Contributions to the Theory of Natural Selection Part 8

_Remarks on the facts of Local variation._

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 (Papilionidae and Pieridae) acquire, in a single island, a characteristic modification of form distinguis.h.i.+ng 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 _a priori_ evidence in favour of an ident.i.ty 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 a.s.suming 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 explanation. Mr. Bates has shown that certain groups of b.u.t.terflies 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. 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 Danaidae 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 Satyridae have also probably a means of protection--perhaps their keeping always near the ground and their generally obscure colours; while the Lycaenidae and Hesperidae may find security in their small size and rapid motions. In the extensive family of the Nymphalidae, 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 b.u.t.terflies 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 b.u.t.terflies 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, &c.), 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 b.u.t.terflies on the wing, but it is highly probable that they do so when other food is scarce. Mr. Bates has suggested to me that the larger Dragonflies (aeshna, &c.) prey upon b.u.t.terflies; but I did not notice that they were more abundant in Celebes than elsewhere. However this may be, the fauna of Celebes is undoubtedly highly peculiar in every department of which we have any accurate knowledge; and though we may not be able satisfactorily to trace how it has been effected, there can, I think, be little doubt that the singular modification in the wings of so many of the b.u.t.terflies 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.

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,--and why, in three separate cases, the females of Amboyna species should be less gaily attired than the corresponding females of the surrounding islands,--are questions which we cannot at present attempt to answer. That they depend, however, on some general principle is certain, because a.n.a.logous 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 aeneas-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 b.u.t.terflies peculiar to the island of Sardinia are generally smaller and more deeply coloured than those of the mainland, and the same has recently been shown to be the case with the common tortoisesh.e.l.l b.u.t.terfly in the Isle of Man; while Papilio Hospiton, peculiar to the former island, 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 colour, and thus directly aid in producing the endless variety of nature.

_Mimicry._

Having fully discussed this subject in the preceding essay, I have only to adduce such ill.u.s.trations of it, as are furnished by the Eastern Papilionidae, and to show their bearing upon the phenomena of variation already mentioned. As in America, so in the Old World, species of Danaidae are the objects which the other families most often imitate. But besides these, some genera of Morphidae 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 Papilionidae of the Malayan region and India:--

Mimickers. Species mimicked. Common habitat.

DANAIDae.

1. Papilio paradoxa Euploea Midamus } Sumatra, &c.

(male & female) (male & female) } 2. P. Caunus E. Rhadamanthus Borneo and Sumatra.

3. P. Thule Danais sobrina New Guinea.

4. P. Macareus D. Aglaia Malacca, Java 5. Papilio Agestor Danais Tytia Northern India.

6. P. Idaeoides Hestia Leuconoe Philippines.

7. P. Delessertii Ideopsis daos Penang.

MORPHIDae.

8. P. Pandion Drusilla bioculata New Guinea (female)

PAPILIO (POLYDORUS- and c.o.o.n-groups).

9. P. Pammon (Romulus, Papilio Hector India.

female) 10. P. Theseus, var. P. Antiphus Sumatra, Borneo.

(female) 11. P. Theseus, var. P. Diphilus Sumatra, Java.

(female) 12. P. Memnon, var. P. c.o.o.n Sumatra.

(Achates, female) 13. P. Androgeus, var. P. Doubledayi Northern India.

(Achates, female) 14. P. Oenomaus P. Liris Timor.

(female)

We have, therefore, fourteen species or marked varieties of Papilio, which so closely resemble species of other groups in their respective localities, that it is not possible to impute the resemblance to accident. The first two in the list (Papilio paradoxa and P. Caunus) are so exactly like Euploea Midamus and E. Rhadamanthus on the wing, that although they fly very slowly, I was quite unable to distinguish them.

