CHAPTER VIII

THE LITTORAL PLANTS AND THE CURRENTS OF THE PACIFIC

The working value of the currents as plant-dispersers.—The relation between the currents and the distribution of shore-plants.—The clue afforded by the American plants.—Two regions of tropical shore-plants, the American and the Asiatic.—America, the home of the cosmopolitan tropical shore-plants that are dispersed by the currents.—Hawaii and the currents.—Summary.

ACTIVE as the currents are in dispersing seeds and fruits over the Pacific, it should be remembered that those plants that owe their distribution to this agency are only shore-plants, and not, indeed, all the shore-plants, but only those with buoyant seeds or fruits. Even the coral atoll owes a great deal to the agency of the fruit-pigeon and of other birds; for instance, their species of Ficus, Eugenia, and Pisonia. In order, therefore, not to form an exaggerated notion of the efficacy of the currents, it will be necessary to obtain some numerical idea of what they have really accomplished in transporting seeds and seedvessels over the oceans in a state fit for successful germination on the shores upon which they are stranded. It is requisite to make this proviso, because in some cases the currents work to no purpose. Thus, the empty nuts of Aleurites moluccana are carried far and wide over the Indian and Pacific Oceans, and are stranded on the beaches of the various islands, as I have found myself in the cases of Keeling Atoll, Java, and Fiji. The Coco-de-Mer, or the Double Coco-nut Palm, is another apt instance. Though its fruits have been carried far and wide over the Indian Ocean, the species is restricted to the Seychelles. So also the acorns of various species of Quercus are widely but ineffectually distributed by the currents both in temperate and tropical regions. (This subject of useless dispersal is dealt with in Chapter XIII.)

It is essential to bear in mind at the outset that for their inland plants the Pacific islands can draw on the floras of a relatively large portion of the globe. Such plants, having as a rule fruits or seeds that sink in sea-water, or are incapable of floating for long periods, could only have arrived at these islands, where man’s interference is excluded, through the agencies of winds and birds, assisted by other lesser agencies, as those of bats, insects, &c. On the other hand, for their littoral plants, which are for the most part dispersed by the currents, the source of supply is very restricted. The shore-plants with buoyant seeds or fruits of the islands of the tropical Pacific, that are here dealt with, number only about seventy, and it is not likely that this number will be greatly increased, since, whatever may be the deficiencies in our acquaintance with the inland floras of these islands, we have a fairly complete knowledge of the strictly littoral plants.

I do not suppose, indeed, that the number of such plants with seeds or fruits capable of being transported unharmed over wide tracts of sea would much exceed 100 for the whole Indo-Pacific region from India to Tahiti. Professor Schimper gives a list containing 117 tropical plants distributed far and wide over the shores of this region, and made up of species dispersed by currents, birds, and man. Taking a liberal estimate, not over two-thirds of the plants mentioned in this list are dispersed by currents. Then, again, if the flora of a coral atoll, like that of Diego Garcia or of the Keeling Islands, is taken as affording an index of the work of the currents, the number of plants dispersed by the currents would appear to be indeed restricted, since in either case their indigenous flowering plants, including those of both the buoyant and non-buoyant groups, do not exceed fifty.

About twenty years ago, Mr. Hemsley, who, in his work on the botany of the _Challenger_ Expedition, prepared the way for the investigation of this subject, made a list of not less than 120 plants, almost all tropical, that are “certainly or probably dispersed” by the currents (Introd. _Chall._ Bot., p. 42). This is admittedly only a preliminary list, and as the result of recent investigations some plants have to be omitted and others to be added; but I doubt whether, numerically, it is far below the mark. The relative efficacy of the currents seems to have been first systematically discussed by De Candolle in his _Géographie Botanique_, which was published in 1855. Data were then very scanty, and out of a list of nearly 100 inter-tropical species (Old World plants found in the New World and New World plants found in the Old World) he designates nine only as exclusively dispersed by the currents. Even this list, in one respect, needs correction (see Note 33); but it is of interest to note that this eminent botanist from the first never looked upon the agency of the currents as a very important factor in plant-dispersal; and, finding in the specially directed and carefully performed experiments of Thuret confirmation of his views, he reiterated his opinion in a note to that author’s paper in 1873 (cited in Chapter III.).

