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.
CHAPTER II
THE FLORAS OF THE PACIFIC ISLANDS FROM THE STANDPOINT
OF DISPERSAL BY CURRENTS
The initial experiment.—The proportion of littoral plants.—The two great principles of buoyancy.—The investigations of Professor Schimper.—The investigations of the author.—The great sorting process of the ages.—Preliminary results of the inquiry into the buoyancy of seeds and fruits.
In the previous introductory chapter some of the numerous questions affecting insular floras were briefly referred to. I will now ask my reader, if he has had the patience to read it, to consign that chapter for the time at least into oblivion, and to proceed with me to our Pacific island with the intention of investigating its flora from the standpoint of dispersal. We will together take up the subject de novo, after banishing from our minds all preconceptions that we may have possessed.
After having been over the island gathering specimens of all the seeds and fruits, we return to our abode on the beach. But we are puzzled where to begin. The problem presents itself as a tangled skein, and our difficulty is to find an “end” that we can follow along with some chances of success. In our trouble we look around us; and at that moment we see a number of floating seeds and fruits carried by the current past the beach. This presents us with a clue and our investigation begins.