페이지 이미지

6th. Tailed species in India and the western islands lose their tails as they spread eastward through the archipelago.

Details were given and diagrams, as well as specimens exhibited, showing the peculiarities of size and form above recapitulated. The most interesting case, however, was that of the Island of Celebes, almost all the Papilionidse, Pierida?, and some of the Nymphalida; of which had acquired a peculiar curve of the upper wings, amounting in some instances to an abrupt bend.

In no less than thirteen species of Papilio, ten Pieridae, and five or six Nymphalidse, the difference of form between the species or varieties peculiar to Celebes, and those most closely allied which inhabit all the other parts of the archipelago, is of exactly the same character and of nearly equal amount. The difference is the same whether we compare the small species of Java or the large ones of the Moluccas with those of Celebes, showing that the causes which have produced it are distinct from those which have led to increase of size. The difference is equally perceptible both in the varieties and in the species peculiar to Celebes; and this is held to be in favour of the doctrine that species and varieties are really of the same nature, and differ only in degree, since the local causes which have been at work in Celebes have acted on both in a manner perfectly identical.

In attempting to explain the possible origin of this curious phenomenon, is was pointed out that one species (Papilio ■polyphonies, Bd.), inhabiting Celebes, did not offer any difference of form when compared wkh its nearest allies in the surrounding islands; and that this unchanged species formed part of a group which, from being slow of flight, abundant in individuals, and especially from being the object of mimicry by other groups, was considered to have some special and hidden protection independent of flight. If, therefore, the butterflies of Celebes acquired these longer and more curved wings, owing to the persecution of bird or insect enemies from whom they could only escape by increased powers of flight, it is evident that those which had already some other means of protection wuiild receive no benefit from a change in the form of their wings, and therefore could not acquire it by the action of "natural selection." This also explains why none of the Danaidae are so modified, for they are universally the objects of mimicry by other groups, and are therefore already protected. The large thick-bodied Nymphalidae also fly very rapidly by the power of their muscles rather than the length of their wings, and are probably never captured on the wing; they have, therefore, not received any similar modification, because they did not require it.

In like manner the weak, obscure, brown SatyridaB and the small active Lycenidse and Hesperidse, all of which have unmodified wings, secure themselves from attack rather by concealment and peculiar habits than by direct power of flight. Although, therefore, we are not able to point out the peculiar enemies to Lepidoptera which have existed in Celebes alone, we may be sure that the singular alteration in the form of wing in so many of the butterflies of that island, as well as the changes of form and size induced in other parts of the archipelago, are the effects 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 ever-varying conditions of the surrounding universe.

The subject of mimicry was then explained, and it was shown by illustrative specimens that a number of the Malayan and Indian Papilios resembled very closely species of Danaidffi inhabiting the same districts. These Danaidse are almost a nuisance to the collecting entomologist, from their abundance and ubiquity. Every garden, every roadside, the suburbs of every village are full of them—indicating very clearly that their existence is an easy one, and that they are free from persecution by the foes that keep down the population of less favoured races. Their strong and peculiar odour is believed to be the cause of their safety, aud they are for this reason habitually passed over by insectivorous creatures. When, therefore, auother insect which has not this peculiar scent resembles one of the Danaidse of the same district, it also is passed over by mistake, aud thus gains an advantage. When this resemblance is general and slight, the benefit will be correspondingly small. Such a slight general resemblance may, however, evidently occur accidentally between very differently constructed animals; and what is important to observe is, that when once this happens the resemblance and the corresponding advantage will necessarily go on increasing by the action of "natural selection." For example, let a Pap'lio resemble a Danais so slightly that it is only mistaken for it by very unobservant birds, Ac, or at a considerable distance. Even this will be some advantage to it, for many individuals that would otherwise have been devoured will now live and leave offspring. But every animal varies more or less, sooner or later. Among the varieties of this Fapilio, some must be more like, some less like the Danais. The former will escape persecution more than the latter—will increase, therefore, while the latter will diminish. In each succeeding generation this preservation of the more like and the destruction of the less like will go on, which must slowly, but surely, produce a gradually increasing likeness, till the one insect can hardly be distinguished from the other. Such cases occur everywhere. Some of the more remarkable have been poiuted out by Mr. Bates as occurring in South America, where every streak aud spot, as well as the exact outline, has been copied by insects of a quite different structure. In tropical Africa cases quite as extraordinary occur, two or more insects of widely different structure coming outwardly to resemble a third equally different from both; and, in every case whether there are two or three, one of them is a Danais, or belongs to one or two other groups which have also Bome special protection. In every case, too, the insects which resemble each other inhabit the same country, and, in cases where their habits are known, are found

always to frequent the same places. This takes the phenomenon altogether out of the region of accidental resemblance, for it is evident that the chances of one insect resembling another are much greater when we compare those of all parts of the world than when we confine ourselves to those inhabiting the same country, the same locality, and even the same station. Yet cases of this very close resemblance between species of distinct families and genera are very rare in the former case-—just ns rare, in fact, as accidental resemblances should be; whereas, in the latter case, where, according to the law of chances, they should be infinitely more rare, they arc so frequently to be met with in every part of the world that they may almost be called common. There must be, therefore, in the cases pointed out by Mr. Bates and Mr. Wallace, a true relation between the species so resembling each other; and, if the explanation given by them be objected to, it should be shown that it is inadequate to account for the observed facts. Instead of this, however, the objection is made that there ought to be more cases of mimicry if this explanation is true; that almost all Pieridse should imitate Heliconias, and almost all Papilios should have come to resemble Danaidce. To this the answer appears sufficient, that it is only when, among the countless variations of insects, a remote resemblance accidentally occurs between the distinct groups, that "natural selection" can begin to act to increase that resemblance for the creature's benefit. The remaining species of the group will be preserved in a different manner. Some will acquire increased powers of flight, others facilities for concealment, others, again, increased reproductive powers, which will enable them to resist the persecution of their enemies. And it will depend, at some critical period of the existence of the species, on the proportionate amount and rapidity of variation in these different directions to determine the special means by which its perpetuation shall be secured. The comparative isolation of cases of mimicry is therefore no argument either against its serving as a means of protection, or against its having been produced in the manner here indicated. No less than fourteen Indian and Malayan Papilios mimic species of distinct groups, in several cases so closely that they have been placed by entomologists in the same species, although really having no close affiuity; and the writer was constantly deceived by them on the wing. We cannot wonder, therefore, that birds and insects also confound them.