However, De Candolle was quite right in minimising the effect of currents on the distribution of plants. His extensive survey of the plant-world from the standpoint of dispersal gave him that sense of proportion in assigning values to dispersing agents which enabled him to feel his way almost intuitively, even where exact data were often lacking. It is, however, a little disappointing to find such a slight treatment of the subject in Kerner’s great work on the Natural History of Plants, though one can scarcely controvert his opinion that the dispersion of plants, as a whole, is not appreciably affected by this process. Numerically speaking, this is in the main correct; yet it is here that the genius of Schimper led him to recognise and to mark out a line of investigation, fruitful in important results, in connection with the weighty question of “Adaptation.” If the author of this work has been able to add a little to our acquaintance with this subject, he owes much to the inspiration he received from Schimper’s memoir on the Indo-Malayan Strand-Flora.

Still, it must be admitted that the effectual operations of the currents as plant-dispersers are limited to the shore-plants with buoyant seeds or fruits. If we were to include in our list the shore-plants of temperate regions that possess seeds or fruits capable of floating in sea-water for long periods, and of afterwards germinating, the total for the whole world would not, I imagine, reach 200. We cannot here concern ourselves with those purely river-side plants that contribute their buoyant seeds and seed-vessels to river-drift, since there is no evidence indicating that river-side plants are effectively dispersed by the currents unless they also frequent the estuary and the coast-swamp; and in that case they come under the head of littoral plants. The total for the whole British flora would probably not far exceed a dozen, and nearly all of them are very widely dispersed.

The working value of the currents as plant-dispersers in the Pacific can be rudely estimated by the number of littoral plants with buoyant seeds or fruits that occur in the various groups. Most of these plants hail from the Indo-Malayan region. Speaking generally of the extension eastward of the Indo-Malayan strand-plants over the Pacific, Prof. Schimper (page 195) remarks that they become fewer and fewer in number as they extend farther from their original home, their number shrinking to a very few in the most remote groups of the Marquesas and the Hawaiian Islands. This is well illustrated in the following numerical results that I have prepared. Of the whole number, some seventy in all, of the littoral plants of the tropical Pacific with buoyant seeds or fruits, Fiji possesses about sixty-five, Tahiti about forty, and Hawaii only about sixteen. As shown, however, in Chapter VII., some of the Hawaiian littoral trees that are useful to the aborigines were probably introduced by them. The number actually introduced through the currents into Hawaii in all likelihood therefore does not exceed ten. There is a method in this diminution in numbers, as the plants migrate eastward and northward over the Pacific, which has been described in detail in the preceding chapter. The efficacy of the currents as plant-dispersers in the tropical Pacific therefore diminishes as we proceed eastward.

In the South Pacific the littoral plants preserve their Old World origin as far as the Polynesian archipelagoes extend eastward across to Pitcairn, Elizabeth, and Ducie Islands, where we find in one or other of them such characteristic Indo-Malayan beach trees as Barringtonia speciosa, Cerbera Odollam, Guettarda speciosa, Hernandia peltata, and Tournefortia argentea (see Note 34). In the more distant Easter Island there is a suspicion, for the first time, of immigration from South America in the presence of Sophora tetraptera. In the islands relatively close to the American continent, as in Juan Fernandez and in the Galapagos group, the Indo-Malayan strand-plants are no longer represented.

We come now to consider the relation between the distribution of the shore-plants and the currents. It is quite legitimate to discuss the currents of the Pacific from the botanist’s point of view, that is to say, from the standpoint of the distribution of littoral plants with buoyant seeds or fruits. For ages the buoyant seeds and fruits of the strand-plants of the tropical Pacific have been drifting over that ocean, and we have the results now before us in the dispersal of the species to which they belong. There is no necessity to endeavour to make the distribution of such littoral plants square with the arrangement of the currents as shown in a chart. The usual result of such a comparison has been to lead the investigator, whether an anthropologist, a zoologist, or a botanist, to find his facts at variance with the course of the prevailing currents. Man, animals, and plants have entered the Pacific from the west, whilst the most available currents are from the east; and one may be perhaps permitted the solecism that the Pacific islands have apparently been stocked with their shore-plants, with their aborigines, and with much of their fauna by currents running in the wrong direction. These Pacific islands could only have had a direct communication with the Old World, from which they have mainly derived their shore-plants, by the currents; but since both the aborigines and the plants have forced their way across the ocean to the Tahitian region in the teeth of the regular currents, indicated as such in the chart, we are compelled to assume that they have availed themselves either of the Equatorial Counter-Current or of the occasional easterly drift currents that mark the prevalence of westerly winds during the short season of the year when the easterly trade-winds do not prevail.

