Chapter XVII.).

He will also find much to puzzle him in the mode of dispersal of the Hawaiian residual genera of the Convolvulaceæ (Breweria, Jacquemontia, and Cuscuta) that possess only endemic species, and he will speculate as to the manner in which seeds that would seem to possess but little attraction for birds and have no capacity for transportation by the currents could ever have reached these islands, and he will ask himself why it is that the agencies of dispersal, whatever they are, have now ceased to be active. He will perhaps see a way out of his difficulties when he perceives that if isolation has led to the development of peculiar species in Hawaii, it has strangely enough in the case of the Myrsinaceous genus Embelia produced the same effect over the whole range of the genus, and that Hawaii has in this respect derived no advantage from being an oceanic group. According to Carl Mez, nearly all the ninety species of this Old World genus are restricted in their areas, whether continental or insular (“Myrsinaceæ,” _Das Pflanzenreich_, 1902); and indeed we do not seem justified in assuming that the isolating influences in the case of this genus have been more effective in Hawaii in the mid-Pacific, or in Mauritius in the Indian Ocean, than they have been in continental regions like the Deccan and Nyassa Land, in all of which localities endemic species occur.

The remarkable development of the Cucurbitaceous genus Sicyos, in Hawaii alone of all the tropical Pacific groups, will attract his attention, and he will find here another instance of that predominant principle in the distribution of Pacific plants, where in a widely-ranging genus we find one of its species covering most of its area, whilst the other species are more or less localised. He will wonder at the limitation to Hawaii of a genus like Dracæna, that is so well adapted for dispersal over the Pacific by frugivorous birds; and in endeavouring to explain the presence in the Hawaiian forests of the gigantic Rumex, R. giganteus, he will remember that the small group of Tristan da Cunha, equally isolated in the South Atlantic, possesses an endemic species of the same genus. He will discover in the recognised dispersing agencies of wild ducks and other waterfowl an explanation of the occurrence in Hawaii of the aquatic genera Naias and Potamogeton; but he will be puzzled at their restriction to this group alone of the three tropical Pacific archipelagoes here especially discussed.

Amidst these various perplexities he will probably look with relief on the appearance of Phytolacca brachystachys, an endemic species of the American “pokeweeds”; and he will feel grateful to the American botanists like Professor Weed when they tell him that in the United States crows, blackbirds, and other birds successfully disperse these plants, the seeds of which are sometimes able to pass through the alimentary canal undigested.

But by far the most significant lesson that the student of distribution will carry away from his study of the Hawaiian residual genera will be that which he learns from the genera Embelia and Naias. He perceives here that not only with a typical land-genus has specific differentiation occurred to much the same extent in the continental and insular localities of its range, but that even with a typical genus of submerged aquatic plants, where the conditions of existence are as uniform as they are varied in the case of land plants, the process of differentiation has proceeded on the same broad lines in the interior of a continent and in an island in mid-ocean.

The following notes on some of the residual genera refer more particularly to matters connected with distribution and dispersal.

_Osmanthus_ (Oleaceæ).—This genus, according to the _Index Kewensis_, contains six species localised in their several habitats of North America, Hawaii, Japan (two), Hongkong, and the Himalayas. Its representative in this group is the Hawaiian Olive, the Olea sandwicensis of Gray, a prevailing tree in the lower and middle woods (1,000 to 4,000 feet) of all the islands, which, like other Hawaiian plants, such as those of the genera Eurya and Antidesma, indicates that the group has been sometimes independently stocked from the regions of the northern hemisphere. The drupe of this tree contains a stone two-thirds of an inch (17 mm.) in length, and suitable for dispersal by frugivorous birds; and birds have evidently distributed the tree all over the group. In fact Mr. Perkins in mentioning the favourite food of birds of the Hawaiian genus, Phaeornis, refers to the fruits of this tree as well as of the Opiko (Straussia) and of the Olapa (Cheirodendron). When, however, we come to consider the feasibility of the stones of the genus having been thus originally carried to Hawaii either from Japan or from North America, we meet with the difficulty presented to us by other Hawaiian genera with stone-fruits, such as Elæocarpus, or with berries containing large seeds, such as Sideroxylon.

