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Show 372 DARWINIS.l\1 CHAP. or perish, and n.ll which come within sight of a~ island ~ll struggle to reach it as their only r~fuge. But, with ~ounta.m summits the case is altogether different, because, bemg . nrrounded by land instead of by sea, no bird would nee~l to fly, or to be carried by the wind, for several hundred miles at a stretch to another mountain summit, but would find a refnge in the surrounding uplands, ridges, valleys, or plains. As a rule the birds that frequent lofty mountain tops a,re peculiar species, allied to those of the surrounding distr~ct; and there is no indica.tion whatever of the passage of bu·cls from one remote mountain to another in any way comparable with the flights of birds which are known to reach the Azores annually, or even with the few regular migrants from Australia to New Zealand. It is almost impossible to con ceive that the seeds of the Himalayan primula should have been thus carried to Java; but, by means of gales of wind, and intermediate stations from fifty to a. few hundred miles apart, where the seeds might vegetate for a yea.r or two and produce fresh seed to be again carried on in the same manner, the transmission might, after ma.ny failures, he at last effected. A very important consideration is the vastly larger scale on which ·wind-carriage of seeds must act, as compared with bird-carriage. It can only be a few birds which ca.rry seeds attached to their feathers or feet. A very small proportion of these would carry the seeds of Alpine plants; while au almost infinitesimal fraction of these latter would convey the few seeds attached to them safely to an oceanic island or remok mountain. But winds, in the form of whirlwinds or tornadoes, gales or hurricanes, are perpetually at work over large ar ea~ of lund and sea. Insects and light particles of matter an· often carried up to the tops of high mountains ; a~d, from tlH· very nature and origiu of winds, they usually consist of ascending or descending currents, the former capable of suspending such small and light objects as are many seeds long enough for them to be carried enormous distances. For each single seed ca,rried away by external attachment to tho feet or feathers of a bird, countless millions are probably carried away by violent winds; ancl the chance of convcyanc to a great distance a.nd in <L definite direction must be ma11Y XII GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 373 times grea.ter by the latter mode than by the former. 1 \V e have seen that inorganic particles of much greater specific gravity than . eeds, and neal'ly a.s heavy as the smallest kinds, are carried to great distances through the air, and we can therefore hardly doubt that . ome seeds are carried as far: The direct agency of the wind, as a supplement to birdtran. port, will help to expla.in the presence in oceanic islands of plants gro,ving in dry or rocky places whose small seeds are not likely to become attached to birds; while it seems to he the only effective agency possible in the dispersal of those species of alpine or sub-alpine plants found on the summits of clistant mountains, or still more widely separated in the , temperate zones of the northern and southern hemispheres. Concluding Remarks. On the general principles that have been now laid clown it w~ll b? found t.hat all the chief facts of the geographical distnbntwn of ammals and plants can be sufficiently understood. There will, of course, be many cases of difficulty and some see~ing anomalies, but these can usually be seen to depend Oll o~r Ignorance of some of the essential factors of the problem. Either.we ~o not know the distribution of the group in recent geologiCal t1mes, or we are still ignorant of the special methods by which the organisms are able to cross the sea. The latter difficulty applies especially to the lizard tribe, which are found 1 ~ vcrr remarkable case of wind conveyance of ,eeds on a large scale is desc~·tbed m a l ett~r from Mr. 'l'homas Hanbury to his brother, the late Danwl Hanbury, wh1ch has been kindly communicated to me by Mr. Hemsley of Kew. ~he letter is dated "Shanghai, 1st l\Iay 1856," and the passage referred to 1s as follows :- "~or the pat three days we have had very warm weather for this time of y~ar, 111 fact almo t a. warm as the middle of summer. Last evening the wtud ~uduenly cha?ged round to the uorth anu blew all night with con ·iuer· able vwlence, malong a great change in the atmosphere. . '' This r~10miug, ~tyriads of small white particles are floating about in the at~: there JS not a st~1gle cloud. and no ~li t, ye.t the sun i. quite ob cured by tl11s substance, an<l 1t looks hke a whtte fog m England. I enclose thee a saJ.uple, thinking it may interest. It is evidently a vegetable production; I thmk, apparently, some kiuu of seed." .Mr. Hems~ ey auds, that this substance proves to be the plumose seeds of a poplar~!' WIIIO\~, r.n order to produce the effects described- q1tite obSClt?'ing th~ snn ltke a whtte jog,-the seeds must have tilled the air to a very great he1g!Jt; and they must have been brought from some district where there were extensive tracts covered with the tree which produced them. |