CHAPTER XVI

THE RELATION BETWEEN LITTORAL AND INLAND PLANTS (_continued_)

The Fijian difficulty.—Inland species of a genus possessing fruits not known to have any means of dispersal through agencies now at work in the Pacific.—Pandanus.—Its remarkable distribution in oceanic groups.—To be attributed perhaps to extinct Columbæ or extinct Struthious birds.—Barringtonia.—Guettarda.—Eugenia.—Drymispermum.—Acacia laurifolia.—Conclusions to be drawn from the discussion.—Summary of chapters XIV, XV, XVI.

SECTION IV

HERE we deal with two genera, Pandanus and Barringtonia, where inland endemic species occur in the same group with the wide-ranging coast species, but possess fruits concerning which it is either difficult or almost impossible to suggest a mode of dispersal by existing agencies. This section is especially concerned with Fiji, and represents the peculiar “Fijian difficulty” that is illustrated by other genera as—for instance, the Coniferous genus Dammara—which are not in any sense littoral. Further investigation is, however, requisite in the case of Barringtonia, and to a less degree with Pandanus; and I can only here point to the general indications of the data at my disposal. We have in these genera to assume either that the inland species are derived from the coast species, or that the seeds were brought by one of the extinct birds of the Western Pacific, by a megapode or by one of the Columbæ, or by some Struthious bird like the moa or the cassowary, or, if these two assumptions fail, that there has been a continental connection through the islands to the westward with the mainland beyond.

PANDANUS.

I take this genus first because the recent monograph on the Pandanaceæ by Dr. Warburg (Engler’s _Das Pflanzenreich_, 1900) enables me to tread on relatively safe ground in making my deductions. The three genera of the order, Freycinetia, Pandanus, and Sararanga, each tell their own story; and in each and all of them I have taken an especial interest from the standpoint of their dispersal. Freycinetia is fully discussed in Chapter XXV., and presents no difficulties respecting its dispersal. In the discovery of Sararanga the author has had a share. It was first established by Mr. Hemsley from specimens sent by me to Kew in 1885; and it has received from the botanist the name given to it by the natives of the islands of Bougainville Straits in the Solomon Group, where I first collected it. It contains only one species and was also discovered by Dr. Beccari, the celebrated Italian botanist, in Jobie Island, New Guinea. From the other two genera of the order, Pandanus and Freycinetia, it stands quite apart; and it apparently presents us with a relic of some ancient flora on the western borders of the Pacific. Its fleshy drupes (one-half to three-quarters of an inch in size) inclosing several small osseous pyrenes seem suited for dispersal by birds; and it is not at first sight easy to understand why its distribution should be so limited, unless this is connected with its diœcious habit (see Guppy’s _Solomon Islands_, p. 302; _Journ. Linn. Soc. Bot._ vol. xxx.; and Warburg’s monograph).

It is, however, with the genus Pandanus that we are here especially concerned. If the advocate of the previous continental connections of Fiji and the groups around were to look for evidence in support of his views, he apparently could not do better than take this genus. Whilst P. odoratissimus, the littoral species of tropical Asia and Malaya, is found on the coasts of almost all the Pacific islands from Fiji to Tahiti and northward to Hawaii, it is only in the archipelagoes of the Western Pacific, namely, in Fiji and Samoa, that inland endemic species have been found. (Such species occur also in the more western islands not dealt with here—New Caledonia, Solomon Islands, &c.) Not even in Hawaii, with all its botanical evidence of antiquity, has an inland endemic species been found, although the coast species extends miles inland, and for nearly 2,000 feet up the mountain slopes. When, however, we turn to Fiji and Samoa, we find in each group two endemic inland species. To endeavour to connect the inland species of Fiji and Samoa with the widespread littoral Pandanus odoratissimus, that owes its dispersal largely to the currents, is out of the question, at least for the student of plant-dispersal, since they belong to different sections of the genus, and in their characters are often far removed (see Note 58).

As regards the agency of birds, it is of course possible that fruit-pigeons that can disperse the “stones” of Canarium and Elæocarpus could transport the smaller drupes of Pandanus to oceanic islands like the Fijis, Samoa, and the Mascarene Islands; and in Note 58 reference is made to the size of the drupes of the endemic species of Pandanus in those groups. But my difficulty is that I have not come upon any record of birds eating these fruits; and I should imagine that amongst living birds only those like the cassowary and its kin would prefer such a kind of diet; whilst the only pigeon that could have ever attempted it must have been able to swallow pebbles like the dodo. It is remarkable that the Mascarene Islands, the home of the extinct Columbæ, possess more endemic species of Pandanus than any other groups.