_Sicyos_ (Cucurbitaceæ).—This genus comprises about thirty-five known species, of which three-fourths are confined to the New World, being mainly South American, whilst the remainder are restricted to Hawaii, with the exception of two species in the Galapagos Group and Norfolk Island, and a widely-ranging species, S. angulatus. The plant just named, the small fruits of which possess hooked spines, adapting them for dispersal in a bird’s plumage, occurs in Africa, Australia, New Zealand, and America, but has only been recorded in the Pacific islands from the Kermadec Group.

North America was probably the home of the original Hawaiian species. Hillebrand describes eight species, of which five are not found in more than one island, whilst one species is spread over most of the islands. The fruits vary much in size, and only in a couple of species do they now possess any fitness for attaching themselves to plumage, some of them being pubescent or even glabrate, so that deterioration in the capacity for dispersal has here taken place. Their size is usually a quarter to half an inch (6-12 mm.); but it is noteworthy that the species with the largest fruit (Sicyos cucumerinus, one to two inches, or 25 to 50 mm.) is the species most widely dispersed over the group. This appears to indicate that there is some other means of inter-island dispersal in this archipelago than by attachment to birds’ plumage. The isolation of the genus in Hawaii from the rest of the world is, however, complete, since all the species are endemic; and when, therefore, we come to ask how Sicyos angulatus, that has been dispersed in the recent era over America, Australia, and New Zealand, is not found in these islands, we are brought face to face with the ever-recurring difficulty, the suspension in later times of the agency of dispersal in the tropical North Pacific.

_Jacquemontia_ (Convolvulaceæ).—This genus, which is chiefly American, is represented in Hawaii by a peculiar species, J. sandwicensis. This species grows occasionally on the sandy beaches associated with Heliotropium anomalum and Tribulus cistoides; but it is most at home on rocky ground and on old lava-flows near the sea-border, making its abode often in the pockets of black sand produced by the disintegration of the lava. Its small seeds sink in sea-water even after prolonged drying; and it can perhaps be supposed that the original seeds were brought from North America in the crevices of a drifting log. According to Ridley, Fernando Noronha possesses a peculiar species also growing near the sea; and it may be that the drifting log has here been the agent also: but in neither case would this explanation account for the endemic character of the species.

_Cuscuta_ (Convolvulaceæ).—It would seem that with the exception of Hawaii, where an endemic species, C. sandwichiana, occurs, no other oceanic group in the globe possesses a peculiar species of the Dodders. With the exception of an endemic species in New Zealand, and an introduced species in Fiji which is found usually near the gardens of the white residents on Viti Levu, the genus takes but little part in the Pacific floras. The Hawaiian species is a characteristic beach-plant growing on Ipomœa pes capræ, Scævola Kœnigii, Tribulus cistoides, and on other plants that find a permanent or a temporary abode on the beaches. We learn from Ridley and Moseley that Cuscuta americana in Fernando Noronha finds its host also in Ipomœa pes capræ. Since the seeds of the Hawaiian plant and of the introduced Fijian species possess no buoyancy, even after drying for years, we cannot look to the agency of the current unless we call the drifting log to our assistance, and in that case the endemic character of the Hawaiian species would present the difficulty already alluded to in the case of Jacquemontia. The seeds of the Hawaiian plant are about one-twelfth of an inch (2 mm.) in diameter, and as far as size is concerned they might have been transported in a bird’s stomach; but, on account of the rapidity with which the seeds of the genus absorb moisture and swell up, it is most unlikely that they would escape injury. This is one of the several difficulties in plant-dispersal which New Zealand and Hawaii share in common. Further remarks on the germination of the Hawaiian species are made in Note 69.