The first is a very interesting case, because the male and female differ considerably, and each mimics the corresponding s.e.x of the Euploea. A new species of Papilio which I discovered in New Guinea resembles Danais sobrina, from the same country, just as Papilio Marcareus resembles Danais Aglaia in Malacca, and (according to Dr. Horsfield's figure) still more closely in Java. The Indian Papilio Agestor closely imitates Danais Tytia, which has quite a different style of colouring from the preceding; and the extraordinary Papilio Idaeoides from the Philippine Islands, must, when on the wing, perfectly resemble the Hestia Leuconoe of the same region, as also does the Papilio Delessertii imitate the Ideopsis daos from Penang. Now in every one of these cases the Papilios are very scarce, while the Danaidae which they resemble are exceedingly abundant--most of them swarming so as to be a positive nuisance to the collecting entomologist by continually hovering before him when he is in search of newer and more varied captures. Every garden, every roadside, the suburbs of every village are full of them, indicating very clearly that their life is an easy one, and that they are free from persecution by the foes which keep down the population of less favoured races. This superabundant population has been shown by Mr. Bates to be a general characteristic of all American groups and species which are objects of mimicry; and it is interesting to find his observations confirmed by examples on the other side of the globe.

The remarkable genus Drusilla, a group of pale-coloured b.u.t.terflies, more or less adorned with ocellate spots, is also the object of mimicry by three distinct genera (Melanitis, Hyantis, and Papilio). These insects, like the Danaidae, are abundant in individuals, have a very weak and slow flight, and do not seek concealment, or appear to have any means of protection from insectivorous creatures. It is natural to conclude, therefore, that they have some hidden property which saves them from attack; and it is easy to see that when any other insects, by what we call accidental variation, come more or less remotely to resemble them, the latter will share to some extent in their immunity.

An extraordinary dimorphic form of the female of Papilio Ormenus has come to resemble the Drusillas sufficiently to be taken for one of that group at a little distance; and it is curious that I captured one of these Papilios in the Aru Islands hovering along the ground, and settling on it occasionally, just as it is the habit of the Drusillas to do. The resemblance in this case is only general; but this form of Papilio varies much, and there is therefore material for natural selection to act upon, so as ultimately to produce a copy as exact as in the other cases.

The eastern Papilios allied to Polydorus, c.o.o.n, and Philoxenus, form a natural section of the genus resembling, in many respects, the aeneas-group of South America, which they may be said to represent in the East. Like them, they are forest insects, have a low and weak flight, and in their favourite localities are rather abundant in individuals; and like them, too, they are the objects of mimicry. We may conclude, therefore, that they possess some hidden means of protection, which makes it useful to other insects to be mistaken for them.

The Papilios which resemble them belong to a very distinct section of the genus, in which the s.e.xes differ greatly; and it is those females only which differ most from the males, and which have already been alluded to as exhibiting instances of dimorphism, which resemble species of the other group.

The resemblance of P. Romulus to P. Hector is, in some specimens, very considerable, and has led to the two species being placed following each other in the British Museum Catalogues and by Mr. E. Doubleday. I have shown, however, that P. Romulus is probably a dimorphic form of the female P. Pammon, and belongs to a distinct section of the genus.

The next pair, Papilio Theseus, and P. Antiphus, have been united as one species both by De Haan and in the British Museum Catalogues. The ordinary variety of P. Theseus found in Java almost as nearly resembles P. Diphilus, inhabiting the same country. The most interesting case, however, is the extreme female form of P. Memnon (figured by Cramer under the name of P. Achates), which has acquired the general form and markings of P. c.o.o.n, an insect which differs from the ordinary male P.

Memnon, as much as any two species which can be chosen in this extensive and highly varied genus; and, as if to show that this resemblance is not accidental, but is the result of law, when in India we find a species closely allied to P. c.o.o.n, but with red instead of yellow spots (P.

Doubledayi), the corresponding variety of P. Androgeus (P. Achates, Cramer, 182, A, B,) has acquired exactly the same peculiarity of having red spots instead of yellow. Lastly, in the island of Timor, the female of P. Oenomaus (a species allied to P. Memnon) resembles so closely P.

Liris (one of the Polydorus-group), that the two, which were often seen flying together, could only be distinguished by a minute comparison after being captured.