Dr. Warburg points out that, with the exception of some three or four species dispersed by the currents (P. dubius, P. leram, P. polycephalus, P. odoratissimus), almost all the species (156 in number) are very restricted in their areas. When we look at his table of the distribution of the genus we notice that, excepting the islands of the Hawaiian and Tahitian regions, nearly all the elevated or mountainous islands of the tropical and subtropical latitudes of the Indian and Pacific oceans have their peculiar species, whether in the case of Mauritius, Rodriguez, Réunion, and the Seychelles in the one ocean, or of Lord Howe Island, New Caledonia, Fiji, and Samoa in the other. The student here hesitates even to raise the question of present plant-dispersal in the face of such evidence of isolation all over the area of the genus. He is almost inclined to evade the issue and to place the matter beside that of the dying or extinct Columbæ that have been found in some of these islands, as in Mauritius, Rodriguez, Réunion, and Samoa.

For reasons above given in the instance of Fiji and Samoa, it would seem futile to attempt to connect in their origin the inland with the coast species; and it may be inferred that, excepting the few dispersed by the currents, the species are in the main inland in their stations. Those peculiar to Fiji, for instance, occur in the swampy forests of the lower regions of the interior, as well as high up towards the mountain summits. When traversing the Fijian forests I often used to speculate on the modes of dispersal of the plants familiar to me; but the sight of a strange Pandanus usually brought my speculations to a close. Many of the enigmas of insular floras would be solved if we could interpret aright the 156 species of Pandanus that are enumerated and described by Dr. Warburg in his monograph. Observers like myself obtain little peeps into the conditions of existence of these interesting plants; and the travelled botanist, who becomes a systematist in his later years, attains to a far more extensive view, yet even he can only penetrate the mystery for a little way.

It is doubtful whether Pandanus odoratissimus, the shore-tree of the tropical beaches of the islands of the Pacific and Indian oceans, of Australia, Malaya, and Southern Asia, can aid us much in any one locality, since its distribution has no doubt been often assisted by man. Yet it is probable that the currents have played a predominant part in its dispersal. Its fruits occur commonly in beach-drift, both in the Indian and Pacific oceans, and are often incrusted with serpulæ, polyzoa, and cirripedes. At certain seasons the currents bring them to Keeling Atoll in abundance. When, however, we come to inquire why it is that this beach species is the only representative of the genus in Hawaii and Tahiti, we are met with the possibility of its having been introduced by the aborigines. The tree is almost as useful to a Polynesian as the coco-nut palm, and it has been cultivated by him in some of the atoll-groups, as in the Marshall and in the Radack archipelagoes. In Chapter VII. good reasons are advanced for regarding it as an aboriginal introduction into Hawaii. When, therefore, we learn that in the group just named it extends from the sea-coast to nearly 2,000 feet above the sea, that in Samoa it may at times be found at a similar elevation though usually restricted to the sea-border, and that in the same way in Tahiti and in Fiji it may leave the coast-region and extend into the heart of the islands, we are not inclined to look for any marked differentiation in its character. This indeed appears to be the case. Numerous varieties in different regions are referred to by Dr. Warburg; but the only important one in the Pacific islands here mentioned is a cultivated form from the Marshall Group. A variety from Hawaii is distinguished chiefly by the smaller size of its drupes.

Assuming, therefore, that the inland species are as a rule not derived from littoral species originally brought by the currents, and that no birds of our own time are in the habit of carrying the drupes of Pandanus to oceanic islands, in order to explain the distribution of such species we have to choose between the possibility of the agency of extinct Columbæ and birds similar in their habits and the alternative of a continental connection. Dr. Warburg, who says but little of the mode of dispersal of Pandanus drupes, regards the genus as having now two centres, one in the East African islands (Madagascar, the Mascarenes, and the Seychelles), and the other in Papuasia (New Guinea, extending doubtless to New Caledonia). My readers will recall to their minds that zoologists have at times felt bound to postulate a continent in both the centres of the genus Pandanus. There is the well-known Lemuria of the Indian Ocean, and then we have in the Western Pacific Forbes’ Antipodea and Hedley’s Melanesian Plateau.