_Rumex_ (Polygonaceæ).—Hawaii possesses two peculiar species of Rumex, a genus not recorded from any other of the Polynesian groups. One of these species, R. giganteus, is a very remarkable plant, growing to a height of thirty or forty feet when supported by trees. It is noteworthy that the small group of Tristan da Cunha in the South Atlantic possesses a species, R. frutescens, confined to those islands (_Bot. Chall. Exped._, ii. 154). Both Hawaii and Tristan da Cunha lie in mid-ocean, cut off from the nearest continent by some 1,800 or 2,000 miles of sea; and we may have to choose between the bird and the current in selecting the agency concerned with the transportation of the original seeds; or perhaps they have co-operated. Birds could disperse the nutlets of Rumex as readily as they do those of Polygonum, and I have found these fruits at times in the stomachs of partridges. On the other hand, Rumex fruits occur amongst the drift stranded on beaches in England and in Scandinavia; and, as indicated by the observations of Sernander and myself in these two localities, they float through the winter in ponds and rivers, germinating afloat in the spring. The nutlets sink, but they owe their buoyancy to the persistent perianth. In my sea-water experiments the fruits of Rumex hydrolapathum and R. conglomeratum were still afloat after from six to twelve months’ immersion, and their seeds subsequently germinated. It is quite possible, therefore, that currents can carry these fruits unharmed to oceanic island-groups like Hawaii and Tristan da Cunha.

_Dracæna_ (Liliaceæ).—This Old World genus, which on account of its berries is eminently suited for dispersal by frugivorous birds, is represented in Polynesia by a solitary species (D. aurea) peculiar to the Hawaiian Group. Attaining a height of twenty to twenty-five feet, it often forms a striking feature in the vegetation of the open wooded regions up to altitudes of 3,000 feet. I found it growing in abundance in the large island of Hawaii between Waimanu and Waipio, and on the northern slopes of Hualalai. It grows in a variety of stations, and I came upon it once in the broken-down caverns of an old lava-flow that were frequented by pigeons which no doubt brought the seeds. Its conspicuous yellow berries have hard rounded seeds a quarter of an inch (6 mm.) across and weighing two to three grains when dry, which would probably withstand injury in a bird’s stomach, the minute embryo being protected by a very tough albumen. Neither the entire berry nor the seed could be transported by currents, the last sinking even after drying for six years.

_Naias_ (Naiadaceæ).—If we except New Caledonia, where two or three species have been found, Hawaii is the only island-group in the tropical Pacific from which this interesting world-ranging genus of submerged aquatic plants has been recorded. Chamisso, the celebrated naturalist of Kotzebue’s expedition, collected Naias marina in Oahu in the early part of last century; but apparently it did not come under Hillebrand’s observation in the group. However, in 1897 I found it in another locality, namely, just within the mouth of the Waipio, a river on the north-west side of the island of Hawaii. The mature fruits of this genus have never been experimented on by me; but there is nothing in the structure of the fruits to indicate that they have any buoyancy, or to show that they differ in this respect from the fruits of other completely submerged aquatic plants like Ceratophyllum, Ruppia, and some of the Potamogetons. It is to ducks and other waterfowl that we must attribute the dispersal of this and the other genera just mentioned over wide tracts of ocean, a subject dealt with in discussing those plants.

The Hawaiian Group probably represents the most isolated locality occupied by this genus, since none of the other islands from which species have been recorded, such as New Caledonia, Mauritius, and Bourbon, are so far removed from continental regions. The source of the Hawaiian form of Naias marina lies evidently on the Asiatic side of the Pacific, since it is referred by Mr. Rendle to the variety “angustifolia,” an Asiatic plant found also in the island of Bourbon and in West Australia, but not recorded from the New World. The important little monograph of the genus by Mr. Rendle (“Naiadaceæ,” in Engler’s _Das Pflanzenreich_, 1901) is full of suggestiveness for the student of plant-distribution. His interest is excited when he discovers that one of the most typical genera of aquatic plants displays the same principle of differentiation at work that is so well illustrated by many of the land genera of the Pacific islands. I refer to the principle implied in the existence of a widely-ranging genus comprising “a polymorphic species occurring over almost the whole area of the genus,” as well as a number of less widely distributed species, most of which have “restricted areas and fall for the most part into small geographical groups.” I have just been quoting Mr. Rendle’s description of the distribution of Naias, the “polymorphic” species concerned being N. marina; but it need scarcely be remarked that it would apply just as well to several of the land genera dealt with in the previous chapter (XXVI.), such as Alphitonia, Metrosideros, Pisonia, &c.