The Equatorial Counter-Current hypothesis would involve a preliminary crossing of the whole breadth of the Pacific Ocean, that is to say, a voyage of some 8,000 miles, before the drifting seed doubled back to the Polynesian Islands. The other view is a much more probable one, as is sufficiently indicated by the following extract from the “Admiralty Sailing Directions for the Pacific Islands” (II., p. 25, 1900).... “In the western part of the Pacific these trades ... are frequently interrupted by winds which blow from west or north-west, especially during the months of January, February, and March, when the north-west monsoon of the Indian Ocean extends out in the Pacific as far as the Samoa Islands.” In various works on this region one may find reference to canoes blown off the shore during this season and carried some hundreds of miles to the eastward. A ship can then sometimes sail with a fair wind from the southern end of the Solomon Group to the Fijis; and as we learn from Mariner, the crocodile may be at such times carried away from the Solomon Islands and stranded in Fiji. Mr. Hedley, in his exceedingly interesting paper on a zoogeographic scheme for the mid-Pacific (_Proc. Linn. Soc. N.S.W._, 1899), gives many details of this nature; but there is no space to deal further with the matter here.

After all, the botanist must take his cue from the drifting seed and the distribution of the plant. He finds the seed floating in the open sea as well as stranded on the beach. He then discovers the plant growing on the beaches, and by experiment he tests the floating capacity of the fruit or seed. Finally he ascertains the home of the plant. He does this for all the littoral plants with buoyant seeds or fruits, and he forms his own conclusions of the efficacy of the currents independently of the current-chart, remembering that he has in Time an important factor that the geographer does not possess in dealing with the currents. The effect of time has often been to obscure the differential results of the operations of the currents in the case of those species that, like Barringtonia speciosa, are almost universally distributed in the islands of the Pacific. It is obvious that such plants cannot aid us much in the matter of ascertaining the track followed by the drifting seed in entering this ocean. But if we find a littoral plant with buoyant seed or fruit that has only partially performed the traverse we shall possess in the interrupted operation an important piece of evidence.

Several years ago, in my paper on Polynesian plant-names, read before the Victoria Institute, I developed this argument when endeavouring to find in the floating seed a clue to the route pursued by the Polynesians in entering the Pacific. Since that time my acquaintance with these islands and their plants has been considerably extended; but no important modification of the principal argument is now needed. It was then pointed out that in Nipa fruticans, the swamp-palm of the Malayan Islands and of tropical south-eastern Asia, we have a plant well fitted for the purpose and one well known to be dispersed by the currents over small tracts of ocean. The Nipa Palm has attempted to enter Polynesia from the Malayan region by two routes, namely, by Melanesia and by Micronesia. Along the first route it has in the course of ages reached the Solomon Islands, where I found it in 1884. Along the second route it has extended its range to Ualan at the eastern end of the Caroline Group, where it was observed by Kittlitz many years ago, as indicated in the narrative of his voyage (_Reise nach russische America, nach Mikronesien_, etc., 1858, ii. 35), and in Dr. Seemann’s English edition of the same author’s _Vierundzwanzig Vegetationsansichten ... des stillen Oceans_.

The question now arises as to which of these two routes was taken by the drifting seed. In my paper I adopted the view that the shore plants reached Fiji and Samoa by Micronesia, that is to say, by the Caroline, Marshall, and Gilbert Groups. This is the route which, as mentioned by Mr. Hedley in the paper above quoted, Mr. Woodford prefers for some of the Lepidoptera; and it is the one that is favoured by Mr. Wiglesworth for the birds, since in his memoir entitled _Aves Polynesiæ_ he remarks that certain indications tend to show that the Pelew Islands have served as a sort of bridge for the spread of species from Indo-Austro-Malaya right across the Pacific. Though I still think that the beach trees, most of which would find a home on the numerous coral atolls of the Marshall, Gilbert, and Ellice Groups, often followed that track, yet I am now inclined to consider that the mangroves and their associates, plants which find their most suitable home in the estuaries of large elevated islands, like those of the Solomon Group, in all probability reached Fiji in the mass by the Melanesian route.

