bestowed upon them, yet the various species have not thereby been accurately limited and defined so as to satisfy the majority of botanists.

Dr. Hooker seems to have found the same thing in his study of the Arctic flora. For though he has had much of the accumulated materials of his predecessors to work upon, he continually expresses himself as unable to do more than group the numerous and apparently fluctuating forms into more or less imperfectly defined species[6].

Footnote 6:

In his paper on the “Distribution of Arctic Plants,” Trans. Linn. Soc. xxiii. p. 310, Dr. Hooker says:—

“The most able and experienced descriptive botanists vary in their estimate of the value of the ‘specific term’ to a much greater extent than is generally supposed.”

“I think I may safely affirm that the ‘specific term’ has three different standard values, all current in descriptive botany, but each more or less confined to one class of observers.”

“This is no question of what is right or wrong as to the real value of the specific term; I believe each is right according to the standard he assumes as the specific.”

Lastly, I will adduce Mr. Bates’s researches on the Amazons. During eleven years he accumulated vast materials, and carefully studied the variation and distribution of insects. Yet he has shown that many species of Lepidoptera, which before offered no special difficulties, are in reality most intricately combined in a tangled web of affinities, leading by such gradual steps from the slightest and least stable variations to fixed races and well-marked species, that it is very often impossible to draw those sharp dividing-lines which it is supposed that a careful study and full materials will always enable us to do.

These few examples show, I think, that in every department of nature there occur instances of the instability of specific form, which the increase of materials aggravates rather than diminishes. And it must be remembered that the naturalist is rarely likely to err on the side of imputing greater indefiniteness to species than really exists. There is a completeness and satisfaction to the mind in defining and limiting and naming a species, which leads us all to do so whenever we conscientiously can, and which we know has led many collectors to reject vague intermediate forms as destroying the symmetry of their cabinets. We must therefore consider these cases of excessive variation and instability as being thoroughly well established; and to the objection that, after all, these cases are but few compared with those in which species can be limited and defined, and are therefore merely exceptions to a general rule, I reply that a true law embraces all apparent exceptions, and that to the great laws of nature there are no real exceptions—that what appear to be such are equally results of law, and are often (perhaps indeed always) those very results which are most important as revealing the true nature and action of the law. It is for such reasons that naturalists now look upon the study of _varieties_ as more important than that of well-fixed species. It is in the former that we see nature still at work, in the very act of producing those wonderful modifications of form, that endless variety of colour, and that complicated harmony of relations, which gratify every sense and give occupation to every faculty of the true lover of nature.

_Variation as specially influenced by Locality._

The phenomena of variation as influenced by locality have not hitherto received much attention. Botanists, it is true, are acquainted with the influences of climate, altitude, and other physical conditions in modifying the forms and external characteristics of plants; but I am not aware that any peculiar influence has been traced to locality, independent of climate. Almost the only case I can find recorded is mentioned in that repertory of natural-history facts, ‘The Origin of Species,’ viz. that herbaceous groups have a tendency to become arboreal in islands. In the animal world, I cannot find that any facts have been pointed out as showing the special influence of locality in giving a peculiar _facies_ to the several disconnected species that inhabit it. What I have to adduce on this matter will therefore, I hope, possess some interest and novelty.

On examining the closely allied species, local forms, and varieties distributed over the Indian and Malayan regions, I find that larger or smaller districts, or even single islands, give a special character to the majority of their Papilionidæ. For instance: 1. The species of the Indian region (Sumatra, Java, and Borneo) are almost invariably smaller than the allied species inhabiting Celebes and the Moluccas; 2. The species of New Guinea and Australia are also, though in a less degree, smaller than the nearest species or varieties of the Moluccas; 3. In the Moluccas themselves the species of Amboyna are the largest; 4. The species of Celebes equal or even surpass in size those of Amboyna; 5. The species and varieties of Celebes possess a striking character in the form of the anterior wings, different from that of the allied species and varieties of all the surrounding islands; 6. Tailed species in India or the Indian region become tailless as they spread eastward through the archipelago.

