CHAPTER I

INTRODUCTION

The study of insular floras.—Their investigation in this work from the standpoint of dispersal.—The significance of plant-distribution in the Pacific.—The problems connected with the mountain-flora of Hawaii.—The persistence of dispersing agencies at the coast, their partial suspension on the mountain-top, their more or less complete suspension in the forest, and the effect on the endemic character of plants.—The connection between the endemism of birds and plants.—The relative antiquity of plants of the coast, forest, and mountain-top.—The genetic relation between coast and inland species of the same genus.—The ethics of plant-dispersal.—Evolution takes no heed of modes of dispersal.—The seed-stage is the price of Adaptation.

TO proceed from the general to the special is the only method of dealing with insular floras. A broad and comprehensive grasp of plant-distribution, such as is only acquired by a life-time of research aided by travel and the handling of large collections, is a necessary foundation for the study; yet in the nature of things such qualifications can be possessed by but a few. To direct an inquiry in the opposite direction, and endeavour to attack the problem of continental floras through the insular floras would result merely in the investigation of a few of the many questions connected with plant-distribution.

The panoramic sketch of the surveyor on the mountain-top aids him in a thousand ways when after months of tedious labour he plots the details in his chart. Without such a panoramic view of the plant-world in his mind’s eye, an observer like myself can only look for guidance to the writings of those who have generalised on the foundations of a far broader experience, such as those of Bentham, De Candolle, Gray, Hooker, Schimper, and others.

It would be quite possible for a botanist possessing a profound general acquaintance with the plant-world to dispense altogether with actual observation and experiment on modes of dispersal. It would be quite possible for him to arrive at conclusions, which, even if they did not always come into line with results of observation and experiment, we should be compelled to prefer. It is only from his more elevated position that a general can follow the course of a battle; whilst the private with his experience confined to a limited area of the field of conflict may form the most erroneous ideas of the progress of the fight. So it is with observers whose employment it is to struggle with the details and secondary principles of plant-distribution, and so it is with the generaliser who has already roughly mapped out the principal features of the main problem.

When Mr. Bentham in 1869, remarking on the paucity of species common to tropical Asia and America, characterised them either as plants wholly or partially maritime and spread by the currents, or as weeds dispersed by cultivation over the warm regions of the globe, he mentioned amongst the plants in the former category, Gyrocarpus jacquini. This tree presents one of the mysteries connected with the Pacific islands; and I don’t imagine that this eminent botanist could have known anything except inferentially as regards the mode of dispersal of its fruits. Yet experiment shows how well founded the inference was, whilst behind it lay a life-time of botanical research.

The author thus approaches the subject of the floras of the Pacific islands rather as a plotter of detail than as a delineator of great designs. However much we may study the means of dispersal, we have behind them the great facts of distribution, serving like the main stations of a trigonometrical survey, and with these we have to make our lesser facts and observations square. One is conscious all the time that much of what seems new in one’s researches has already been foreseen by the generaliser, and that one can do little else than assist in confirming some of his results. This is all that I can lay claim to in this work.

The floras of the islands and coasts of the tropical Pacific are here regarded entirely from the standpoint of plant-dispersal. The fruits and seeds rather than the flowers have been the subject of my investigations; and although there is much to please the eye in the flora of a Pacific island, it was always with a sense of disappointment that I turned away from some pretty flowering plant that failed to present me with its seed. Amongst the wonders of the plant-world rank the Tree Lobelias of the Hawaiian Islands; yet their greatest charm to me lay not so much in their giant-flowers and their arborescent habit as in the mystery surrounding the home of their birth and their mode of arrival in these islands. When I first stood under the shade of the lofty Dammara vitiensis, the Kauri Pine of Fiji, all my interest lay in its cones lying on the ground; and I remember how eagerly I handled my first specimen, and how anxiously I watched its behaviour when experimenting on its capacity for different modes of transport. When a strange plant presented itself on a beach, my first care was to ascertain the fitness of its fruits or seeds for transport by the currents; and all inland plants with fruits likely to attract frugivorous birds were at once invested with a special interest for me.