Although the Old World has supplied to the Pacific islands most of their littoral plants that are dispersed by the currents, that is to say, the plants with buoyant seeds or seed vessels, yet there is an appreciable American element, and it is with the plants occurring in the New World that we are now concerned. The total number of the littoral plants of these islands that possess buoyant seeds or fruits is, according to the lists given under Note 35, about seventy. Of these about forty-five are exclusively Old World species, sixteen occur in both the Old and New Worlds, three are exclusively American, and six are Polynesian.

The question we have now to ask ourselves is whether the shore plants common to both the Old World and America have their homes in America, or whether they have been derived from the other hemisphere. With one or two exceptions, as in the cases of the Australian genera Dodonæa, Scævola, and Cassytha, which, as shown in a later page in this chapter, present no great difficulty, there does not seem to be any serious objection, as far as the numerical distribution of the species is concerned, in regarding America as a possible home of the genus. It is not often we shall come upon such a striking instance of the principle that where the species are most numerous there is the home of the genus, as in the instance of Cocos. The Coco-nut palm has been carried around the world through the agencies of man and the currents, whilst the home of the genus is in America.

Now assuming that in having to choose between the Old World and the New World as the home of most of the genera in the list we selected the latter, we have to ask ourselves in what degree this would be consistent with the place America holds with regard to the distribution of tropical shore-plants dispersed by the currents and with reference to the arrangement of the currents. If we except the African continent, there is no part of the world that bears such a definite relation to the currents as America, and with an ordinary chart of these regions their arrangement is to be understood at a glance. Yet strange to say, as far as the distribution of tropical littoral plants is concerned, America holds a position that the present system of the currents on its coasts will not altogether explain. Within the lifetime of the species of mangroves and other plants of the coast swamps that are found on both the Pacific and Atlantic coasts of tropical America the two continents of this name have been united by the emergence of the Isthmus of Panama.

Few things are more significant in plant-distribution than the arrangement of the tropical littoral plants with buoyant seeds or fruits, a subject that is discussed with some detail by Professor Schimper in his work on the Indo-Malayan strand-flora (page 190). These plants group themselves into four sections:—

(_a_) Those of the Pacific and Atlantic coasts of tropical America (including the West Indies) and of the West Coast of Africa. They include mostly plants of the mangrove-swamps and their vicinity, such as Anona paludosa, Avicennia tomentosa, A. nitida, Conocarpus erectus, Laguncularia racemosa, Rhizophora mangle, etc.

(_b_) Those of the Old World excluding the African West Coast and extending from the East Coast of Africa eastward to the Pacific islands. This is much the largest group and comprises many of the plants named in the list given in Note 35 under Old World species. One may cite as examples of plants ranging almost all over this area, Barringtonia speciosa, B. racemosa, Bruguiera gymnorhiza (in its most comprehensive sense), Carapa moluccensis, Derris uliginosa, Guettarda speciosa, Hernandia peltata, Heritiera littoralis, Pemphis acidula, Rhizophora mucronata, etc. Plants of the mangrove-swamp and of the beach are, therefore, here included.

(_c_) Those occurring all around the tropics and including many of the plants mentioned under Note 35 as Pacific island shore-plants found also in America. Most of them belong to the Leguminosæ, and there may here be mentioned Canavalia obtusifolia, Cæsalpinia Bonducella, Entada scandens, Gyrocarpus jacquini, Ipomœa pes capræ, Sophora tomentosa, and Vigna lutea.

(_d_) Those confined to a portion of the two great regions, such as Nipa fruticans in the Old World, and the Manchineel (Hippomane mancinella) to tropical America.

It is to be noted that the ubiquitous species do not include any of the mangroves. Each of the two regions has its own species, none being common to both the American and Asiatic regions, although, as is shown in Chapter XXX., the American species of Rhizophora is now seemingly breaking its bounds and intruding into the Pacific islands. On the other hand, some of the mangrove genera, Avicennia, Carapa, and Rhizophora, are found all round the globe, whilst others are restricted to one or other of the two regions, Bruguiera, Lumnitzera, and Sonneratia, for instance, to the Old World region, and Laguncularia to the American and West African region.

For convenience we may designate the two great regions of tropical strand-plants, with buoyant seeds or fruits, the American and the Asiatic regions, remembering that the first includes both coasts of America as well as the African West Coast, whilst the second extends from the East Coast of Africa to Polynesia. Excluding the ubiquitous species, these two regions are well distinguished from each other. If we look at the chart of the currents we perceive the reason of the American region including the West African Coast, and we see why none of the indigenous plants of this region occur on the African East Coast. So also with the Asiatic region, a glance at the chart will show that all the portions of its area are in connection with each other directly or indirectly through the currents, and that only time is required for the transport of buoyant seeds over most of the region.

