Since this essay was put in type Dr Ernst's striking account of the "New Flora of the Volcanic Island of Krakatau" (Cambridge, 1909.) has reached me. All botanists must feel a debt of gratitude to Prof. Seward for his admirable translation of a memoir which in its original form is practically unprocurable and to the liberality of the Cambridge University Press for its publication. In the preceding pages I have traced the laborious research by which the methods of Plant Dispersal were established by Darwin. In the island of Krakatau nature has supplied a crucial experiment which, if it had occurred earlier, would have at once secured conviction of their efficiency. A quarter of a century ago every trace of organic life in the island was "destroyed and buried under a thick covering of glowing stones." Now, it is "again covered with a mantle of green, the growth being in places so luxuriant that it is necessary to cut one's way laboriously through the vegetation." (Op. cit. page 4.) Ernst traces minutely how this has been brought about by the combined action of wind, birds and sea currents, as means of transport. The process will continue, and he concludes:--"At last after a long interval the vegetation on the desolated island will again acquire that wealth of variety and luxuriance which we see in the fullest development which Nature has reached in the primaeval forest in the tropics." (Op. cit. page 72.) The possibility of such a result revealed itself to the insight of Darwin with little encouragement or support from contemporary opinion.
One of the most remarkable facts established by Ernst is that this has not been accomplished by the transport of seeds alone. "Tree stems and branches played an important part in the colonisation of Krakatau by plants and animals. Large piles of floating trees, stems, branches and bamboos are met with everywhere on the beach above high-water mark and often carried a considerable distance inland. Some of the animals on the island, such as the fat Iguana (Varanus salvator) which suns itself in the beds of streams, may have travelled on floating wood, possibly also the ancestors of the numerous ants, but certainly plants." (Op. cit. page 56.) Darwin actually had a prevision of this. Writing to Hooker he says:--"Would it not be a prodigy if an unstocked island did not in the course of ages receive colonists from coasts whence the currents flow, trees are drifted and birds are driven by gales?" ("More Letters", I. page 483.) And ten years earlier:--"I must believe in the...whole plant or branch being washed into the sea; with floods and slips and earthquakes; this must continually be happening." ("Life and Letters", II. pages 56, 57.) If we give to "continually" a cosmic measure, can the fact be doubted? All this, in the light of our present knowledge, is too obvious to us to admit of discussion. But it seems to me nothing less than pathetic to see how in the teeth of the obsession as to continental extension, Darwin fought single-handed for what we now know to be the truth.
Guppy's heart failed him when he had to deal with the isolated case of Agathis which alone seemed inexplicable by known means of transport. But when we remember that it is a relic of the pre-Angiospermous flora, and is of Araucarian ancestry, it cannot be said that the impossibility, in so prolonged a history, of the bodily transference of cone-bearing branches or even of trees, compels us as a last resort to fall back on continental extension to account for its existing distribution.
When Darwin was in the Galapagos Archipelago, he tells us that he fancied himself "brought near to the very act of creation." He saw how new species might arise from a common stock. Krakatau shows us an earlier stage and how by simple agencies, continually at work, that stock might be supplied.
It also shows us how the mixed and casual elements of a new colony enter into competition for the ground and become mutually adjusted. The study of Plant Distribution from a Darwinian standpoint has opened up a new field of research in Ecology. The means of transport supply the materials for a flora, but their ultimate fate depends on their equipment for the "struggle for existence." The whole subject can no longer be regarded as a mere statistical inquiry which has seemed doubtless to many of somewhat arid interest. The fate of every element of the earth's vegetation has sooner or later depended on its ability to travel and to hold its own under new conditions. And the means by which it has secured success is an each case a biological problem which demands and will reward the most attentive study. This is the lesson which Darwin has bequeathed to us. It is summed up in the concluding paragraph of the "Origin" ("Origin of Species" (6th edition), page 429.):--"It is interesting to contemplate a tangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent upon each other in so complex a manner, have all been produced by laws acting around us."