The mangrove swamps were always great places of interest, and months of my sojourn in the Pacific must have been passed in exploring their creeks and in examining their vegetation. Botanists usually avoid these regions; but the observation of the germination of the Rhizophora fruits on the trees and the inquiries connected with their methods of distribution over the oceans were pursuits so engrossing that I ignored the numerous discomforts connected with the exploration of these gloomy regions. The magnificent mangrove forests of the Ecuador coast of the Pacific will live longest in my memory, though the risks were considerably greater and the discomfort of existence extreme. But the mangrove swamps present us with glimpses into the conditions of plant life during the warmer epochs of the earth’s history, when perhaps the seed-stage was largely dispensed with, whilst an atmosphere, laden with moisture and screening off much of the sun’s light, enveloped most of the circumference of the globe.

The plant world viewed only from the standpoint of dispersal may lack much that is pleasing to the eye, though it abounds with small and great problems fascinating to the reason. Matters of great moment are here involved, and in the case of the Pacific islands they concern not only the source of the oceanic floras, but the story of the islands themselves; whilst behind these there rise up questions of yet deeper import, questions that are bound up with the beginnings of genera and species, and with other mysteries of life on the earth. The distribution of plants presents something more than a problem of means of dispersal, or a problem of station, or a problem of plant migration connected with climatic changes. It is something a great deal more than all three, since it is indissolubly connected with a past, of which unfortunately we know very little. Let us take it to be a question of means of dispersal, and then in imagination transporting ourselves to the Scandinavian coast, let us gather up the stranded West Indian beans of Cæsalpinia, Mucuna, and Entada, that have been drifted there for ages by the Gulf Stream, and lie in some cases semifossilised in the adjacent peat-bog. Was ever dispersal so utterly purposeless as this? Yet here lies a principle of plant-dispersal that is fundamental. We see it in the thistle-seed floating seaward in the wind. Nature never intended its pappus for such an end. It was formed for quite another purpose, yet it aids largely the dispersion of the plant. What can be more significant than that?

Or let us take it to be a matter of station. Given time and the recurrence of the same conditions, with others I once imagined that we could explain most things in plant-distribution, whether of plants at the coast or of plants inland, whether of plants of the alpine peaks or of plants of the plains, or of plants of the river or of the pond. Time, it was held, had long since discounted the means of dispersal, and distribution became merely an affair of station. But the supplanting of many indigenous species of a flora by introduced species is a common story in the plant-world; and such a view needs no further discussion here. Nor is distribution only concerned with plant-migration. Any theory of the origin of alpine floras on tropical mountains will have to explain the presence of the temperate genera, Geranium and Sanicula, not alone on the summits of the mountains of Equatorial Africa and Madagascar, but on the uplands of Hawaii in mid-Pacific, where also are found Ranunculus, Vaccinium, Fragaria chilensis (the Chilian strawberry), and Drosera longifolia.

Taking genera of different stations each in their turn, and following up the clues thus afforded, it would be possible to find support for all the reputable views relating to plant-distribution. The wide range of aquatic plants under conditions that completely change the character of the terrestrial vegetation, such, for instance, as Myriophyllum and Ceratophyllum, might be plausibly attributed to the relative uniformity of the conditions of aquatic life both in time as well as space. The occurrence of Vaccinium on mountain-tops over most of the world, even on the highlands of Samoa, Tahiti, and Hawaii in the Pacific Ocean, would be rightly regarded as evidence of active dispersal of the seeds through the agency of birds from one mountain-summit to another, whether in mid-ocean or in the centre of a continent. The prevalence of the same beach-plants over most of the globe in the same climatic zones would point unmistakably to the predominant agency of currents. But with many plant-genera, some of which range the world, whilst others again may be restricted to a single group of islands in the Pacific, there is often no question either of means of dispersal, or of station, or of plant-migration, and problems of a very different nature are opened up.

When we leave the beach and the mountain-top, the river and the pond, all the troubles of distribution begin; and since but a small proportion of plants in a typical flora belong to these stations, it follows that difficulties will dog our steps with the large majority of the plants. The agencies of dispersal now working around us, the current, the wind, the insect, the bird, and the bat, will explain many of the features of littoral and alpine floras and of the vegetation of ponds and rivers. Here we have in so many cases wide-ranging genera with the means of dispersal ready to hand. We can connect the wide range of Vaccinium with the wide range of birds of the grouse and other families that feed on the berries. We can associate the great areas of aquatic or sub-aquatic genera, like Potamogeton and Sparganium, with the migratory habits of the ducks in the stomachs of which we find their seeds. We can connect the great ranges of beach plants like Ipomœa pes capræ in the tropics, and Convolvulus soldanella in the temperate regions with the currents, and the almost cosmopolitan range of many ferns and lycopods with the winds and other agencies.