Hitherto I have mainly followed Professor Schimper in this matter; but since my visit to Ecuador and the Panama Isthmus some further considerations have presented themselves to me. If the reader will look again at the map of the currents, he will observe that there is little reason for supposing that the Asiatic region can lend its littoral plants to the American region. On the other hand there are greater facilities, as far as currents are concerned, for America supplying the Asiatic region, namely by means of the great equatorial currents that course westward across the Pacific to the tropics of the Old World.

It would therefore seem that the American region can receive nothing by the currents from the Asiatic region. If accordingly it gives but gets nothing back, we are compelled to assign an origin in the American region to all littoral plants dispersed by the currents that are found in the tropics around the globe. This is what we have already regarded on other grounds as possible for nearly all the littoral plants of the tropical Pacific with buoyant seeds or seedvessels that are found in America. These plants are practically the same as those distributed around the tropical zone which are enumerated in the list given under Note 35, _b_. With their home in America, by crossing the Pacific they would ultimately arrive at the East African coast, where their course westward would terminate; whilst commencing their journey from the east side of the American continent they would reach the West African coast; and their distribution around the tropics of the world would be explained. There follow from these considerations the corollaries that a tropical strand-plant dispersed by the currents which has its birthplace in Asia could never reach the American region, and that American strand-plants are for the most part native-born, excepting those, if there are any, that hail originally from the African West Coast.

It is necessary in passing to explain the similarity of shore plants on the Pacific and Atlantic coasts of Tropical America. For the mangroves and their accompanying plants inter-communication between the two coasts is now impossible; and a communication between the two oceans must be postulated within the lives of the existing species. For the plants like Entada scandens and Ipomœa pes capræ, which occur inland as well as at the coast, it is easy to show that in the case of the Panama Isthmus, their seeds could be readily carried into the Atlantic and Pacific Oceans by rivers draining the opposite slopes of the same “divide,” so that the dispersal of the same species from a common centre into two oceans may be seen in operation in our own day. My observations on this subject are given in Chapter XXXII., to which the reader is referred.

I have now gone far enough to indicate the place that America holds with regard to the distribution of tropical shore-plants dispersed by the currents and with regard to the currents. There is every probability, as I venture to think I have shown, that the Pacific islands have derived most of their ubiquitous shore-plants with buoyant seeds or fruits from America. But one of the results of our discussion of America in this double aspect was that excepting in the case of the African West Coast it gives but does not receive plants from the Old World. We apply this test, with perhaps a little hesitation, to the shore-plants of the Pacific islands that are dispersed by the currents; and we find, as will be seen below, that it is responded to in a remarkable manner.

It has been observed in the previous chapter that scarcely any of the large-fruited beach-plants of the South Pacific islands, that could only have been dispersed by the currents, have reached Hawaii. We do not find amongst the truly indigenous coast flora of this group any of the following trees: Barringtonia speciosa, Calophyllum Inophyllum, Cerbera Odollam, Guettarda speciosa, Hernandia peltata, Ochrosia parviflora, Pongamia glabra, Terminalia Katappa, Terminalia littoralis, &c. It was also noted that the currents had not only failed to establish these plants in Hawaii, but that they had also failed to establish them in America, the suggestion being that the Hawaiian Islands had been, in part at least, stocked by the currents from America. That the Indo-Malayan strand-plants in their extension eastward over the Pacific should have failed to reach America, is a result we might have expected from the arrangement of the currents. Yet mingled with them we have plants like Ipomœa pes capræ, Canavalia obtusifolia, and Sophora tomentosa, that also occur in America. Since, however, their seeds are not better adapted for accomplishing the passage across the Pacific from the Old World to America than the equally buoyant fruits of the above-named littoral trees that have failed, the presumption arises that their home is in America, and that they have performed the easier passage across the Pacific westward from America to the Old World.

The exclusion of so many characteristic shore-trees from America that range often over the whole tropical region from the African East Coast to the islands of the Central Pacific, is not a matter of seed or fruit-buoyancy, but a matter concerned with the home of the species, and with the arrangement of the currents. Those shore-plants of this region that occur also in America have their home in that continent, and have subsequently been carried across the Pacific by the currents westward to the Asiatic shores.