The arrangement and geographical distribution of the Papilionidas formed the concluding section of the paper. The family is very richly represented in the Malay islands, more than a quarter of all the known species beiDg found there. In Africa about 40 species of Papilio exist, in Tropical Asia 65, in South America 120, or about the same number as in the Malay Archipelago. The area of the two countries is, however, very different; for, while South America contains more than 5,000,000 square miles, a line encircling the whole of the Malay islands would only include an area of 2,700,000 square miles, of which the actual land surface would not exceed 1,000,000 square miles. This comparative richness was shown to be partly due to the breaking up of the area into numerous isolated tracts, which led to the separation of many forms which, on a continental area, would probably exist as widely diffused and variable species. Dividing the Malayan Papilionida; into twenty groups of the most closely allied species, it was shown that seven of those groups were confined to the Indo-Malayan and three to the Austro-Malayan region, indicating the same division of the archipelago between the Indian and Australian regions which has already been established from a consideration of the distribution of Mammalia and Birds. The degree of the relationship of the islands with each other was also indicated by the relative proportion of the species common to them, aud was often opposed to that indicated by their geographical position or their physical characteristics. A striking example was that of Java and Sumatra, which are so closely connected by intervening islands in the Straits of Sunda, and have such a similarity in the continuous chain of volcanoes which passes through them, that we can hardly avoid the idea of their recent separation. This idea, however, is erroneous, for they have really less resemblance in their natural productions than either has with Borneo—separated from both by a wide extent of sea, and differing from both in its non-volcanic structure. The relations of the Papilionidae of these islands are as follow:—

Sumatra 21 species ) 20 species common to

Borneo 29 species J both islands.

Java 27 s]>ecies I 20 species common to

Borneo 29 species i both islands.

Sumatra 21 species ( 11 species common to

Java 27 species S both islands.

Showing that both Sumatra and Java have a much closer relationship to Borneo than they have to each other.

This exactly confirms a similar deduction from the Birds and Mammalia of the three islands, and may therefore be held to prove that the only recent connexion of Sumatra and Java has been through Borneo.

Again, Borneo and Java have each only two species quite peculiar to them, and Sumatra not one. Celebes, however, not separated from them geographically more than they are from each other, has seventeen species altogether restricted to it. Further east, no island has more than five species confined to it. Celebes, therefore, stands alone, and has an individuality comparable with that of extensive groups rather than with single islands; and, though eituated in the very midst of the archipelago, and surrounded on every side by islets which seem to afford the greatest facilities for the migration and intercommunication of the productions of it and the rest of the archipelago, yet retains a character of its own.

The great peculiarities of Celebes in its mammals, birds, and insects were then recapitulated, as well as the singular modifications of form in its Lepidoptera, which stamps the most dissimilar species with a mark distinctive of their common birthplace. It was argued that such phenomena could not be explained by the simple doctrine of special creations. There were so many signs of gradual modification and dependence upon physical and organic changes that we could not believe it to be all a delusive appearance, any more than we could believe that strata were never deposited in primaeval oceans, or that the fossils collected by the geologist are no true record of a former living world, but were all created just as they now appear.

All the curious phenomena here brought forward were believed by the author to be immediately dependent on the last series of changes, organic and inorganic, in these regions; and, as the phenomena presented by the island of Celebes differ from those of all the surrounding islands, it could, he conceived, only be because the past history of Celebes has been to some extent unique and different from theirs. More evidence was wanted to determine in what that difference consisted. At present one deduction only could be made—viz., that Celebes represented one of the most ancient parts of the archipelago; that it has been formerly more completely isolated both from India and from Australia than it now is; and that, amid all the mutations it has undergone, a relic of the fauna and flora of some more ancient land has been here preserved to us. The full peculiarity and interest of the country had only been understood since Mr. Wallace had been able to compare the productions of Celebes side by side with those of the rest of the archipelago; and he concluded by expressing a hope that some enterprising naturalist might devote himself to its more detailed examination, since he was sure that no single island on the globe promised so well to repay a careful research inio its past and present history.

April 7th.

Dr. H. Carter exhibited leaves of Mcus glomerata, Eoxb., in which a double foliaceous development had formed along some of the secondary veins on the under surface of the leaf, in a manuer recalling the indusium of ferns. The papers read were:—1. "On the Garcinia yelding Gamboge in Siam, by Daniel Hanbury, Esq., F.L.S.; with Remarks on the Structure of the Anther in the section Hebradendron of the genus Garcinia," by Professor Oliver, F.R.S. This Garcinia was regarded as a variety, with pedicellate flowers, of G. Morella of Ceylon. The authers were stated to be truly circumscissile in their dehiscence, and, at the same time, multilocular. 2. "Dimorphism in the Flowers of Monochoria vaginalis" by John Kirk, M.D.

« 이전계속 »