Although there is such a contrast in the degree of uniformity of their life-conditions between land and water plants, a strictly aquatic plant being but slightly affected by changes in the physical conditions that are accompanied by a complete transformation in the character of the terrestrial vegetation, yet—and this is the important point—we find the same principle of differentiation at work with both land and water plants. If one wished to produce proof of the contention that the production of new species is largely independent of external conditions, one could not do better than take the cases of Elæocarpus, Metrosideros, and Naias. In all cases we see a widely-ranging polymorphous species settling down and “differentiating” in particular localities or regions, and forming subcentres for the distribution of the genus.

_Potamogeton_ (Potameæ).—Though well suited for dispersal by waterfowl, the Potamogetons have been recorded from the Hawaiian and Marianne Islands alone among the tropical groups of the open Pacific. The genus, though not so well represented in insular floras as we might have expected, is still not infrequently to be found. Widely-ranging species have been observed in the Azores, Madeira, and the Canaries in the Atlantic, as well as in Hawaii in the Pacific; whilst species have been recorded that are peculiar to Martinique, the Mascarene Islands, and to the Marianne Group. Hillebrand gives for Hawaii, Potamogeton fluitans, a plant of the Old and New Worlds, and P. pauciflorus, a North American species; whilst in the _Index Kewensis_ a peculiar species, P. owaihiensis of Chamisso (which is, however, regarded by Hillebrand as a form of P. fluitans), is also accredited to the group. Owing, however, to the paucity of streams and rivers this genus takes no prominent part in the Hawaiian flora, and the species seem to have been recorded alone from Oahu. As they were discovered by Chamisso in the early part of last century they are in all probability truly indigenous in Hawaii, even if none are peculiar to the group.

That ducks and similar birds are the agents in carrying the seeds of Potamogeton to oceanic islands cannot be doubted. About twelve years ago I examined the stomachs and intestines of thirteen wild ducks obtained in the London market. Three of them contained in all forty-one Potamogeton seeds, or rather “stones,” most of which subsequently germinated in water. In one of my experiments, carried out in the month of December, I fed a domestic duck with the fruits of Potamogeton natans. They appeared in quantity in the droppings, for the most part divested of their soft coverings, but otherwise uninjured. Sixty per cent. germinated in the following spring; whilst of those left in the vessel, from which the duck had been fed, only one per cent. germinated in the next spring, and another year elapsed before any number did so. These results were published in _Science Gossip_ for September, 1894.

One often reads in books of travel interesting remarks bearing indirectly on the dispersal of the Potamogetons. Thus, when Sir Joseph Hooker (then Dr. Hooker) noted in his _Himalayan Journals_ the occurrence of P. natans in the Neongong Lake in the Himalayas, and the presence of coots, he most probably mentioned the bird that brought the plants, coots being active distributors of the seeds of water plants. It is of importance to remember that (as shown in my experiment on the duck) seeds of water-plants are voided in a condition peculiarly favourable to early germination. Ducks, coots, and other water birds might often be characterised as “travelling germinators.” My experiment showed that seven to eight hours at least were occupied by Potamogeton nutlets in passing through the digestive canal of a duck, and that probably nine or ten hours would be required after an average full meal. But this does not represent the possible maximum period, since the bared “stone” may remain in the gizzard for a long time with ordinary gravel. Most of the Potamogeton fruits found by me in wild ducks were obtained from the gizzard, where they were mixed with gravel and other hard seeds or seedvessels, as described in Chapter XXXIII. Such fruits afterwards germinated. With regard to the chances, therefore, of the fruits of Potamogeton being carried by a bird without injury across an ocean, we may infer that, whether they are retained in its body for only ten hours or for as long as three or four days, they will preserve in some cases their germinating power.