The only exceptions, that I can recall, to the rule that America does not receive shore-plants dispersed by the currents from the Old World, are presented by the three Australian genera, Dodonæa, Scævola, and Cassytha, of which widely spread littoral species occur in America, namely, Scævola Lobelia, Dodonæa viscosa, and Cassytha filiformis. They offer, however, but little difficulty, since, as pointed out in other parts of this work, Dodonæa viscosa has probably been in part dispersed by man, whilst the other two species are as well fitted for dispersal by birds as by currents. The occurrence therefore of these species in America does not necessarily raise the question of the currents.

The same exclusive principle is illustrated in the scanty littoral flora of Hawaii. Deprived, like America, of the characteristic large-fruited beach-trees of the South Pacific, species that could only have reached it through the agency of the currents, it is scarcely to be expected that it would have received its few littoral plants with buoyant seeds from the source which has failed it in the cases of the numerous absentees. It is to America therefore that we look for the source of its littoral plants as far as the agency of the currents is concerned.

The Hawaiian Islands contain about twelve plants, named in the list given in Note 36, that possess seeds or fruits known to be dispersed by the currents, and capable, as experiments indicate, of floating in sea-water for prolonged periods. Not all of them are at present littoral in their station in this group; but their claim to be considered such in other regions is established in the Note above mentioned. Of these plants, seven at least are found in America, five in the Old World also, and two exclusively in America. This proportion of American plants is far greater than that characterising the whole littoral flora of the Pacific islands dispersed by currents, where out of some seventy species only nineteen are found in America (see Note 35). As far as the distribution of the plants is concerned, it is therefore quite possible that Hawaii has received most of its plants that are dispersed by the currents from tropical America.

We will now consider how such a possibility is in accordance with the arrangement of the currents in the North Pacific. If we look at the Quarterly Current Charts for this ocean published by the British Admiralty we notice that all through the year the Hawaiian Group lies more or less within the area of currents flowing from the West Coast of America, the Northern Equatorial Currents as they are collectively named. Except in the winter months these currents come from the N.E. and E.N.E., and bring drift from the coasts of British Columbia, Oregon, and Northern California. It is then that they pile up huge pine logs on the shores of the Hawaiian Islands, as I have described in Chapter VII. and in Note 30; and, according to Dr. Hillebrand, they transport this drift timber much farther south to the shores of the Marshall and Caroline Groups. One might cite other facts illustrative of the working of these currents, such as one finds in the pages of Fornander and other authors; but this would scarcely come within the province of this work. I may here remark that when in Honolulu I was informed that a bell-buoy which had got adrift on the Californian coast was subsequently washed up on the coasts of Kauai. It is stated in Findlay’s “North Pacific Directory” (1886, p. 1068), that a junk carrying nine hands that had been blown off the south coast of Japan in a typhoon, anchored, after ten or eleven months at sea, in December, 1832, near Waialea in Oahu, the view taken of its course being that after drifting along in the Japan Current it came within the range of the south-west current that carries pine timber to Hawaii from the West Coast of America.

The portion of the Northern Equatorial Current that strikes the Hawaiian Group during the greater part of the year is no doubt a south-westerly deflection of the Japan Current from the American West Coast; and it would be impossible to find any tropical drift mingled with the pine logs stranded on the islands during that period. However, in the winter months, centering in January, the Japan Current flows down the West Coast of America to about the latitude of Cape Corrientes on the coast of Mexico, before being deflected westward. Here it meets with a portion of the Peruvian Current, and both flow westward, the united stream striking probably only the southernmost islands of the Hawaiian Group. It is at this season alone that there would be any likelihood of drift from tropical America being stranded on the Hawaiian beaches, and it is quite possible that at such a time the Northern Equatorial Current may carry intermingled in its stream pine logs from Oregon and seed-drift from Panama.

