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Show 78 ISLAND LIFE. (I'AllT I. for migration have been pointed out by eminent botanists, and a considerable space might be occupied in giving a summary of what has been written on the subject. In the present work, however, it is only in two or three chapters that I discuss the origin of insula.r floras in any detail; and it will therefore be advisable to adduce any special facts when they are required to support the argument in particular cases. A few general remarks only will therefore be made here. Special adaptability of Seeds for dispersal.-Plants possess many great advantages over animals as regards the power of dispersal, since they are all propagated by seeds or spores, which are hardier than the eggs of even insects, and retain their vitality for a much longer time. Seeds may lie dormant for many years and then vegetate, while they endure extremes of heat, of cold, of drought, or of moisture which would almost always be fatal to animal germs. Among the causes of the dispersal .of seeds De Candolle enumerates the wind, rivers, ocean currents, icebergs, birds and other animals, and human agency. Great numbers of seeds are specially adapted for transport by one or other of these agencies. Many are very light, and have winged appendages, pappus, or down, which enable them to be carried enormous distances. It is true, as De Candolle remarks, that we have no actual proofs of their being so carried; but this is not surprising when we consider how small and inconspicuous most seeds are. Supposing every year a million seeds were brought by the win~ to the British Isles from the Continent, this would be only ten to a square mile, and the observation of a life-time might never detect one; yet a hundredth pa~t of this number would serve in a few centuries to stock an island like Britain with a great variety of continental plants. When, however, we consider the enormous quantity of seeds produced by plants,-that great numbers of these are more or less adapted to be carried by the wind,-and that winds of great violence and long duration occur in most parts of the world, ,,.e are as sure that seeds must be carried to great distances as if we had seen them so carried. Such storms carry leaves, hay, dust, and many small objects to a great height in the air, w bile OilAP. V.] DISPEUSA L OF ANIMALS AND PLANTS. 70 many insects have been conveyed by them for hundreds of miles out to sea and far beyond what their unaided powers of flight could have effected. Birds as agents in the dispersal of Plants.-Birds are undoubtedly import~nt agents in the dispersal of plants over wide spaces. of ocean, either by swallowing fruits and rejecting the seeds In a state fit for germination, or by the seeds becoming attached .to t~e plumage of ground-nesting birds, or to the feet of aquat.w buds embedded in small quantities of mud or earth. !llustrat10ns of these various modes of transport will be found m Chapter XII. when discussing the origin of the flora of the Azores and Bermuda. Ocean-currents as agents in Plant-dispersal.-Ocean-currents are undoubtedly more important agents in conveying seeds of plants t~an they are in the case of any other organisms, and a consider.able body of facts and experiments have been collected provmg that seeds may sometimes be carried in this way .many thousand miles and afterwards germinate. Mr. Da:wm made a series of interesting experiments on this subJect, some of which will be given in the chapter above referred to. Dispersal along mountain chains.-These various modes of transport ar~, as will be shown when discussing special cases, ~mply su:ffi~Ient to account for the vegetation found on oceanic Islands, whwh almost always bears a close relation to that of the nearest con~inent ; but there are other phenomena presented. by the dispersal of species and genera of plants over very. w1de areas, especially when they occur in widely separated portions of the northern and southern hemispheres that are not _ea~il~ explained by such causes alone. It is here that transmissiOn along n:ount~in chains has probably been effective; and the exact mode m whwh this has occurred is discussed in Chap.ter XXIII., w~ere a considerable body of facts is given, sho';mg that extensive migrations may be effected by a successiOn of moderat~ steps, owing to the frequent exposure of frf£e sh. surfaces. of sml or debris on mountain sides and summi' ts , o en~g statiOns on which foreign plants can tern oraril estabhsh themselves. p y |