Before, however, we accept the indications of the distribution of Pandanus as favouring a continental hypothesis for either area it is essential to exclude the agency of the extinct Aves. In this connection it is of prime importance to notice that the Mascarene Islands are remarkable, when contrasted with all other oceanic islands, not only for the predominance of peculiar species of Pandanus, but also as having been the home of extinct Columbæ like the dodo and the solitaire. The dodo’s habit of swallowing pebbles of the size of a nutmeg (_Encyclopædia Britannica_, vii., 322), and the solitaire’s inclination for swallowing stones as large as a hen’s egg (_Birds_, by A. H. Evans, p. 331), doubtless represent, as explained below, a capacity for the dispersal of large fruits and seeds that would be regarded as “impossible” for distribution by birds now. It is quite possible that at some time the ancestors of these birds possessed the powers of flight now owned by the Nicobar pigeon, in the gizzard of which, in the Solomon Islands, I found quartz pebbles half an inch across (_Solomon Islands_, p. 324). In the work just quoted I refer on page 325 to the observation of Messrs. Chalmers and Gill that the Goura pigeon of New Guinea usually carries a good-sized pebble in its gizzard. We do not, however, seem to possess any record of extinct Columbæ in the tropical islands of the Western Pacific. The nearly extinct Didunculus of Samoa apparently prefers berries and soft fruits. Dr. Reinecke says that it especially favours the berries of Cananga odorata, the seeds of which are not over a third of an inch (8 mm.) in length.

It would appear from Mr. Hamilton’s note in the _Transactions and Proceedings of the New Zealand Institute_ (vol. 24) that the extinct Struthious birds of New Zealand, as in the case of the moa, carried crop-stones sometimes as large as a pigeon’s egg. These pebbles are, of course, swallowed by birds to enable them to crush the hard seeds, and “stones” of fleshy fruits, on which they feed. In the Solomon Islands I noticed that the Nicobar pigeon was able in this way to crack the seeds of Adenanthera pavonina, which for their fracture require a blow with a hammer. The implication is that the extinct Columbæ were able to transport to oceanic groups seeds and “stones” which no existing pigeon could now carry over a tract of ocean. I am inclined to extend this view also to extinct Struthious birds, and to suppose that they were able, like the cassowary (see page 152), to fly across tracts of sea in ages gone by. Though such an agency would come under discussion in connection with the floras of New Zealand and Madagascar, we have no evidence to show that birds of this family ever reached the tropical islands of the open Pacific.

The Megapodidæ of the Western Pacific are a family of birds that suggest themselves in this connection. Their distribution corresponds with that of Pandanus in the Western Pacific, excepting the littoral species; and like Pandanus the Megapodes have “differentiated” in every group. The limited powers of flight possessed by existing species would unfit them for crossing wide tracts of sea; but the parent form or forms of all these species must have been able to traverse broad tracts of ocean. These birds subsist on fallen fruits, seeds, &c.; but I have no data relating to them as seed-dispersers.

It is evident from the endemic character of most of the species of Pandanus in oceanic islands that, except with a few widely-spread littoral species, the dispersal of the genus has been for ages suspended. Whether the explanation is to be found in the isolation and differentiation of the extinct Columbæ of the Mascarene Islands, where the endemic species of Pandanus are most numerous, has yet to be established. It seems to offer the only way out of the difficulty, unless we accept the old view concerned with the continent of Lemuria.

BARRINGTONIA.

There are two littoral species of this genus in the Pacific, B. speciosa and B. racemosa, both widely spread over the Old World, but only the first is generally distributed over the Polynesian region reaching east to Ducie Island, whilst the second does not extend east of Fiji and Samoa. With the exception of one or two inland species in Fiji and Samoa no inland species have been recorded from the groups of the open Pacific, and the genus is not represented at all in Hawaii. If it were not for a suspicion that the aborigines may have aided in the distribution of the inland species, the advocate of the previous continental connections of the islands of the Western Pacific would receive from their occurrence in these islands considerable support for his views. The fruits of the inland Fijian species are large, the smallest being three inches in length; and the agency of birds seems to be out of the question.