I am not inclined to attach any value except in the Western Pacific to the agency of the Equatorial Counter-Current in transporting seeds and fruits over the Pacific. It presents seemingly the only opportunity of the transportal of the seeds and fruits of Asiatic littoral plants to America; but if at all effective in this way, it would have endowed the littoral flora of the western shores of tropical America with many of the trees so characteristic of the coral islands of the Pacific. In this sense, it has failed completely as an effective agency in plant-dispersal; and judging by results we may, I think, dismiss it from our consideration. However, Dr. Hillebrand (p. xv.) assumes that during the prevalence of south-westerly gales in winter in the Hawaiian Islands, the Equatorial Counter-Current would be pushed northward so as to mingle to the east of the group with the North Equatorial Current. In this manner it is supposed that seed-drift brought direct from the Asiatic side of the Pacific would be stranded on these islands. This appears to me to be most improbable, since some ten or twelve degrees of latitude usually intervene between the Hawaiian Group and the Equatorial Counter-Current (_see_ Admiralty Sailing Directions, Pacific Islands, 1900, II., 31, and the Quarterly Current Charts; also Encyclopædia Britannica, vol. 18, p. 118).

The most serious objection from the botanist’s standpoint against such a view as that of Dr. Hillebrand is the absence from Hawaii of most of the shore-plants that we should expect the currents to have brought from the Old World. It is also evident that as far as the currents are concerned the Hawaiian Islands are far more likely to receive littoral plants from America than from the Old World. Though no tropical drift has yet been found stranded on the coasts of these islands, yet it is not unlikely that future investigators may find some seed-drift from Central America on the most southerly coasts of the group, as on the south-east shores of the large island of Hawaii. It would only be stranded in the winter months and then probably in small quantities.

_Summary of the Chapter._

(_a_) Since the effective operations of the currents are limited to the shore-plants with buoyant seeds or fruits, such plants forming but a small proportion of any flora, it must be acknowledged that, numerically speaking, the results of the dispersing-agency of the currents on plant-distribution in general are but slight.

(_b_) Yet the importance of the subject is by no means to be measured by a numerical scale of results, a line of inquiry being here opened up leading to fields of investigation full of promise for the student of plant-distribution.

(_c_) Whilst dealing with the relation between the distribution of shore-plants and the arrangement of the currents, it is quite legitimate to discuss the currents of the Pacific from the point of view of the botanist, who, after all, must take his cue from the drifting seed and the resulting distribution of the plant.

(_d_) The shore-plants of the Pacific islands that are dispersed by the currents being mainly Indo-Malayan in origin, it follows that they have extended eastward over the Pacific to the Tahitian islands against the stream of the South Equatorial Current and against the trade-wind. It is, however, shown that they could have availed themselves of the interval between January and March when the North-west Monsoon reaches the Pacific.

(_e_) It is claimed that whilst the mangroves and their associated plants have for the most part entered the Pacific by the Melanesian route through the Solomon Islands, the beach-plants have also followed the route through Micronesia by the Caroline, Marshall, and Ellice Groups.

(_f_) A small number of the strand-plants of the Pacific islands that are dispersed by currents occur in America as well as in the Old World; and questions of prime importance arise when we have to decide whether their home is in the Old World or in the New World.

(_g_) Good reasons are given for regarding them as chiefly of American origin; and it is shown that America with regard to the arrangement of the currents stands in the singular relation of being a disperser but not a recipient of shore-plants.

(_h_) It is pointed out that the tropical shore-plants that are distributed by currents belong to two great regions which are the effect of the present arrangement of the currents, viz., the American including the West Coast of Africa, and the Asiatic comprising the remainder of the tropical zone. Each region has its own plants, and those that occur in both, being in fact distributed all round the tropics, are regarded, according to the principle above stated, as having their home in the American region.

(_i_) The occurrence of the same strand species on the Pacific and Atlantic coasts of tropical America is regarded as indicating that the arrangement of the existing species of its shore-plants, more particularly of the mangroves, antedates the emergence of the Panama Isthmus. This hypothesis is not needed for the coast plants like Entada scandens that occur inland, since we can now observe their seeds being carried down into the Atlantic and Pacific Oceans by rivers draining the opposite slopes of the same “divide” in the Panama Isthmus.

(_j_) It is shown that the currents of the Pacific have failed to establish the numerous beach-trees (possessing buoyant fruits) of the Pacific islands, not only in the Hawaiian Group, but also on the coast of America; and it is therefore argued that we should expect the Hawaiian Group to have received through the currents its shore-plants with buoyant seeds or fruits from the tropical west coasts of America.

(_k_) In support of this contention it is pointed out that most of the Hawaiian strand-plants that are dispersed by the currents are found in America, and some indeed in America to the exclusion of the Old World.

(_l_) The arrangement of the currents in the North Pacific also favours the view that the Hawaiian Islands are more likely to receive plants by the agency of the currents from America than from the Asiatic side of the Pacific.