HAWAIIAN GENERA FOUND IN TAHITI TO THE EXCLUSION OF FIJI.

Taking only the genera that are strictly indigenous, and excluding therefore all those introduced by the aborigines, the number available for establishing an independent connection between the Hawaiian and Tahitian regions is exceedingly few. Amongst the Hawaiian shore-plants not found in Fiji proper but occurring in the Tahitian region are Heliotropium anomalum and Sesuvium portulacastrum. The last-named, however, has been recorded from Tonga, which lies within the Fijian area; whilst the first will probably be found in the same region. Amongst the Hawaiian and Tahitian mountain genera not recorded from Fiji proper are Nertera, Vaccinium, Cyathodes, and Luzula. As is pointed out in Chapter XXIII., the absence of these genera from Fiji is connected with the relatively low elevation of those islands, though it is quite possible that one or more of them may yet be found on the highest summits of Fiji; and indeed Nertera depressa and Vaccinium have been discovered in the more elevated uplands of Savaii in Samoa.

After removing the littoral plants and the mountain genera, there are probably not more than half a dozen inland genera that connect the Hawaiian lowlands with the Tahitian region to the exclusion of the Fijian Group; and Byronia (Ilicineæ), Reynoldsia or Trevesia (Araliaceæ), Phyllostegia (Labiatæ), and Pseudomorus (Urticaceæ) may be taken as examples. Of these, Pseudomorus, which has a small drupaceous fruit suitable for dispersal by frugivorous birds, has been recorded from New Caledonia, and not improbably it exists in the Fijian area; and the same may be postulated of Reynoldsia, which is discussed in a later page, since it has been found in Samoa. We may almost form the same opinion of Byronia, since it exists in Australia. This genus of small trees contains only three known species, one in Australia, one in Tahiti, and one in Hawaii. Its fleshy drupes, about a third of an inch (8 mm.) in size, would attract birds, and their numerous cartilaginous pyrenes would probably pass unharmed through a bird’s alimentary canal. Phyllostegia, a Labiate genus with fleshy nucules that might attract birds, is, with the exception of a solitary Tahitian species, entirely confined to Hawaii (see Chapter XXII.).

From these data it may be inferred that the interchange of plants between the regions of Hawaii and Tahiti to the exclusion of Fiji has been very slight. The facts of distribution are just such as we might look for in the case of a general dispersal over the oceanic groups of the tropical Pacific, with the altitudes of the islands playing a determining part. In this general dispersal Hawaii has shared; and except in the case of Phyllostegia it is evident that this group has kept nearly all it received and has distributed but little.

HAWAIIAN GENERA FOUND IN FIJI TO THE EXCLUSION OF TAHITI.

We shall be able to throw further light on the floral history of Hawaii by discussing the few tropical genera, not a score in all, that it possesses in common with Fiji to the exclusion of the Tahitian region. The following genera offer themselves for treatment:—Eurya (Ternstrœmiaceæ), Gouania (Rhamnaceæ), Maba (Ebenaceæ), Sideroxylon (Sapotaceæ), Antidesma (Euphorbiaceæ), Pleiosmilax (Smilaceæ), and Ruppia (Potameæ).

These seven genera, which with the exception of Ruppia, an aquatic genus, are only represented in Hawaii by peculiar species, possess in all cases, except Gouania and the last-named genus, drupaceous or baccate fruits likely to attract frugivorous birds. Two of them, Eurya and Antidesma, have their home in Malaya and in the Asiatic continent; three of them, Gouania, Maba, and Sideroxylon, are found on both the Asiatic and the American sides of the Pacific Ocean; whilst Pleiosmilax should, strictly speaking, be regarded as a Polynesian subgenus of Smilax, a world-ranging genus; and Ruppia is a cosmopolitan brackish- and salt-water genus.