The fruits of the littoral species possess dry buoyant husks that enable them to be carried by the currents over wide tracts of ocean. Those of the Fijian inland species display only a trace of these buoyant coverings and the floating power is much diminished or absent altogether. These inland species are two or three in number. One of them, described as a new species by Seemann under the name of B. edulis, has edible kernels and is sometimes cultivated. A species that I found growing in the plantations of the Solomon Islanders in Bougainville Straits may be near the Fijian tree just named (_Solomon Islands_, pp. 85, 297). Its kernels are edible; and I may add that the Solomon Islanders cultivate other species with edible fruits. We cannot, therefore, exclude the agency of the aborigines in the distribution of the inland species of this genus. Horne found an undescribed species in Fiji, which may be that which I found on the slopes of Mount Seatura in Vanua Levu, as described in Note 50; and it is quite possible that it was originally a cultivated tree, though not necessarily within the memory of the later generations of the aborigines.

This retrocession to the wild state of cultivated plants and the resulting production of apparently new species is a point on which Dr. Beccari lays considerable stress in the English edition of his book on the Great Forests of Borneo. He takes the case of Nephelium and other fruit-trees and shows how in old clearings, long since abandoned, they have undergone singular alteration in characters. For these reasons, therefore, Barringtonia can scarcely be regarded as offering in its inland species unequivocal evidence of a previous continental condition of the islands of the Western Pacific. Nor, as shown in Note 50, should we be justified in establishing a genetic connection between the inland and coast species; but a great deal of research is needed before we can handle the numerous interesting problems connected with the genus; and indeed it cannot be said that the specific limits of the inland Polynesian trees have been definitely determined, or the species themselves diagnosed.

SECTION V.

In this section are included those genera where within the same genus some inland species have been derived from the coast species whilst others have been originally brought by birds. Guettarda alone belongs here. In this genus we find, as is so frequently the case, a littoral tree (G. speciosa) widely spread in the Old World and ranging over the whole tropical Pacific as far east as Pitcairn and Elizabeth islands, but absent from Hawaii. Here also as with Pandanus it is only in the Western Pacific that we find inland endemic species so distinct in character from the littoral tree that they may be regarded as of independent origin.

Since, however, there is an inland form of the coast species in Tahiti (Guettarda speciosa, var. tahitensis) which, according to Drake del Castillo, is distinguished only by its more rounded leaves and by the more marked pubescence of the under leaf-surfaces, we evidently have there an inland species in process of development from the littoral species. This inland tree is found at elevations as great as 600 metres or almost 2,000 feet above the sea; and indeed if we follow Nadeaud the specific differentiation is complete. However, there is no doubt raised as to its close affinity to the beach tree; and we are almost compelled for another reason to regard it as a derivative of the shore species, because, as pointed out in Chapter XXVII., there are very few inland plants in the Tahitian flora possessing fruits as large as those of Guettarda that owe their presence in those islands to frugivorous birds.

Of the two inland species of the genus found in Fiji, G. inconspicua and G. vitiensis, it may at once be said that, as indicated in Dr. Seemann’s work, their characters are far from suggesting any connection in origin with G. speciosa, the shore-species, the inland and littoral plants belonging to different sections of the genus. In their case we can only look to the frugivorous bird for the explanation of their existence in the group. The fruits would be probably small; and in this connection it is to be noted that Mr. H. N. Ridley in his paper on the flora of Fernando Noronha evidently looks to birds to account for the presence of a species of Guettarda on the island, a species not found elsewhere.

But another inland Fijian form of Guettarda found by me in Vanua Levu at elevations of 1,000 to 1,400 feet above the sea, and dubbed by the natives with the name of the littoral tree (Mbua-mbua), corresponds in its close relation to G. speciosa with the inland Tahitian form of that tree, and is to all appearance a derivative of it. It is chiefly distinguished by its thinner, more hairy leaves, which taper at each end and are not subcordate at the base as is often the case with the leaves of G. speciosa. The coverings of the fruit are less fibrous and the putamen is not so deeply notched or grooved. The difference also extends to the buoyancy of the fruits in accordance with the principle laid down in Chapter II. Whilst those of G. speciosa float for many months and are of common occurrence amongst the stranded drift of tropical beaches, as for instance in the Keeling Islands, in the Solomon Group, and in Fiji, those of the inland species float only for a few weeks, their softer coverings decaying more rapidly in sea-water.

