H. B. Guppy

“ 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. ”

“ Then come a flood of old memories and old trains of thought that came to us years before on some mountain-top or in a shady gorge or on some river-bank, in regions Pacific and non-Pacific, and by degrees our ideas shape themselves and we begin to think the matter over in an orderly fashion. ”

“ 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. ”

“ Observers at the coast often little imagine, when looking at a cloud-concealed mountain peak, that although the cloud-belt from below looks black and lowering and rain is falling heavily in the gloomy forests, there is on the upper side a region of bright sunshine, and that the peak stands out, unseen by them, above a sea of clouds sparkling brilliantly in the sun and dazzling in their whiteness. It will be seen from the table given in Note 61, that during my sojourn on the summit of Mauna Loa the sky was cloudless or almost free from cloud during nearly half the time. ”

“ 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. ”

“ 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. ”

“ If we look over the Pacific islands in search of a critical locality for the investigation of the genetic relation between the littoral and coast species of Erythrina, we discover it, as far as I can gather, only in one group. In Fiji, Tonga, and Samoa we find only the littoral species; in Hawaii there is only an inland species; whilst in Tahiti occur both the littoral and the inland species—E. indica, the wide-ranging shore-tree of the South Pacific, and E. monosperma, the inland tree of Hawaii—the last found nowhere else in Polynesia, and confined to the Pacific. ”

“ The mangrove-formation comprises, it may be pointed out, many plants other than mangroves, plants that find a home in the mangrove-swamps of Fiji, either within their limits or at their borders. It presents, indeed, a world in itself. When the mangroves establish themselves in a new locality they carry along with them a host of hangers-on, both plants and animals, that only find a home under the favourable conditions of a mangrove-swamp. Thus, the absence of the mangrove-formation from a Pacific island deprives its littoral flora of many very striking features. ”

“ 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. ”

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

“ But if we look a little more closely at the sunken and floating seeds, we find that in the same genus there are species with seeds or seed-vessels that sink and species with those that float. We look again and then perceive that the same general principle is true of different species of the same genus growing inland and at the coast. We learn now that as a rule when a genus possesses both littoral and inland species, the seeds or fruits of the former float in sea-water for a long time, whilst those of the latter have little or no floating power. ”

“ The problem of man in the Pacific bristles with difficulties differing in degree but not in kind from those relating to the flora. Whenever a particular theory seems on the point of being well established, some disturbing question arises, and as with the plant, we are never able to push our facts quite home. Since I first visited the Solomon Islands, now twenty-four years ago, the Pacific islander and his flora have deeply interested me. The history of man and of the plant cannot be separated in the Pacific; and the same determining principles of distribution have affected both. ”

“ De Candolle many years ago, in his Géographie Botanique, gave a list of nearly 100 plants, made up of Old World species naturalised in America and of American species naturalised in the Old World, and quite half of them were classed as plants distributed in one way or another through man’s agency. Now this is either a subject of supreme importance or it is of no interest to the student of man’s history. If it should prove that birds have done most of this dispersal, then the story of the aboriginal weed would be of little interest in connection with the races of man in the New World. ”

“ Buoyancy of seed or fruit is only one of many other qualities that is concerned with distribution by currents. Nature does not act in this way in seed-distribution, and there can be little doubt that the author of the Origin of Species would have been the first to abandon this view, if his researches had been continued. It should be especially noted that plants of the sea-beach, where the floating power happens to be nil, or limited only to a week or two, would have derived great advantage from the drying of their seeds or fruits if it was really effective in aiding dispersal by currents. ”

“ By following up the path of inquiry that is concerned with the flotation of seeds and seedvessels, we are guided into other fields of research that give promise of interesting discoveries in connection with plant-life. We are led in the first place to consider the question of utility, and to ask whether the buoyancy of the seed or fruit has been a matter of moment in the history of the species. Nature is ever engaged in telling off the plants to their various stations. ”

“ Whatever view we adopt, we must apply the same view to all, whether it be a question of dispersal by birds or by currents. We cannot choose between two sets of principles determining the buoyancy of seeds and fruits any more than we can regard a fleshy drupe and a buoyant seed as illustrating different principles regulating the dispersal of plants. Nature works with uniformity in these matters, and if the Natural Selection theory is held to explain one case it ought to account for all. ”

“ There is for me no more pregnant fact in plant-life than the thistle-seed blown before the wind, or the seed of our sea-convolvulus floating in the sea. It proves to my mind that the evolutionary power in nature is checked and 120hampered by conditions not of its own creation, and that two opposing forces are ever at work, the one creating and the other limiting the creative power, the actual mode of dispersal being but a blind and accidental result of the struggle. ”

“ The isolation of the oceanic archipelagoes may not explain the endemic character of the flora, but only the extreme degree to which the endemism is carried. When a genus is in its prime, it can defy all the limiting conditions imposed by similarity of station and by free and unchecked means of dispersal, the essential marks of a species or a genus having probably in their development little or no connection with environment. ”