It is highly probable that Fiji received almost all these genera from the Old World through Malaya; and in some cases the resemblance between the Malayan and the Fijian species is so close that, as in Gouania, Dr. Seemann questioned if they were not forms of the same species. In other instances, as with Maba, we find a widely-ranging Asiatic and Malayan species, like Maba buxifolia, extending into Western Polynesia, where it is accompanied by other species peculiar to that region. But if the genera were able subsequently to extend their range thence to Hawaii, it is difficult to understand why they have not reached the Tahitian region. It is therefore likely that Hawaii received most of these genera by a northern route and not through the South Pacific; and it is legitimate to suppose that when Old World genera like Eurya and Antidesma occur in north-eastern Asia, as in Japan and in the neighbouring mainland, Hawaii received the genus by that route. In the case of Eurya it is noteworthy that Fijian and Samoan forms, regarded by Seemann and Gray as distinct species, are viewed by Reinecke as forms of E. japonica, an extremely variable species found in Japan. With genera like Gouania and Maba, that exist on both sides of the Pacific, it is possible that they may have originally reached Hawaii from America.

A noticeable feature in the instance of genera like Maba and Sideroxylon is that hard seeds or pyrenes 3/4 to 1 inch (18 to 25 mm.) in length have seemingly been transported by frugivorous birds across the ocean to Hawaii. This at first sight seems improbable; but it is known that fruit-pigeons can swallow very large drupes, as in the case of those of Canarium, Dracontomelon, and Elæocarpus, afterwards disgorging the “stones.” They have carried such stones to Fiji, across some 500 or 600 miles of ocean; and unless we impute a continental origin to Hawaii we must assume that in some cases, as with Elæocarpus, Maba, and Sideroxylon, they have been able to transport these large stones or pyrenes to that group. The extent of ocean to be crossed is no doubt much greater, but this area of the Pacific is not without some small half-way groups that would serve as resting-places.

That fruits of the order Sapotaceæ are much appreciated by fruit-pigeons is already known. We learn from Kirk that the fruits of Sideroxylon costatum (Sapota costata) are a favourite food of the New Zealand fruit-pigeon, the fruits, about an inch long, containing three hard crescentic bony seeds nearly as long as the fruit. The natives of Vanua Levu informed me that a Fijian species of Sideroxylon with hard seeds about an inch long was much appreciated on account of its fruit by the pigeons. I found the hard, sound seeds of a species of Sapota, two-thirds of an inch (or 16 mm.) in size, in the crop of a Fijian fruit-pigeon. The similarly large seeds of a species of Achras were identified by Mr. Charles Moore, of Sydney, amongst a collection of seeds, &c., found by me in the crops of fruit-pigeons shot in the Solomon Islands (Guppy’s _Solomon Islands_, p. 293). It may be added that the difficulty concerned with Sideroxylon in Hawaii is the difficulty concerned with other large-seeded Sapotaceous trees in Fiji and New Zealand, and the same explanation must be applied to all. Some further remarks on the Sapotaceæ in the Pacific are given below.

The mode of dispersal of some of these genera is illustrated in other regions. The berries of Pleiosmilax, a subgenus of Smilax, are well suited for aiding the dispersal of the genus by frugivorous birds; and we learn from Prof. Barrows (Weed, p. 42) that in the United States crows feed on the fruits of Smilax rotundifolia and disperse the seeds. On the other hand, it is not at first sight easy to understand how a genus like Gouania has been distributed over the tropics of the globe, since it possesses dry capsular fruits about half an inch across, separating into three woody cocci that appear most unlikely to attract birds. The same difficulty exists, however, with other dry-fruited widely-ranging genera like Alphitonia and with many of the Euphorbiaceæ.