When, however, we enter the forests we find genera that are often much more restricted in their areas, and species that are yet more limited in their range. There is very little dispersal going on here. The birds are strange. Their distribution is usually very local. They look lazily down at us from the branches, as they disgorge the seeds and stones of the fruits they have eaten, which cover the ground around. We can almost fancy that they say:—“Our work is done. We rest from the toil of our ancestors. They carried seeds to far-distant Hawaii, Tahiti, and Savaii. Our work is done.” And as we walk through those noiseless forests, where the machinery of species-making is ever in silent motion, we become aware that we are treading one of Nature’s great workshops for the manufacture of species and genera. Outside the forest all is bustle and hurry. We are in the streets, or rather in the distributing areas of the plant-world. We hear the noise of the breaker, the roar of the gale, the cry of the sea-gull, the flapping of a myriad pairs of wings of some migrating host overhead, and we know that the current, the wind, and the bird are actively at work; but their operations are confined mainly to the beach, the mountain-top, the river, and the pond.

Let us take a well-wooded Pacific island several thousand feet in height. We find on its beaches the same littoral plants that we have seen before on the tropical shores of Malaya, of Asia, of Africa, and of America. We find in its ponds and rivers the same species of water-plants, such as Ceratophyllum demersum, Ruppia maritima, and Naias marina, that are familiar to us in the cool and tepid waters of much of the globe. On its level summit, if it remains within the clouds we find in the boggy ground, where Sphagnum thrives, genera that are represented in Fuegia, New Zealand, and the Antarctic islands, such as Acæna, Lagenophora, and Astelia, and the world-ranging Drosera longifolia. In other elevated localities we find Ranunculus, Geranium, Sanicula, Artemisia, Vaccinium, and Plantago, chiefly genera of the temperate regions of the northern hemisphere; whilst there are also found Gunnera, Nertera, and Uncinia, all hailing from the south and belonging to the Antarctic flora characterising all the land-area around the globe in the latitude of New Zealand and Fuegia. The Hawaiian species of Nertera and of Uncinia occur also in New Zealand, and the first-named is found also in Tristan da Cunha and in South America. In the Hawaiian uplands there is also to be seen Deyeuxia, a genus of grasses found in the Tibetan highlands and in the Bolivian Andes at elevations of 16,000 to 19,000 feet; and the same species that exists in Australia may be found in the mountains of Hawaii. Here also, both in Hawaii and Tahiti, occurs Luzula campestris.

In making the foregoing remarks on the alpine plants of a Pacific island, I have had Hawaii in my mind, but we find the elements of a similar widely-distributed mountain-flora in the less lofty peaks of Tahiti and Samoa, and traces even in Fiji, where the mountains, however, have only a moderate elevation. But the point I wish to lay stress on is the cosmopolitan yet temperate character of the mountain-flora of an island lying in the midst of the tropical Pacific. As he shifts his station on this mountain-summit, the observer might at different times imagine himself in the Sierra Nevada of California, on a Mexican tableland, on a peak of the Andes, or in the lowlands of Fuegia. Other plants that I have not mentioned, such as Coprosma, would bring back to him New Zealand. He might even be on a mountain-top in Central Africa, or on a Madagascar plateau; whilst in the boggy region of an elevated Hawaiian tableland he would meet with not only the physical conditions, but also several of the plants found on the higher levels of Tristan da Cunha.

It is, however, to be noted that although these mountain-tops in the mid-Pacific have been stocked with genera from the four quarters of the compass, the species as a rule are restricted to that particular archipelago. Whilst the beach and the river in most cases possess plants that have very wide ranges over the earth, a good proportion of the species on the mountain-summit are not found elsewhere. This implies a partial suspension of the means of dispersal on the mountain-top, whilst the currents and waterfowl are still actively distributing the seeds of the littoral tree and of the aquatic plant. We here get a foreshadowing of another great principle, or of another line along which Nature has worked in stocking these islands of the Pacific with their plants, a subject concerning which much will be said in later pages.