Having preserved the finest and largest specimens of Butterflies in my own collection, and having always taken for comparison the largest specimens of the same sex, I believe that the tables I now give are sufficiently exact. The differences of expanse of wings are in most cases very great, and are much more conspicuous in the specimens themselves than on paper. It will be seen that no less than fourteen Papilionidæ inhabiting Celebes and the Moluccas are from one-third to one-half greater in extent of wing than the allied species representing them in Java, Sumatra, and Borneo. Six species inhabiting Amboyna are larger than the closely allied forms of the northern Moluccas and New Guinea by about one-sixth. These include almost every case in which closely allied species can be compared.

PAPILIONIDÆ.

Species of the Moluccas and Celebes│Closely allied species of Java and (large). │ the Indian region (small). Expanse. │ Expanse. inches. │ inches. Ornithoptera Helena 7·6│O. Pompeus 5·8 (Amboyna) │ „ „ │O. Amphrisius 6·0 ───────────────────────────────────┼─────────────────────────────────── Papilio Macedon 5·8│P. Peranthus 3·8 (Celebes) │ P. Philippus (Moluccas) 4·8│ „ „ ───────────────────────────────────┼─────────────────────────────────── P. Blumei (Celebes) 5·4│P. Brama 4·0 P. Alphenor (Celebes) 4·8│P. Theseus 3·6 P. Gigon (Celebes) 5·4│P. Demolion 4·0 P. Deucalion (Celebes) 4·6│P. Macareus 3·7 P. Agamemnon, var. 4·4│P. Agamemnon, var. 3·8 (Celebes) │ ───────────────────────────────────┼─────────────────────────────────── P. Eurypilus (Moluccas) 4·0│P. Jason 3·4 P. Telephus (Celebes) 4·3│ „ „ ───────────────────────────────────┼─────────────────────────────────── P. Ægisthus (Moluccas) 4·4│P. Rama 3·2 P. Miletus (Celebes) 4·4│P. Sarpedon 3·8 P. Androcles (Celebes) 4·8│P. Antiphates 3·7 P. Polyphontes 4·6│P. Diphilus 3·9 (Celebes) │ Leptocircus Curtius 2·0│L. Meges 1·8 (Celebes) │ │ Species inhabiting Amboyna (large).│ Allied species of New Guinea and │ the North Moluccas (smaller). Papilio Ulysses 6·1│P. Penelope 5·2 „ „ │P. Telegonus 4·0 ───────────────────────────────────┼─────────────────────────────────── P. Polydorus 4·9│P. Leodamas 4·0 P. Deiphobus 6·8│P. Deiphontes 5·8 ───────────────────────────────────┼─────────────────────────────────── P. Gambrisius 6·4│P. Ormenus 5·6 „ „ │P. Tydeus 6·0 ───────────────────────────────────┼─────────────────────────────────── P. Codrus 5·1│P. Codrus, var. 4·3 │ _papuensis_ Ornithoptera Priamus, ♂ 8·0│Orn. Poseidon, ♂ 7·0

The differences of form are equally clear.

_Papilio Pammon_ everywhere on the continent is tailed in both sexes. In Java, Sumatra, and Borneo, the closely allied _P. Theseus_ has a very short tail, or tooth only, in the male, while in the females the tail is retained. Further east, in Celebes and the South Moluccas, the hardly separable _P. Alphenor_ has quite lost the tail in the male, while the female retains it, but in a narrower and less spatulate form. A little further, in Gilolo, _P. Nicanor_ has completely lost the tail in both sexes.

_Papilio Agamemnon_ exhibits a somewhat similar series of changes. In India it is always tailed; in the greater part of the archipelago it has a very short tail; while far east, in New Guinea and the adjacent islands, the tail has almost entirely disappeared.

In the _Polydorus_-group two species, _P. Antiphus_ and _P. Diphilus_, inhabiting India and the Indian region, are tailed, while the two which take their place in the Moluccas, New Guinea, and Australia, _P. Polydorus_ and _P. Leodamas_, are destitute of tail, the species furthest east having lost this ornament the most completely.

Western species, tailed. Eastern species (closely allied), less tailed.

Papilio Pammon (India) tailed. P. Thesus (islands) very short tail.