Amongst these genera found in Hawaii and Fiji to the exclusion of Tahiti we can at times detect indications of the operations of a polymorphous species as described in Chapter XXVI., when a widely-ranging highly variable species is associated in some groups with peculiar species. We see some evidence of this in the genera Gouania, Maba, and Eurya, alluded to on a previous page. (See also _Bot. Chall. Exped._, iii. 134, under “Gouania.”)

One of the mysteries of the Pacific is concerned with the distribution of the Sapotaceæ, the dispersal of which by frugivorous birds has been dealt with above. It is strange that whilst the order seems to have found a _rendezvous_ in Tonga, no one except Horne appears to have recorded any of the genera from Samoa. They are fairly well represented in Fiji; but it is in Tonga that we especially note the gathering together of several Sapotaceous trees with large heavy seeds, of the genera Bassia, Mimusops, and Sideroxylon. Besides owning one or two species of Sideroxylon in common with Fiji (Burkill), this small group possesses Bassia amicorum and Mimusops kauki, both of which were found there by Forster at the time of Cook’s visit. In a list of a small collection of plants made by him in Upolu in the Samoan Group about 1879, Horne includes two species of Sideroxylon (_Year in Fiji_, p. 286); and according to Seemann there is a Sapotaceous tree in Wallis Island. A species of Bassia exists in Rarotonga, the seeds of which, from Mr. Cheeseman’s description of the fruit, must be almost an inch long. Drake del Castillo refers to an endemic Tahitian tree near Mimusops; but its fruit was not known to him.

As already indicated, the difficulties connected with the Sapotaceæ affect the whole Pacific from New Zealand north to Hawaii and from Fiji east to Tahiti. We are driven to appeal to the agency of frugivorous birds, at least in the case of Sideroxylon, since some fruits experimented on by me in Fiji sank at once or in a day or two, the seeds having no buoyancy. That birds actually disperse the seeds of this and other genera of the order has been already pointed out, yet it is possible that currents have at times aided in the dispersal of some of the genera. This is indicated by the circumstance that, as we learn from Schimper, some Sapotaceous trees are to be included in the Malayan strand-flora, namely, Sideroxylon ferrugineum, Mimusops kauki, and M. littoralis, all occurring as well on the Asiatic mainland, the first growing also in the Liukiu Islands, and the last in the Andaman and Nicobar Groups.

_Ruppia maritima_ (Potameæ).—This cosmopolitan aquatic plant has only been recorded in Polynesia from Hawaii, Samoa, and Fiji. It had not been collected in Fiji before my discovery of it in 1897. Amongst other oceanic islands where it occurs may be mentioned the Bermudas, where, according to Hemsley, it exists as an indigenous plant in the lagoons. Chamisso first noticed it in Hawaii, and Hillebrand remarks that it grows in shallow waters along the coasts. Amongst other localities where I noticed it in this group may be mentioned the north-west coast of the large island of Hawaii between Kailua and Keahole Point. Here in 1896 it was thriving in brackish-water ponds, with Sesuvium portulacastrum growing at the edges. Reinecke observes that it occurs in similar ponds in Samoa. In 1897 I found it in abundance in the Rewa estuary (Fiji), both in the creeks and in the main channel. In the following year it was not to be found in this locality, a circumstance noticed both by the natives and by resident whites. The fruits of this plant possess no floating power, sinking, even after prolonged drying, in a few hours. It is to ducks and to birds of similar habit that its dispersal must be attributed.

THE ABSENTEES FROM HAWAII.

It has been before remarked that of the 330 or 340 genera of flowering-plants recorded from Fiji some 200 are not known in Hawaii. It will only be possible to deal with the absent genera in a cursory manner; but enough will be done to show that we are face to face here with a multitude of the seeming inconsistencies that so often beset the study of plant-distribution.