“ When from a canoe on a Fijian river we lift up the germinating fruit of Barringtonia racemosa from amongst the drift floating past in the stream and pull down from the branches overhead the seedling a foot in length of Rhizophora, we hold in our hands the two extremes of the series of vivipary. With many of the plants of the mangrove-formation there is a fine adjustment with respect to the germinating capacity of the seed, or in other words a delicate balancing of organisation on one side and of physical conditions on the other. ”

“ The botanist, again, finds a climber like Strongylodon in the forests of Fiji, Tahiti, and Hawaii, and he picks up the seeds on the beaches of those islands and notices that they float unharmed for many months in the sea, yet when he pays heed to the distribution of the genus he finds that it only comprises four or five species, and that it occurs outside the Pacific only in the Philippines, Ceylon, and Madagascar. ”

“ Each smaller area breaks up again, and forms, yet more specialised, are produced; and thus the process of subdivision of range and of differentiation of form goes along until each island in an archipelago owns its bird and each hill and valley has its separate plants. This is not the path that Evolution takes, since beyond lies extinction whether of plant or of bird. ”

“ On the one hand we have the tranquil working through the ages of the winds and currents. On the other hand there has been the revolutionary influence of the bird. One cannot doubt that many of the species of flowering plants now growing on the beach and many of the ferns on the upper mountain-slopes have witnessed changes within the forest-zone of the island, such as an antediluvian might record if he had lived through the ages to the present time. ”

“ 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. ”

“ In Vanua Levu I formed the opinion that it is only an intruder amongst the littoral vegetation. In accounting for its distribution we have to choose between man, the bird, and the current. Though it may sometimes be noticed in native plantations, as I observed in the Solomon Islands, the tree has no special use; and the Solomon Island natives themselves indicated to me that the parrots that fed on the fruits of the tree aided in distributing the plant. The buoyant behaviour of the seeds, which are freed by the dehiscence of the bladder-capsules on the tree, is not constant. ”

“ Yet, although we might term the sinking of a seed or fruit an accidental attribute of certain plants, just as we might regard the floating of a log as an accidental attribute of a pine, since in either case the specific weight might have been acquired without any direct relation to the density of water, still the sinking of the seed or fruit signifies a profound distinction not only, as is stated below, in plant distribution, but, as we shall see later on, in plant-development. ”

“ We arrive now at the close of the era of the general dispersal of tropical plants, mainly Malayan, over the Pacific, and this brings us down to our own age. The few genera that are still dispersed have no peculiar species in particular groups. The species which often range over all the groups, and retain as a rule their characters in most of them, do not therefore display, except in a few cases, that extreme variation which would give them a place in the ranks of the polymorphous species. ”

“ The tendency in the course of ages has been towards the differentiation of climate, bird, and plant, the range of the bird being largely controlled by the climate, and the range of the plant being mainly dependent on the range of the bird.
It is evident that in some cases the plants themselves may make the endemism of a flora more pronounced by creating their own difficulties and by standing in the way of their own dispersal to outside regions. ”

“ 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. ”

“ Nature is represented as working blindly, and the result of such “fortuitous variation” is termed an adaptation. We cannot, however, pick and choose only adaptations that are very evident in their character. We must include everything in the organic world as an adaptation, whether apparent or not, that is in direct relation with the organism’s conditions of existence. It is not conceivable that an organism can be adapted to conditions outside its environment, and yet many so-called adaptations are of this character. ”

“ It is unnecessary to dwell longer here on the subject of the Hawaiian absentee-genera, since many of the absent plants will be discussed when dealing with the peculiarities of the Fijian flora. The data there given all go to show that mere lack of capacity for dispersal over the Pacific often counts for little in supplying us with an explanation of the absence of so many likely genera from the Hawaiian flora. Hawaii has only been stocked with those genera common to Fiji and Tahiti that could have reached it during each age of general dispersal over the Pacific. ”

“ We are, however, compelled to appeal to the bird, since, as my experiments in Fiji indicate, the genus has little or no capacity for dispersal by currents, the “stone” when containing a seed always sinking, whilst the entire fruit either sinks at once or floats heavily for a few days.
The degree of fleshiness of the drupes of Elæocarpus varies in different species, being sometimes slight and at other times pronounced, but, speaking generally, they would be expected to attract frugivorous birds. The colour of the fruits of some species is dark and purplish, whilst in others it is a bright blue. ”

“ We learn also from Hochstetter and from Sir W. Buller that the drupes of the “Hinau” (Elæocarpus) form a favourite food of the parrots and fruit-pigeons of New Zealand (Hochstetter’s New Zealand; Buller’s Birds of New Zealand) .
The question of size acquires considerable importance when we come to consider the transport of the seeds of the genus to a group of islands lying, like Hawaii, in the middle of the Pacific Ocean. The protection of the seed is also another important matter. There can, however, be no doubt that the hard woody or often osseous “stone” sufficiently protects the seed. ”

“ The principles of evolution and adaptation rule the world except in matters of dispersal. Take, for instance, the fleshy fruits which the gardener often makes more attractive to birds than they are in the wild condition. The result is certainly to increase their facilities for dispersal by birds; but such a result was as little intended by man as it was by Nature when species of Cornus, Ficus, Prunus, Viburnum, and other genera matured their drupes, berries, and fleshy fruits in the Cretaceous epoch. ”