We seem therefore to have had two principles at work in Fiji in determining the origin of the inland species of Guettarda. Whilst in one case the inland species is so sharply distinguished from the coast species as to require the independent agency of frugivorous birds to explain its presence, in the other the inland form, as in the instance also of the Tahitian variety, is so much akin to it that the probability of derivation from it is very great.

SECTION VI.

In this section are contained genera possessing littoral species restricted to the Western Pacific islands, and dispersed by birds, but having little or no capacity for dispersal by the currents. They are regarded as derived from the inland species of the genus in the western part of the Pacific, and as distributed from thence over the islands in that part of the ocean. We are here only concerned with Fiji, Tonga, and Samoa and the neighbouring islands. The genera Eugenia, Drymispermum, and Acacia are here comprised.

The genus Eugenia, though essentially inland in its station, is apt to lend species to the beach-flora in different parts of the tropics. Such species, being dispersed by frugivorous birds and other animals, and possessing but slight capacity for distribution by the currents, are usually restricted in their areas. Thus, Schimper (p. 118) names two or three species, including E. javanica, as amongst the Indo-Malayan strand-flora. Ridley notices that E. grandis is a common sea-shore tree in the Malay peninsula; and the author observed two littoral trees of the genus in the islands of Bougainville Straits in the Solomon Group, the fruits of one of them that flourished in the interior of the coral islets being found in the crops of fruit-pigeons. So also in Fiji, some of the inland species, as E. rariflora, appear at times amongst the strand vegetation and in the coral islets. There is, however, one Fijian species found also in Samoa and Tonga that is a characteristic beach tree, namely E. richii (Gray), and it is more or less confined to that station. The fruits will float a fortnight in sea-water, which is nearly twice as long as most other Eugenia fruits will float; and it is quite possible that the currents may assist the pigeons in distributing the species. This genus is dealt with more in detail in Chapter XXVI.

The genus Drymispermum (Thymeleaceæ) comprises in the Western Pacific a number of species, of which two range over the groups of Fiji, Tonga, and Samoa, whilst some four or more are peculiar to Fiji. All are inland plants with the exception of D. Burnettianum, a characteristic littoral shrub of these three groups. Its bright red drupes float only from five to ten days, even after some weeks of drying; and like those of the inland species they are well suited for dispersal by fruit-pigeons. This beach-plant may be regarded as probably an intruder in the strand-flora from the interior of one of the islands of the Western Pacific, whence birds, perhaps assisted a little by currents, have carried it to the neighbouring groups.

The very remarkable coast tree, Acacia laurifolia, alone represents its genus in the littoral flora of the Pacific islands. It is confined to the Western Pacific, having been found in New Caledonia, the New Hebrides, Fiji, Tonga, and Samoa; but it is doubtful whether it is truly indigenous in all these localities. Thus, in Samoa, though restricted to the coast districts, as we learn from Reinecke it seldom flowers, and according to that botanist it was probably introduced through cultivation. It is, however, evidently regarded by the Samoans as a tree of their group, as is shown in a curious legend, given by Dr. George Turner in his latest book on those islands, which I have quoted in my book on the Solomon Islands, p. 287. Both in Fiji and Samoa it bears the name “tatangia” or “tatania,” whilst its hard wood was employed for various purposes, the leaves being used as spoons. The tree flowers and seeds freely on the Fijian beaches. The pods dry up on the plant, and do not dehisce, but are apt to break across between the seeds into article-like portions, the seeds being ultimately liberated by the decay of the pod or its fragments. The seeds either sink at once or in the course of a day or two; whilst the pods or their fragments float at first in sea-water, but all are at the bottom in a week or less. With its absence of any apparent means of dispersal this small tree presents quite an anomaly in the strand-floras of the Western Pacific, and can only be regarded as a loan from the inland flora, though probably of a very ancient date, and perhaps going back like Acacia koa, the forest-tree of Hawaii to some early epoch in the history of these islands.

_The conclusions to be drawn from the discussion of the relations between the littoral and inland species of the same genus in the Pacific islands._ (Chapters XIV., XV., XVI.)

In ten of the twenty-two genera here dealt with (Calophyllum, Hibiscus, Colubrina, Morinda, Scævola, Cordia, Ipomœa, Vitex, Tacca, Casuarina) the shore and inland species have their own independent modes of dispersal, usually by currents in the case of coast plants, and by birds in that of inland plants; and the relations between the two are not such as to suggest a derivation of one from the other.