A host of plants are unrepresented in Hawaii, of which it may be said that their seeds or fruits are not less suited for being carried across the Pacific than those of many that are now in that group. On the other hand, a number of genera exist there which we should never expect to have been endowed with the capacity, and to have received the opportunity, of crossing nearly 2,000 miles of ocean. Yet perhaps when Nature acts in a wholesale fashion and excludes entire orders we may be able to perceive the dim outlines of a principle of exclusion at work. But even here much caution and some clearing of the ground are needed.

For example, having regard to the several modes of dispersal possessed by the great variety of fruits and seeds of the Sterculiaceæ, it would be almost meaningless to remark that the order so well represented in Fiji is practically non-existent in Hawaii as far as truly indigenous plants are concerned. It is true that two species of Waltheria are here present, but one of them W. americana, is a weed probably introduced by the aborigines whilst the other, W. pyrolæfolia, recorded from a solitary locality by the Wilkes Expedition, has seemingly never been found since. From the standpoint of dispersal the genera Sterculia, Heritiera, Kleinhovia, Melochia, and Commersonia, that are represented in Fiji but not in Hawaii, cannot be discussed together. With Sterculia is concerned the dispersal by birds of large seeds, an inch in length, not particularly well protected, the genus being confined to Fiji alone of all the oceanic Pacific groups. Heritiera is only represented by a littoral species, the large fruits of which are carried great distances by the currents; and no other agency of dispersal is here possible. The last three genera are distributed over the South Pacific, their relatively small seeds being probably in the main dispersed by granivorous birds; whilst the setose fruits of Commersonia may have been at times transported in birds’ plumage.

It is more legitimate, perhaps, to speak collectively of the orders Meliaceæ and Melastomaceæ as absent from Hawaii; but even here the issue raised is one concerned rather with opportunities than with capacities for dispersal. Several years ago, M. Casimir de Candolle remarked that “it is hardly credible that the Meliaceæ should be entirely absent from the Sandwich archipelago” (_Trans. Linn. Soc. Bot._, vol. i. 1880). Yet it can scarcely be said that this is a matter connected with means of dispersal. Amongst the Meliaceous genera represented in Fiji, Vavæa and Aglaia have a berry, Melia has a drupe, and Dysoxylum has a capsule. So again with the Melastomaceæ; it possesses at least six genera in Fiji, two in Tahiti, and none in Hawaii. Whilst the genera Melastoma and Medinilla have baccate fruits with minute seeds, Astronia has a capsule with similar seeds, and Memecylon has a single-seeded berry. Since, however, minute seeds are most typical of the order, those of Melastoma denticulatum being about one-fiftieth of an inch or ·5 mm. in size, it would seem that this character has not aided its dispersal in the Pacific so far as Hawaii is concerned. From the circumstance that berries, drupes, and capsules are represented in these two Fijian orders we may form the opinion that their non-occurrence in Hawaii is due not so much to lack of capacities for dispersal as to failure of opportunities.

This opinion is much strengthened when we come to deal with the individual genera, where the predominant cause of the absence of so many Fijian genera from Hawaii is concerned with the failure of the agencies of dispersal. It is not a question of a difference in size between the groups, since, although the surface-area is approximately the same in both groups, Hawaii possesses only two-thirds of the number of genera occurring in Fiji. It is not a question of capacity for dispersal across an ocean, since birds have transported across the Pacific to Hawaii the “stones” and large seeds of genera like Elæocarpus and Sideroxylon, a feat that would have been deemed impossible by many botanists. It is no lack of capacity for dispersal that has excluded Loranthus from Hawaii and has admitted Viscum.

Few genera, indeed, would seem to be better fitted for dispersal by frugivorous birds in the Pacific than that of Ficus. Its fruits are known to be eaten by birds all over the area of the genus; and we find the species distributed over the South Pacific from Fiji to Tahiti, but they are quite absent from Hawaii. This is the more remarkable on account of the occurrence of a species of Ficus resembling a banyan in Fanning Island about 900 miles south of the group (_Bot. Chall. Exped._,