The last six cases of mimicry are especially instructive, because they seem to indicate one of the processes by which dimorphic forms have been produced. When, as in these cases, one s.e.x differs much from the other, and varies greatly itself, it may happen that occasionally individual variations will occur having a distant resemblance to groups which are the objects of mimicry, and which it is therefore advantageous to resemble. Such a variety will have a better chance of preservation; the individuals possessing it will be multiplied; and their accidental likeness to the favoured group will be rendered permanent by hereditary transmission, and, each successive variation which increases the resemblance being preserved, and all variations departing from the favoured type having less chance of preservation, there will in time result those singular cases of two or more isolated and fixed forms, bound together by that intimate relations.h.i.+p which const.i.tutes them the s.e.xes of a single species. The reason why the females are more subject to this kind of modification than the males is, probably, that their slower flight, when laden with eggs, and their exposure to attack while in the act of depositing their eggs upon leaves, render it especially advantageous for them to have some additional protection. This they at once obtain by acquiring a resemblance to other species which, from whatever cause, enjoy a comparative immunity from persecution.

_Concluding remarks on Variation in Lepidoptera._

This summary of the more interesting phenomena of variation presented by the eastern Papilionidae is, I think, sufficient to substantiate my position, that the Lepidoptera are a group that offer especial facilities for such inquiries; and it will also show that they have undergone an amount of special adaptive modification rarely equalled among the more highly organized animals. And, among the Lepidoptera, the great and pre-eminently tropical families of Papilionidae and Danaidae seem to be those in which complicated adaptations to the surrounding organic and inorganic universe have been most completely developed, offering in this respect a striking a.n.a.logy to the equally extraordinary, though totally different, adaptations which present themselves in the Orchideae, the only family of plants in which mimicry of other organisms appears to play any important part, and the only one in which cases of conspicuous polymorphism occur; for as such we must cla.s.s the male, female, and hermaphrodite forms of Catasetum tridentatum, which differ so greatly in form and structure that they were long considered to belong to three distinct genera.

_Arrangement and Geographical Distribution of the Malayan Papilionidae_.

_Arrangement._--Although the species of Papilionidae inhabiting the Malayan region are very numerous, they all belong to three out of the nine genera into which the family is divided. One of the remaining genera (Eurycus) is restricted to Australia, and another (Teinopalpus) to the Himalayan Mountains, while no less than four (Parna.s.sius, Doritis, Thais, and Sericinus) are confined to Southern Europe and to the mountain-ranges of the Palaearctic region.

The genera Ornithoptera and Leptocircus are highly characteristic of Malayan entomology, but are uniform in character and of small extent.

The genus Papilio, on the other hand, presents a great variety of forms, and is so richly represented in the Malay Islands, that more than one-fourth of all the known species are found there. It becomes necessary, therefore, to divide this genus into natural groups before we can successfully study its geographical distribution.

Owing princ.i.p.ally to Dr. Horsfield's observations in Java, we are acquainted with a considerable number of the larvae of Papilios; and these furnish good characters for the primary division of the genus into natural groups. The manner in which the hinder wings are plaited or folded back at the abdominal margin, the size of the a.n.a.l valves, the structure of the antennae, and the form of the wings are also of much service, as well as the character of the flight and the style of colouration. Using these characters, I divide the Malayan Papilios into four sections, and seventeen groups, as follows:--

Genus ORNITHOPTERA.

a. Priamus-group. } Black and Green.

c. Brookea.n.u.s-group.} b. Pompeus-group. Black and yellow.

Genus PAPILIO.

A. Larvae short, thick, with numerous fleshy tubercles; of a purplish colour.

a. Nox-group. Abdominal fold in male very large; a.n.a.l valves small, but swollen; antennae moderate; wings entire, or tailed; includes the Indian Philoxenus-group.

b. c.o.o.n-group. Abdominal fold in male small; a.n.a.l valves small, but swollen; antennae moderate; wings tailed.

c. Polydorus-group. Abdominal fold in male small, or none; a.n.a.l valves small or obsolete, hairy; wings tailed or entire.

B. Larvae with third segment swollen, transversely or obliquely banded; pupa much bent. Imago with abdominal margin in male plaited, but not reflexed; body weak; antennae long; wings much dilated, often tailed.

d. Ulysses-group.

{Protenor-group (Indian) is e. Peranthus-group. {somewhat intermediate between f. Memnon-group. {these, and is nearest {to the Nox-group.