In six genera the inland species are regarded as derived from the littoral species. In two of them, as in Vigna and Premna, where the coast and inland species occur in the same group of islands and are connected by intermediate forms, there is direct evidence in favour of this conclusion; but such a development of inland species need not have taken place in every group, since in the instance of Premna it has apparently occurred only in the Western Pacific, and the inland and coast species have extended independently to the eastern groups through the agencies of birds and currents.... In the other four genera (Canavalia, Erythrina, Sophora, Ochrosia) we have presented the so-called “Hawaiian difficulty,” that group being alone concerned. Although these genera have no littoral species in Hawaii, they have inland species in those islands, which are in three genera endemic. Since these inland species have non-buoyant seeds or seedvessels, the transport of which by birds half-way across the Pacific Ocean is in the case of the first three genera unlikely and in the last impossible, it is assumed that they are all derived from original coast species with buoyant seeds or fruits, such as are widely distributed over the Pacific but are not now existing in Hawaii. This assumption, in the instance of the Leguminosæ, to which the first three genera belong, derives support from the singular fact in the distribution of the order pointed out by Mr. Hemsley, that it is wanting in many oceanic islands where there is no littoral flora.

In one genus, Guettarda, the inland species are regarded as having been sometimes developed independently of the coast species, and as at other times derived from it, both principles having been at work in Fiji and only the last in Tahiti.

In two genera, Pandanus and Barringtonia, which represent the “Fijian difficulty,” there is no reason on grounds of affinity to connect the inland with the coast species; and since the agency of existing birds is improbable in the first genus and out of the question in the second, whilst the operation of the currents is excluded for the inland species of both genera, it is assumed that we must either appeal to the agency of extinct birds, such as those of the Mascarene Islands, or we must fall back on the hypothesis of a continental connection. In the instance of Barringtonia it is also possible that some of the inland species may have been derived from species spread through cultivation.

Lastly, in three genera (Eugenia, Drymispermum, Acacia) the coast species are viewed as derivatives of the inland flora in the Western Pacific, not necessarily in Fiji, but it may be in New Caledonia or in one of the other large groups. In this case the coast species of all three genera are either unfitted for dispersal by currents, or display the capacity only in a small degree.

We thus see that in only seven of these twenty-two genera, containing both littoral and inland species in the Pacific islands, can it be argued from the standpoint of dispersal that the inland species are or may have been derived from the shore species; and in most instances the evidence is largely presumptive in its character. In three genera the reverse has been the case, and here the coast has borrowed from the inland flora. In twelve, or more than half of the genera, the shore and inland species have been evidently independent in their origin. It is accordingly apparent that in the Pacific the strand flora has lent more to the inland flora than it has borrowed from it; but with a large proportion of these coast genera no interchange has taken place. Two-thirds of the genera of the beach-plants have no inland species, and in their case the question of such a connection cannot be raised. With the remaining genera such a relation can be suggested in only two-fifths of the cases, or in about one-seventh of the total number of beach genera. Where a connection can be traced, it points more frequently to the derivation of the inland from the shore plant. Taking all the evidence together, the beach flora presents itself in the Pacific as practically independent of the inland flora as regards its origin. It has received in these regions but few recruits from inland. It has yielded, except in Hawaii, but few recruits to the inland flora. In this ocean it bears the stamp of a high antiquity, though in the mass no doubt of more recent origin than the mangrove flora.

Yet, as I have remarked in different parts of this work, even with the beach genera possessing no inland species, considerable variety is displayed in the behaviour of the strand species. Thus, whilst some, like Pemphis acidula, Tournefortia argentea, and Triumfetta procumbens, rarely if ever leave the beach, others, like Heritiera littoralis and Excæcaria agallocha, find a home on the borders of the mangrove swamps, and one or two extend inland and take their place in the forests, either as trees (Afzelia bijuga) or as giant climbers (Entada scandens). Others again, like Cassytha filiformis, Cerbera Odollam, and Cycas circinalis, with a number of other beach-plants, may invade the interior of the island wherever arid plains or exposed scantily wooded districts offer conditions conformable to the xerophytic habit of the beach-plants.

