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Show 366 DARWINISM OHAP. per century were carried to an oceanic isla.nd, that i land miaht become rapidly overrun by the plant, if the conditions we~e favourable to its growth and reproduction. It is further objected that search has been made for such seeds, and they have not been found. Professor Kerner of Inusbruck examined the snow on the surface of glaciers, and assiduou ly collected all the seeds he could find, and these were all of plants which grew in the adjacent mountains or in the same district. In like manner, the plants growing on moraines were found to be those of the adjacent mquntains, plateaux, or lowlands. Hence he concluded that the prevalent opinion that seedB may be carried through the air for very great distances "if> not supported by fact." 1 The opinion is certainly not supported by Kerner's facts, but neither is it opposed by them. It is obvious that the seeds that would be carried by the wind to moraines or to the surface of glaciers would be, fir. t a.ncl in the greatest abundance, those of the immediat ly surrounding district ; then, very much more rarely, those from more remote mountains; and lastly, in extreme rarity, those from distant countries or altogether distinct mountain ranges. Let us suppose the first to be so abundant that a. single seed could be found by industrious search on each square yard of the surface of the glacier; the second so scarce that only one could possibly be found in a hundred yard~ square; while to find one of the third class it would be necessary exhaustively to examine a square mile of surface. Should we expect that one ever to be found, and should the fad that it could not be found be taken as a proof that it was not there 1 Besides, a glacier is altogether in a bad position to receive such remote wanderers, since it is generally surroundr<l by lofty mountains, often range behind range, which would intercept the few air-borne seeds that might have been carri <'cl from a distant land. The conditions in an oceanic island, on the other hand, are the most favourable, since the land, especially if high, will intercept objects carried by the wind, and will thus cause more of the solid matter to fall on it than on an equal area of ocean. vVe know that winds at sea often blow violently for days together, and the rate of motion is indicated by the fact that 72 miles an hour was the average velocity 1 See Natwre, vol. vi. p. 164, for a summary of Kerner's paper. XI! GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 367 of the wind observed during twelve hours at the Ben Nevis observatory, while the velocity sometimes rises to 120 miles ~n hour. A twelve hOl~rs' gale ~ight, therefore, carry hght seeds a thousand. m1les as easily and certainly as it could ~arry quartz-grams of much greater specific gravity, rotund1ty, and smoothness, 500 or even 100 miles· and it is diffi cult even. to imagine a sufficient reason why they should not be so carrw~~perhaps ver~ r~rely ancl u_nder exceptionally favourable cond1t10ns,-but this Is all that 1s required. As regards the second objection, it has been observed that orchiclere, which have often exceedingly small and light seeds, are remarkably absent from oceanic isln.nds. Thi , however, may be very largely due to their extreme specialisation and dependence on insect agency for their fertilisation ; while the fact that they do occur in such very remote islands as the Azores, Tahiti, and the Sandwich I. lands, proves that they mus~ have once reach?d. these localities either by the agency of birus or by transmission through the air; and the facts I have given above .render the latter mode at lea_. t as probable as the former. Sir Joseph Hooker remarks on the compo ite plant of Kerguelen Island (Cotula plumosa) being found also on Lord Auckl.and and Ma~Quarrie Islanus, and yet having no pappus, while other specws of the genus possess it. Thi. is certainly remarkable, and proves that the plant must have, or once have had, some other means of disper al across wide oceans.1 One of the most widely dispersed species in the whole world (Sonchus oleraceus) possesses pappus, as do four out of five of the species which are common to Europe and New Zealand, all of which have a very wide distribution. The same. author rema~ks on the limited area occupied by most speCies of Compositre, notwithstanding their facilities for dispersal by means of their feathered seeds; but it has been 1 It seems quite possible that the absence of pappus in this case is a recent ad~ptation, and that it has been brought about by cause sin1 ilar to tho e whiCh Lave redttced or aborted the wiugs of iw:;ects in oceanic islands. For when a plant has once reached one of the 'torm-swept islands of the southern ?cean, the _P~p~us wi_ll he_ iuj_urious for the s:tme reason that the wings of msects are lllJUnous, smce 1t wlll lead to the seec.ls being blown out to sea and destroyed. 'fhe ·eeds whi h are heaviest and l1ave least pappus will have the be~t chance of fallin? on t!1e gron!1d and remaining there to germinate, and thrs process of selectwu n11ght raprdly lead to the entire disappearance of the pappus. |