Hitherto, we have dealt only with a small proportion of the flora, and with but a small portion of the area of the island. We have yet to deal with the intermediate region between the sea-border and the summit of the island, or, in other words, with the forested mountain slopes. This is the home of many of the peculiar species and peculiar genera, both of plants and birds; and it is with this zone that we shall be mainly concerned when we come to contrast the floras of the several archipelagoes of the tropical Pacific. Here the agencies of dispersal have, to a large extent, ceased to act; and the question will arise as to the connection between the endemic character of the plants and the endemic character of the birds. We shall have to ask why this island, after receiving so many plants, ceased to be centres of dispersal to other regions. It is possible that these seeds or fruits have lost their capacity for dispersal; but only a few instances of this change present themselves. Rather it may be supposed that the birds that originally brought the seeds to the island came to stay; and this at once suggests another query as to the cause of the change of habit. I am alluding here not to the plants with minute seeds, such as Sagina and Orchis, which Mr. Wallace, in his _Darwinism_, regards as capable of being transported by strong winds over a thousand miles of sea; but to those numerous plants found in the Fijian, Tahitian, and Hawaiian forests, where the seeds and “stones” are large and heavy, measuring often as much as a quarter of an inch (6 mm.), and sometimes nearly an inch (25 mm.) in size. The reader will be surprised to learn how little “size” has determined the distribution of seeds and fruits in the Pacific. He will have to appeal to the habits of pebble-swallowing of the Dodo, the Solitaire, the Goura pigeon, the Nicobar pigeon, &c., if he desires to find a parallel in the habits of birds.

It is here assumed that the reader is already acquainted with the principles involved in a discussion of island-floras, principles clearly laid down in the writings of Hooker, Wallace, Hemsley, and others. As a general rule in an island or in a group of islands where there are a large number of plants not found elsewhere, there is also a large endemic element in the avifauna, and where none of the plants are peculiar, endemic birds are either few or wanting. As an example of the first we may mention Hawaii, and Iceland affords an instance of the second. But there is no hard and fast rule connecting the endemic character of the plants and birds of an island with its distance from other regions. Even the small group of Fernando Noronha, lying only some 200 miles off the coast of Brazil, possesses its peculiar birds and its peculiar plants; and we can there witness the singular spectacle, as described by Mr. Ridley, of an endemic bird, a frugivorous dove, engaged in scattering the seeds of endemic plants over the little group. This is the only fruit-eating bird in the islands, remarks the same botanist in the _Journal of the Linnean Society_ (vol. 27, 1891); and “when one sees the number of endemic species with edible fruits, one is tempted to wonder if it were possible that they were all introduced by this single species of dove, or whether other frugivorous birds may not at times have wandered to these shores.” This inter-island dispersal in a particular group of peculiar plants by peculiar birds is a common spectacle in the Pacific. The contrast between the large number of plant-genera possessing fruits that would be dispersed by frugivorous birds and the poverty of fruit-eating birds in the avifauna is well displayed in Hawaii.

The island of St. Helena would seem to offer an exception to the rule that endemic birds and endemic plants go together, since, though its flora possesses a very large endemic element, there are scarcely any endemic or even indigenous birds recorded from the island. We can never know, however, how much of the original fauna disappeared with the destruction of the forests. It would nevertheless appear that but few of the genera possessing peculiar species of plants were adapted for dispersal by frugivorous birds. The lesson to be learned from this island concerns the Compositæ, often arboreous, that constitute the principal feature of its flora. St. Helena retains almost more than any other island evidence of the age of Compositæ which has left its impress on many insular floras; and when we discuss the original modes of dispersal of the endemic Hawaiian genera of the same order we shall look to the flora of this Atlantic island for assistance in the matter. To the age of Compositæ belong the beginnings of several insular floras.

To return to the main line of our argument, it would seem that in a Pacific island there is a constant relation between free means of dispersal and the preservation of specific characters. The ocean-current and the aquatic bird are in our own time actively engaged in dispersing the seeds of shore-plants and water-plants, and we see the same species ranging over the world. On the other hand on the mountain-top the agencies of dispersal are beginning to fail, and as a result many a mountain has some of its species restricted to its higher regions. In the forest zone there has been a more or less complete suspension of the activity of the dispersing agencies, and new genera are formed whilst peculiar species abound. Free means of communication with other regions restrains but does not arrest the differentiating process that is ever in progress throughout the organic world. Isolation within certain limits gives it play.

