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Show 2!8 ISLAND LIFE. (PART II. unknown means of conveyance; but however this may be, the general character of the land-molluscs is such as to con~rm the conclusions we have arrived at from a study of the birds and insects -that these islands have never been connected with a continent, and have been peopled with living things by such forms only as in some way or other have been able to reach them across many hundred miles of ocean. TIM Flora of the .A.zores.-The flowering-plan~s of the Azores have been studied by one of our first botamsts, Mr. H. . C. Watson, who has himself visited the islands and made extensive collections · and he has given a complete catalogue of the species in Mr. Godman's volume. As our obj~ct in the ~resent work is to tmce the past history of the more Important Islands by means of the forms of life that inhabit them, and as for this purpose plants are sometimes of more value than any clas~ of animals, it will be well to take advantage of the valuable matenals here available, in order to ascertain how far the evidence derived from the two organic kingdoms agrees in character ;. an~ also to obtain some O'eneral results which may be of serviCe m our 0 discussion of more difficult and more complex problems. There are in the Azores 480 known species of flowering-plants and ferns of which no less than 440 are found also in Europe, Madeira, ~r the Canary Islands; while forty are peculiar to the Azores, but are more or less closely allied to European species. As botanists are no less prone than zoologists to invoke former land-connections and continental extensions to account for the wide dispersal of objects of their study, it will be well to examine somewhat closely what these facts really imply. The Dispersal of Seeds.-The seeds of plants are liable ~o be dispersed by a greater variety of agents than any other orgamsms, while their tenacity of life, under varying conditions of heat and cold, drought and moisture, is also exceptionally great. They have also an advantage, in that the great majority of flowering plants have the sexes united in the same individual, so that a single seed in a state fit to germinate may easily stock a whole island. The dispersal of seeds has been studied by Sir Joseph Hooker, Mr. Darwin, and many other writers, who have made it sufficiently clear that they are in many cases liable to be CIJAr. Xll.] THE AZORES. 24!) carried enormous distances. An immense number are specially adapted to be carried by the wind, through the possession of down or hairs, or membranous wings or processes; while others are so minute, and produced in such profusion, that it is difficult to place a limit to the distance they might be carried by gales of wind or hurricanes. Another class of somewhat heavier seeds or dry fruits are capable of being exposed for a long time to sea-water without injury. Mr. Darwin made many experiments on this point, and he found that many seeds, especially of Atriplex, Beta, oats, Capsicum, and the potato, grew after 100 days' immersion, while a large number survived fifty days. But he also found that most of them sink after a few days' immersion, and this would certainly prevent them being floated to very great distances. It is very possible, however, that dried branches or flower-heads containing seeds would float longer, while it is quite certain that many tropical seeds do float for enormous distances, as witness the double cocoa-nuts which cross the Indian ocean from the Seychelle Islands to the coast of Sumatra, and the West Indian beans which frequently reach the west coast of Scotland. There is therefore ample evidence of the possibility of seeds being conveyed across the sea for great distances by winds and surface currents.1 1 Some of Mr. Darwin's experiments are very interesting and suggestive. Ripe hazel-nuts sank immediately, but when dried they floated for ninety days, and afterwards germinated. An asparagus-plant with ripe berries, when dried, floated for eighty-five days, and the seeds afterwards germinated. Out of ninety-four dried plants experimented with, eighteen floated for more than a month, and some for three months, and their powers of germination seem never to have been wholly destroyed. Now, as oceanic currents vary from thirty to sixty miles a day, such ·plants under the most favourable conditions might be carried 90 x 60 = 5,400 miles I But even half of this is ample to enable them to reach any oceanic island, and we must remember that till completely water-logged they might be driven along at a much greater rate by the wind. Mr. Darwin calculates the distance by the average time of flotation to be 924 miles ; but in such a case as this we are entitJed to take the extreme cases, because sueh countless thousands of plants and seeds must be carried out to sea annually that the extreme cases in a single experiment with only ninety-four plants, must happen hundreds or thousands of times and with hundreds or thousands of species, naturally, and thus afford ample opportunities for 5uccessful migration. (See Origin of Specie.s, 6th Edition, p. 325.) |