Russel Wallace : Alfred Russell Wallace (sic)
Archipelago" (S381: 1885)
Mr. Forbes appears to have had rather more than his fair share of accidents to his collections, and in every case what was lost was of especial interest. His insect collection from the Keeling Islands was destroyed on the way back to Java, and we thus lose the opportunity of comparing the list with that made by Mr. Darwin more than forty years before. In Timor Laut a large part of his herbarium was destroyed by fire, while a smaller collection made in the interior of Buru was actually left behind for want of porters to carry it. The Timor Laut collection is especially to be regretted, as it was obtained with great difficulty in perhaps the least known island of the whole archipelago, while it is probable that many years will elapse before any other naturalist will venture to explore so remote and inhospitable a country.
Mr. Forbes' residence for three weeks in the Keeling Islands enabled him to note what changes had occurred since Darwin's visit nearly half a century earlier. These are very slight, and seem incompatible with the theory that any subsidence has taken place, because the inner margin of some of the islands next the lagoon are sometimes half a mile distant from the outer edge, and the greatest cyclones do not carry the coral débris nearly so far. It is now generally admitted that the celebrated "subsidence theory" of the formation of atolls and barrier reefs is unsound as a general explanation of the facts; yet it so fully and plausibly explained all the details of coral structure known at the time, as to command universal acceptance and unbounded admiration. We have here a remarkable instance of the danger of founding a general explanation of widespread phenomena on an assumed basis, for the fact of long-continued subsidence, which was the very foundation of the whole theory, was in most cases quite incapable of proof. It is also now apparent that the theory was to some extent inconsistent with the views as to oceanic islands which Darwin himself originated and which are now generally admitted to be sound. His great argument, that no single oceanic island possessed ancient stratified rocks or contained a single indigenous mammal, was equally an argument against the view that the widespread coral archipelagoes of the Pacific and Indian Oceans were due to the subsidence of co-extensive tracts of land, since it is almost impossible that all the higher points of these submerged lands, spread over nearly half the surface of the globe, should be without exception of volcanic origin.
Crabs of two or three species were the most abundant terrestrial inhabitants of the Keeling Islands living in narrow corkscrew burrows, which are so numerous that one hundred and twenty of their holes were counted in an area only two feet square. Around these holes little mounds are formed, and the crabs carry into their burrows twigs of trees, pieces of seaweeds, seeds, &c., thus fulfilling in many ways the functions of earthworms in this newly-formed land. Their numbers are enormous, and Mr. Forbes thus describes the curious optical effect produced by them:--
"On placing the foot on the region occupied by them, one perceives an undulation of the surface followed, over a circular area, by a surprising change of the pure white ground into a warm pink colour, which for the moment the stranger puts down to some affection of his eyes from the reflection of the light. He soon perceives that this movement is caused by the simultaneous stampede of the dense crowd of the peopled shore into their dwellings, just within the door of which they halt, with the larger of their two pincer-claws, which is of a rich pink colour, effectually barring the entrance except where one watchful stalked eye is thrust out to take an inquiring look if the alarm is real. As one advances the pink areas again change into white, as the Crustaceans withdraw into their subterranean fastnesses. On traversing a broad field occupied by these crabs, the constant undulations and change of colours produce a curious dazzling effect upon the eyes."
During his long residence in the mountains of Java, Mr. Forbes made many interesting observations on the fertilisation of orchids. He was surprised at the large number of these plants which, though often possessing the combined attractions of showy flowers and fragrant odours, yet never or rarely produce seed-capsules. In one case, for example, out of 360 flowers examined till they withered or dropped off, only six produced capsules. Again, he finds a considerable number of species with showy flowers which are yet specially adapted for self-fertilisation and never seem to be visited by insects. The most extreme and marvellous example of this phenomenon is found in a plant related to Chrysoglossum, which fertilises itself without ever opening its flowers at all. Mr. Forbes observed these plants in the forest as well as in numerous specimens grown in a garden, and all were fertilised in the same way; and he adds:--"In opening the locked-up petals, I found the labellum beautifully marked with lines of purple, carmine, and orange, and the column also; but no insect eye could ever be fascinated or allured by its painted whorls."