It is in this connection interesting to reflect that during the differentiation of the inland flora the littoral plants have lagged behind or have remained relatively unchanged. The currents have been working without a break throughout the ages; and the cosmopolitan Ipomœa, that now creeps over the sand of the beach, or the wide-ranging Rhizophora, that forms the mangroves of the coast-swamp, must have witnessed the arrival of the ancestors of several of the endemic inland genera. The swamp-plants of the littoral flora are probably older, however, than the beach-plants which have been recruited from time to time in one region or another of the tropics from the inland flora. Yet as a body the littoral plants have lagged far behind the inland flora. We might thus expect that in a Pacific island, excluding the wind-distributed plants, such as the ferns and the lycopods, the most ancient types of the plants would be found at the coast, the most modern in the forests, whilst the plants of the mountain-summit would represent an intermediate age.

But true as this may be, the composition of a strand-flora is a very complex one. Although, as Prof. Schimper remarks, the mangrove formation is more isolated than the beach formation, and affords evidence of a much earlier separation, the beach-plants as a body are anything but homogeneous in their character. Their physiognomy varies to some extent with the alteration in the characters of the inland flora, changes to which the mangrove formation makes a very slow response. Yet amongst the plants of the beach we find strangely assorted forms that are as ancient denizens of the coast as the mangroves themselves. Take, for instance, Salsola Kali, that thrives alike on a beach in Chile, on the sea-shore in Devonshire, and in the salt-marshes of the interior of Tibet. Then, again, there is a type of littoral plant, of which Armeria vulgaris and Plantago maritima may be taken as examples, which is equally at home on the beach and on the tops of inland mountains. We might in a sense apply the wrecker’s motto,

“What the sea sends and the land lends,”

to the history of a littoral flora. Yet on the other hand the inland flora in its turn receives a few recruits from the littoral flora; and it is the relation between the inland and coast species of the same genus that offers one of the most fascinating studies in the botany of the Pacific Islands.

This introductory chapter may be concluded with a few remarks on what may be termed “the ethics of plant-dispersal.” Not that this is in any way a suitable phrase, but it best expresses my sense of the lack of propriety in some things connected with this subject. It is odd, for instance, that we speak of the dispersal of plants and animals in the same breath, as if the process was in both cases identical. Seeing that from this point of view we judge a plant only by its seeds and fruits, it is apparent that we are following quite a different method than that which we employ in the study of the dispersal of animals. Whilst the zoologist classifies the units of dispersal, the botanist does nothing of the kind; and the two systems of classification are at the outset fundamentally distinct. The student of plant-dispersal thus often finds himself placed in an awkward dilemma. For him a family is a collection of allied genera having similar seeds or fruits and fitted often for the same mode of dispersal. A family like Sterculiaceæ, possessing such a variety of seeds and fruits suitable for very different modes of dispersal, is from his standpoint a collection of dissimilar units. Genera like Commersonia, Waltheria, Kleinhovia, Sterculia, and Heritiera, that he so often meets with in the Pacific Islands, have in these respects frequently very little in common; and yet one of the earliest determining influences in plant-life must have lain in the capacity for dispersal.

Yet chance seems to reign in the processes of plant-dispersal ever going on around us. In the floating seed, in the achene with its light pappus blown before the gale, in the prickly mericarp entangled in the plumage of a bird, in the “stone” of the drupe disgorged or ejected by the pigeon, in the small grain that becomes adhesive in the rain, in the tiny rush-seed enclosed in the dried pond-mud on the legs of some migratory bird, in all these we see the agencies of dispersal making use of qualities and of structures that were developed in quite another connection and for quite another purpose. That such characters have been so to speak appropriated by these agencies is a pure accident in a plant’s life-history. If the evolutionary force had been in operation here, it would have selected some common ground to work on. There would have been some uniformity in its methods, whereas the modes of dispersal are infinite. The qualities and characters that happen to be connected with dispersal belong to a plant’s development in a particular environment. They can never have been adapted to another set of conditions that lie quite outside that environment. There is a relation of a kind between the specific weight of wood and the density of water, and this, in a sense, sums up the connection between a seed and its distributing agencies.

Evolution has never concerned itself directly with means of dispersal. Evolution and Adaptation represent the dual forces that rule the organic world, the first an intruding force, the last a passive power representing the laws governing the inorganic world. To these laws the intruding power has often been compelled to bend, and it has had to pay its price, and sometimes it has succumbed, and sometimes it has turned its defeat into a victory. Nature, so watchful over the young plant, as represented by the seed, is finally compelled to let it go, and dispersal begins where evolution ends, or rather when the evolutionary power fails. The seed-stage itself is the price of adaptation. The death of the individual may also be regarded from the same standpoint. It represents a defeat of the evolutionary force, which, however, has been retrieved by the gift of reproductive power.