Part 8 (1/2)

_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.