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Show 2!)(3 ISLAND LIFE. [PArt T II paper on the Compositre, 1 Mr. Bentham gives us some valuable remarks on the affinities of the seven endemic species belonging to the genera Commidendron, Melanodendron, Petrobium, ami Pisiadia, which form so important a portion of the existing flora of St. Helena. He says: ''Although nearer to Africa than to any other continent., those composite denizens which bear evidence of the greatest antiquity have their affinities for the most part in South America, while 'the colonists of a more recent character are South African." ... "Comml.J.endron and :Melanodendron are among the woody Asteroid forms exemplified in the Andine Diplostephium, and in the Australian Olearia. Petrobimn is one of three genera, remains of a group probably of great antiquity, of which the two others are Pouanthus in Chile and Astemma in tho Andes. The Pisiadia is an endemic species of a genus otherwise Mascarene or of Eastern Af1ica, presentir1g a geographical connection analogous to that of the St. Helena Melhanim,2 with the Mascarene Trochetia." Whenever such remote and singular cases of geographical affinity as the above are pointed out, the fir.st impression is to imagine some mode by which a communication between the distant countries implicated might be effected; anJ. this way of viewing the problem is almost universally adopted, even by naturalists. But if the principles laid down in this work and in my GeogTaphical Distribution of Animals are sounll, such a course is very unphilosophical. For, on the theory of evolution, nothing can be more certain than that groups now broken up and detached were once continuous, and that fragmentary groups and isolated forms are but the relics of once widespread types, which have been preserved in a few localities where the physical conditions were especially favourable, or where organic competition was less severe. The true explanation of all such remote geographical affinities is, that they date back to a time when the ancestral group of which they are the common descendants had a wider or a different distribution· ' 1 "Notes on the Classification, History, and Geogm phical Distribution of Compositre."-Jou1·nal of the Linnean Society, Vol. XIII. p. 563 (1873). 2 The M.elhanire comprise the two finest timber trees of St. Helena, now almost e'X:tmct, the redwood and native ebony. CHAP. XIV.] ST. HELENA. 297 and they no more imply any closer connection bvtween the distant countries the allied forms now inhabit, than does tho existence of living Equidro in South Africa and extinct Eq uidre in the Pliocene deposits of the Pampas, imply a continent bridging the South Atlantic to allow of their easy communicatio.n. Concluding Rema1·lcs on St. Ilelena.-The sketch we have now given of the chief members of the indigenous fauna and flora of St. Helena shows, that by means of the knowledge we have obtained of past changes in the physical history of the earth, and of the various modes by which organisms are conveyed across the ocean, all the more important facts become readily intelligible. vV e have here an island of small size and great antiquity, very distant from every other land, and probably at no time very much less distant from surrounding continents, which became stocked by chance immigrants from other countries at some remote eroch, and which has preserved many of their more or less modified descendants to the present time. When first visited by civilised man it was in all probability far more richly stocked with plants and animals, forming a kind of natural museum or vivarium in which ancient types, perhaps dating back to the Miocene period, or even earlier, had been saved from the destruction which has overtaken their allies on the great continents. Unfortunately many, we do not know how many, of these forms have been exterminated by the carelessness and improvidence of its civilised but ignorant rulers; and it is only by the extreme ruggedness and inaccessibility of its peaks and crater-ridges that the scanty fragments have escaped by which alone we are able to obtain a glimpse of this interesting chapter in the life-history of our earth. |