P. Agamemnon, var. tailed. P. Agamemnon, var. not tailed. (India) (islands)

P. Antiphus (India, tailed. P. Polydorus (Moluccas) not tailed. Java)

P. Diphilus (India, tailed. P. Leodamas (New not tailed. Java) Guinea)

The most conspicuous instance of local modification of form, however, is exhibited in the island of Celebes, which in this respect, as in some others, stands alone and isolated in the whole archipelago. Almost every species of _Papilio_ inhabiting Celebes has the wings of a peculiar shape, which distinguishes them at a glance from the allied species of every other island. This peculiarity consists, first, in the upper wings being generally more elongate and falcate; and secondly, in the costa or anterior margin being much more curved, and in most instances exhibiting near the base an abrupt bend or elbow, which in some species is very conspicuous. This peculiarity is visible, not only when the Celebesian species are compared with their small-sized allies of Java and Borneo, but also, and in an almost equal degree, when the large forms of Amboyna and the Moluccas are the objects of comparison, showing that this is quite a distinct phenomenon from the difference of size which has just been pointed out.

In the following Table I have arranged the chief Papilios of Celebes in the order in which they exhibit this characteristic form most prominently. (See Plate VIII.)

Papilios of Celebes, having the Closely allied Papilios of the wings falcate or with abruptly surrounding islands, with less curved costa. falcate wings and slightly curved costa.

1. P. Gigon, n. s. P. Demolion (Java).

2. P. Telephus, n. s. P. Jason (Sumatra).

3. P. Miletus, n. s. P. Sarpedon (Moluccas, Java).

4. P. Agamemnon, var. P. Agamemnon, var. (Borneo).

5. P. Macedon, n. s. P. Peranthus (Java).

6. P. Ascalaphus. P. Deiphontes, n. s. (Gilolo).

7. P. Hecuba, n. s. P. Helenus (Java).

8. P. Blumei. P. Brama (Sumatra).

9. P. Androcles. P. Antiphates (Borneo).

10. P. Rhesus. P. Aristæus (Moluccas).

11. P. Theseus, var., ♂. P. Thesus, ♂ (Java).

12. P. Codrus, var. P. Codrus (Moluccas).

13. P. Encelades. P. Leucothoë (Malacca).

It thus appears that every species of _Papilio_ exhibits this peculiar form in a greater or less degree, except one, _P. Polyphontes_, Bd., allied to _P. Diphilus_ of India and _P. Polydorus_ of the Moluccas. This fact I shall recur to again, as I think it helps us to understand something of the causes that may have brought about the phenomenon we are considering. Neither do the genera _Ornithoptera_ and _Leptocircus_ exhibit any traces of this peculiar form. In several other families of Butterflies this characteristic form reappears in a few species. In the Pieridæ the following species exhibit it distinctly:—

1. Eronia tritæa │compared with Eronia Valeria (Java).

2. Iphias Glaucippe, var.│ „ „ Iphias Glaucippe (Java).

3. Pieris Zebuda │ „ „ Pieris Descombesi (India).

4. P. Zarinda │ „ „ P. Nero (Malacca).

5. P., n. s. │ „ „ P. Hyparete (Java).

──────────────────────────┬──────────────────────────────────────────── 6. P. Hombronii │ have the same form, but are isolated │ species. 7. P. Ithome │ „ ──────────────────────────┼──────────────────────────────────────────── 8. P. Eperia, _Bd._ │compared with P. Coronis (Java). 9. P. Polisma │ „ „ P., n. s. (Malacca). 10. Terias, n. s. │ „ „ P. Tilaha (Java).

The other species of _Terias_, one or two _Pieris_, and the genus _Callidryas_ do not exhibit any perceptible change of form.

In the other families there are but few similar examples. The following are all that I can find in my collection:—

Cethosia Æole compared with Cethosia Biblis (Java).

Junonia, n. s. „ „ Junonia Polynice (Borneo).

Limenitis Limire „ „ Limenitis Procris (Java).

Cynthia Arsinoë, var. „ „ Cynthia Arsinoë (Java, Sum., Born.).

All these belong to the family of the Nymphalidæ. Many other genera of this family, as _Diadema_, _Adolias_, _Charaxes_, and _Cyrestis_, as well as the entire families of the Danaidæ, Satyridæ, Lycænidæ, and Hesperidæ, present no examples of this peculiar form of the upper wing in the Celebesian species.