These observations are of extreme interest, and they certainly prove, as Mr. Forbes remarks, that the rule "that the flowers of orchids are fertilised by the pollen of their flowers," is by no means so universal as has been supposed. Yet the phenomenon does not seem so extraordinary if we look upon it as one of the normal phases [[p. 220]] in the developmental life-history of species. The overwhelming amount of evidence which has now been obtained of adaptations for cross-fertilisation, not in orchids only, but throughout the whole series of flowering plants, and the almost constant association of conspicuous form, colour, and odour with adaptations for insect fertilisation, force us to the conclusion that in almost all the cases adduced by Mr. Forbes we have species which were once adapted for insect-fertilisation. But in the terrific struggle for existence ever going on in tropical regions, insects are subject perhaps more than any other group of organisms to excessive fluctuations of numbers, sometimes culminating in the complete extermination of species; because they are equally liable to severe injury by physical and organic causes--by adverse seasons which destroy them in some of their earlier stages, or by the excessive attacks of insectivorous animals in both their larval and perfect states. It must therefore often happen that certain species of insects almost disappear in districts where they are usually abundant, and if any particular plant has had its flowers so highly specialised as to be adapted for fertilisation by one of these insects only, it must become extinct unless it occasionally produces varieties which are capable of self-fertilisation. The species of orchids in which a very small percentage of flowers produce seed capsules are evidently those in which the special insects adapted to fertilise them have become either temporarily or permanently scarce, and if that scarcity goes on increasing one of three things must happen--either the flower must become modified so as to be fertilised by some more abundant insect, or it must become capable of self-fertilisation, or it must become extinct. No doubt all these three cases occur, but it is of the second alone that we can obtain any knowledge, because we there find, as in our own bee-orchis, the special attractions of conspicuous form and colour which have yet ceased to be of service to the species. But no naturalist can doubt that these attractions were once serviceable; and we are thus led to conclude that all such instances are forms of functional degeneration which under changed conditions of the environment have afforded the only means of preserving the species.
Mr. Forbes's record of his thirteen months of travel in Sumatra are perhaps the most interesting portions of his book. He here met with some of the most marvellous productions of the vegetable kingdom--strange parasitical Rafflesiaceæ, an eccentric fig which ran underground and there produced its fruit, just showing their tops above the surface, and the giant arum (Amorphophallus titanum), some of which were seventeen feet high and with tubers six feet six inches in circumference. In the same forest huge earth-worms raised tubes of mud four and a half inches in circumference and eight inches high; and were so numerous as to render the whole surface of the ground as rough and hummocky as that of a newly-ploughed field. Here too, as well as in Java, he found a wonderful case of mimicry in a spider which deceived him even a second time; and he here obtained the rare Ornithoptera brookeana, perhaps the most chastely beautiful of all butterflies. Grand mountains, active volcanoes, glorious forest scenery, strange antique monoliths, and many interesting races of men, combine to render Sumatra one of the finest hunting-grounds yet left for the naturalist, while over the greater part of it there are facilities for travel or for residence rarely to be found in so little known a country.
In his later and more adventurous explorations of Timor Laut and Timor, Mr. Forbes was accompanied by his wife, a lady who seems to have endured all the annoyances, privations, and dangers of such a journey with truly heroic fortitude. Although these islands are far less known to naturalists than almost any other part of the Archipelago, they seem comparatively poor in a natural-history point of view. A considerable proportion of the birds and butterflies of Timor Laut were new species, but the collections were scanty, and there is, no doubt, much still to be done there if a collector could freely explore the country and not be confined, as was Mr. Forbes, to a limited tract owing to tribal warfare. One of the interesting discoveries here was another example of mimicry among birds, in which a new species of oriole mimics a new honeysucker, just as do corresponding species in Ceram, Buru, Gilolo, and Timor. A most interesting case of protective colouration was also observed in the white-headed fruit-pigeon of Timor (Ptilopus cinctus). These birds sat motionless during the heat of the day in numbers on well-exposed branches, yet Mr. Forbes states that it was with the greatest difficulty that either he or his sharp-eyed native servant could detect them, even in trees where they knew they were sitting. The strongly-contrasted white and dark colours of this species are such that any person looking at a specimen in a museum might take it as an example of a defenceless bird with very conspicuous plumage, and might ask triumphantly how our theory of protective colouration can be applied here. Yet it turns out that these strongly-marked colours so exactly harmonise with the colours of the branches of the trees on which it sits, exposed to the glare of the tropical sun, as to be completely protective; and we thus have another illustration of the impossibility of forming any correct judgment on this question unless we are able to observe each species in its native country and among the exact surroundings to which it has become adapted.
The hasty journey through the interior of Timor, among strange scenery and strange people, is full of interest. Most of the mountain tops, where alone a rich and interesting vegetation was to be found, were strictly taboed, and it was often only by stratagem that specimens were collected; while the difficulties of travel in a country absolutely without roads and consisting almost wholly of an endless series of rugged mountains and deep valleys were exceptionally great.
The book is on the whole very well written, and will give the reader an excellent idea of some of the less known parts of the Malay Archipelago. The weakest part of it are the illustrations, which, though numerous, appear to be for the most part reproductions of rough sketches by some unsatisfactory process of photo-zincography. For this the author was probably not responsible, but his readers will regret that the strange and beautiful scenery he has so graphically described is not more effectively presented to the eye. The portraits of many of the natives are, however, very well done, while several good maps and a full index greatly add to the value of the book as a useful work of reference.