It will thus be perceived that although the inland and coast floras of an island are in the mass distinct, the line of separation is by no means always well defined. Beach-plants are something more than salt-lovers in their ways. They are in the first place xerophilous, or, in other words, they will be equally at home in exposed situations away from the coast where the soil is dry and the rainfall scanty. Whenever these conditions are presented by the districts backing the coast, as we find for instance in the plains on the lee or dry sides of many a Pacific island, the shore-plants will often leave the beach and travel far inland.

_Summary of Chapters XIV., XV., XVI._

(1) Though littoral floras are as a rule chiefly made up of two sets of plants, one brought through the agency of the currents from regions outside, and the other derived from the inland flora of the region concerned, the proportion of the two varies much amongst temperate and tropical strand-floras, the current-borne plants forming the majority in the tropics, and those from the inland flora of the region prevailing in the temperate zone.

(2) There is, therefore, far greater uniformity as a rule amongst tropical strand-floras than in the temperate zone, since in temperate latitudes the prevailing constituents of the strand flora vary with the inland flora of every region, whilst in the tropics the predominant plants are those ranging far and wide on the shores of the warm regions of the globe.

(3) Regarding the tropical strand-flora as comprising two formations, that of the beach and that of the mangrove swamp, the last, which is the older of the two, may, it is suggested, be viewed as the remnant of an ancient flora widely spread over the lower levels and coastal regions of the globe, during an age when, in a warm atmosphere charged with watery vapour and heavy with mist and cloud, vivipary or germination on the plant was not the exception but the rule.

(4) But it is contended that even in the beach formation some of the plants may date back to this age of vivipary, as is indicated by the anomalous seed-structures of some of the genera, such as Barringtonia, which seem to indicate a lost viviparous habit.

(5) Since the beach formation of the islands of the tropical Pacific is largely formed of plants ranging over great areas in the tropics, there is no reason to expect that it owes much to recruits from the inland floras of this region. The discussion, therefore, of the relation between the littoral and inland floras is mainly concerned with the possible origin of inland from coast plants in these islands.

(6) Yet there are numerous cases of genera possessing both coast and inland species that are of peculiar interest in determining the true relation between the beach and inland floras.

(7) As the result of a detailed discussion of these genera, the conclusion is formed that the beach and inland floras have been in the main developed on independent lines, the beach flora receiving from the inland flora but few recruits, and except in Hawaii yielding but few plants to the inland flora. Only a third of the genera of the beach flora have also inland species, and in only a few of these genera, or about a seventh of the whole beach flora, can any question of a connection between coast and inland species of the same genus be raised.

(8) Two special difficulties arise in this discussion. The first is the “Hawaiian difficulty,” which is more particularly concerned with genera of the orders Leguminosæ and Apocynaceæ. Here are genera which possess both inland and littoral species, but only the first occur in Hawaii. In the absence of any likely means of dispersal, whether by currents or by birds, it is assumed that the inland species are derived from shore plants, originally brought by the currents, that have since disappeared, a view supported by the fact that Leguminosæ are wanting in oceanic islands where there is no littoral flora. The second is the “Fijian difficulty” which is best represented by Pandanus. From our inability to regard the inland species as derivatives of the coast species, or to supply them with a means of dispersal, we are compelled to regard them either as having been a part of the original continental flora of Fiji or as owing their existence there to the agency of extinct birds having the habits of the Nicobar pigeon and of the extinct Columbæ of the Mascarene Islands. Since the Mascarene Islands are noted not only for their extinct Columbæ but also for their number of peculiar species of Pandanus, the implication seems to lie against the continental view. The subject, however, awaits further investigation. In the Western Pacific the possible agency of the parent forms of the existing species of Megapodidæ is worthy of attention. Like the Columbæ and Pandanus in the Mascarene Islands, the Megapodes and Pandanus have “differentiated” together in the Western Pacific.

(9) The general view of the independent origin of the beach and inland floras of the Pacific islands is supported by the large number of genera in the strand flora that only possess littoral species.

(10) Such shore species, together with other strand plants, sometimes extend into the interior of an island, but only as a rule where the requisite conditions for a plant of xerophilous habit exist.

(11) Shore plants, it is pointed out, are xerophytes first and halophytes afterwards; and under certain conditions the purely xerophilous inclination prevails and the plants travel far inland.