The facts now brought forward seem to me of the highest interest. We see that almost all the species in two important families of the Lepidoptera (Papilionidæ and Pieridæ) acquire, in a single island, a characteristic modification of form distinguishing them from the allied species and varieties of all the surrounding islands. In other equally extensive families no such change occurs, except in one or two isolated species. However we may account for these phenomena, or whether we may be quite unable to account for them, they furnish, in my opinion, a strong corroborative testimony in favour of the doctrine of the origin of species by successive small variations; for we have here slight varieties, local races, and undoubted species, all modified in exactly the same manner, indicating plainly a common cause producing identical results. On the generally received theory of the original distinctness and permanence of species, we are met by this difficulty: one portion of these curiously modified forms are admitted to have been produced by variation and some natural action of local conditions; whilst the other portion, differing from the former only in degree, and connected with them by insensible gradations, are said to have possessed this peculiarity of form at their first creation, or to have derived it from unknown causes of a totally distinct nature. Is not the _à priori_ evidence in favour of the assumption of an identity of the causes that have produced such similar results? and have we not a right to call upon our opponents for some proofs of their own doctrine, and for an explanation of its difficulties, instead of their assuming that they are right, and laying upon us the burthen of disproof?

Let us now see if the facts in question do not themselves furnish some clue to their own explanation. Mr. Bates has shown that certain groups of butterflies have a defence against insectivorous animals, independent of swiftness of motion. These are generally very abundant, slow, and weak fliers, and are more or less the objects of mimicry by other groups, which thus gain an advantage in a freedom from persecution similar to that enjoyed by those they resemble. Now the only Papilios which have not in Celebes acquired the peculiar form of wing belong to a group which is imitated both by other species of _Papilio_ and by Moths of the genus _Epicopeia_, West. This group is of weak and slow flight; and we may therefore fairly conclude that it possesses some means of defence (probably in a peculiar odour or taste) which saves it from attack. Now the arched costa and falcate form of wing is generally supposed to give increased powers of flight, or, as seems to me more probable, greater facility in making sudden turnings, and thus baffling a pursuer. But the members of the _Polydorus_-group (to which belongs the only unchanged Celebesian _Papilio_), being already guarded against attack, have no need of this increased power of wing; and “natural selection” would therefore have no tendency to produce it. The whole family of Danaidæ are in the same position: they are slow and weak fliers; yet they abound in species and individuals, and are the objects of mimicry. The Satyridæ have also probably a means of protection—perhaps their keeping always near the ground and their generally obscure colours; while the Lycænidæ and Hesperidæ may find security in their small size and rapid motions. In the extensive family of the Nymphalidæ, however, we find that several of the larger species, of comparatively feeble structure, have their wings modified (_Cethosia_, _Limenitis_, _Junonia_, _Cynthia_), while the large-bodied powerful species, which have all an excessively rapid flight, have exactly the same form of wing in Celebes as in the other islands. On the whole, therefore, we may say that all the butterflies of rather large size, conspicuous colours, and not very swift flight have been affected in the manner described, while the smaller-sized and obscure groups, as well as those which are the objects of mimicry, and also those of exceedingly swift flight, have remained unaffected.

It would thus appear as if there must be (or once have been) in the island of Celebes, some peculiar enemy to these larger-sized butterflies which does not exist, or is less abundant, in the surrounding islands. Increased powers of flight, or rapidity of turning, was advantageous in baffling this enemy; and the peculiar form of wing necessary to give this would be readily acquired by the action of “natural selection” on the slight variations of form that are continually occurring. Such an enemy one would naturally suppose to be an insectivorous bird; but it is a remarkable fact that most of the genera of Fly-catchers of Borneo and Java on the one side (_Muscipeta_, _Philentoma_), and of the Moluccas on the other (_Monarcha_, _Rhipidura_), are almost entirely absent from Celebes. Their place seems to be supplied by the Caterpillar-catchers (_Graucalus_, _Campephaga_), of which six or seven species are known from Celebes and are very numerous in individuals. We have no positive evidence that these birds pursue butterflies on the wing, but it is highly probable that they do so when other food is scarce[7]. However this may be, the fauna of Celebes is undoubtedly highly peculiar in every department of which we have any knowledge; and though we may not be able to trace it satisfactorily, there can, I think, be little doubt that the singular modification in the wings of so many of the butterflies of that island is an effect of that complicated action and reaction of all living things upon each other in the struggle for existence, which continually tends to readjust disturbed relations, and to bring every species into harmony with the varying conditions of the surrounding universe.

Footnote 7:

Mr. Bates has suggested that the larger Dragon-flies (_Æshna_, &c.) prey upon butterflies; but I did not notice that they were more abundant in Celebes than elsewhere.

But even the conjectural explanation now given fails us in the other cases of local modification. Why the species of the western islands should be smaller than those further east,—why those of Amboyna should exceed in size those of Gilolo and New Guinea—why the tailed species of India should begin to lose that appendage in the islands, and retain no trace of it on the borders of the Pacific, are questions which we cannot at present attempt to answer. That they depend, however, on some general principle is certain, because analogous facts have been observed in other parts of the world. Mr. Bates informs me that, in three distinct groups, Papilios which on the Upper Amazon and in most other parts of South America have spotless upper wings obtain pale or white spots at Para and on the Lower Amazon; and also that the _Æneas_-group of Papilios never have tails in the equatorial regions and the Amazons valley, but gradually acquire tails in many cases as they range towards the northern or southern tropic. Even in Europe we have somewhat similar facts; for the species and varieties of butterflies peculiar to the island of Sardinia are generally smaller and more deeply coloured than those of the mainland, and _Papilio Hospiton_ has lost the tail, which is a prominent feature of the closely allied _P. Machaon_.

Facts of a similar nature to those now brought forward would no doubt be found to occur in other groups of insects, were local faunas carefully studied in relation to those of the surrounding countries; and they seem to indicate that climate and other physical causes have, in some cases, a very powerful effect in modifying specific form, and thus directly aid in producing the endless variety of nature.

I may state that I can adduce facts perfectly analogous to these from other families of Lepidoptera, especially the Danaidæ; but as the greater part of the species are still undescribed, I can only now assert that similar phenomena do occur there.

_Mimicry._

I need scarcely say that I entirely agree with Mr. Bates’s explanation of the causes which have led to one group of insects mimicking another (Trans. Linn. Soc. vol. xxiii. p. 495). I have, therefore, only now to adduce such illustrations of this curious phenomenon as are furnished by the Eastern Papilionidæ, and to show their hearing upon the phenomena of variation already mentioned. As in America, so in the Old World, species of Danaidæ are the objects which the other families most often imitate. But, besides these, some genera of Morphidæ and one section of the genus _Papilio_ are also less frequently copied. Many species of _Papilio_ mimic other species of these three groups so closely that they are undistinguishable when on the wing; and in every case the pairs which resemble each other inhabit the same locality.

The following list exhibits the most important and best-marked cases of mimicry which occur among the Papilionidæ of the Malayan region and India:—

Mimickers[8]. Species mimicked. │Common habitat. DANAIDÆ. 1. Papilio paradoxa, Euplœa Midamus, _Cr._, ♂│Sumatra, &c. _Zink._, ♂ │ —— ——, ♀ —— ——, ♀ │ „ ──────────────────────────────────────────────────────┼──────────────── 2. —— ——, _West._ E. Rhadamanthus │Sumatra, &c. 3. P. Caunus, E., sp. │Borneo. 4. P. Thule, _Wall._ Danais sobrina, _Bd._ │New Guinea. 5. P. Macareus, _Godt._ D. Aglaia, _Cr._ │Malacca, Java. 6. P. Agestor, _G. R. G._ D. Tytia, _G. R. G._ │Northern India. 7. P. idæoides, _Hewits._ Hestia Leuconoë, │Philippines. _Erichs._ │ 8. P. Delessertii, _Guér._ Hestia, sp. │Penang. │ MORPHIDÆ. 9. P. Pandion, _Wall._, ♀ Drusilla bioculata, │New Guinea. _Guér._ │ │ PAPILIO (POLYDORUS- and COON-groups). 10. P. Pammon, _L._ (Romulus, Papilio Hector, _L._ │India. _L._), ♀ │ 11. P. Theseus, _Cr._, var., P. Antiphus, _Fab._ │Sumatra, Borneo. ♀ │ 12. P. Theseus, _Cr._, var., P. Diphilus, _Esp._ │Sumatra, Java. ♀ │ 13. P. Memnon, var. P. Coon, _Fab._ │Sumatra. _Achates_, ♀ │ 14. P. Androgeus, var. P. Doubledayi, _Wall._ │Northern India. _Achates_, ♀ │ 15. P. Œnomaus, _God._, ♀ P. Liris, _God._ │Timor.

Footnote 8: