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Show 362 DARWINISM CTIAI'. a very small portion of mud may servo :o convey seeds, and such an occurrence repeated even at long mtervals may greatly aid in stockincr remote islands with vegetation. Many seed ~ also a.dhere to~ the feathers of birds, and thus, again, may b conveyed as far as birds are ever carried. Dr. Guppy found a small hard seed in the gizzard of a Cape Petrel, taken ahont. 550 miles east of Tristan da Cunha. Dispersal of Seeds by the TVind. In the preceding cases we have been able to obtain direct evidence of transportal; but although we know that many seed" arc specially adapted to be dispersed by t_hc wind, we cannot obtain direct proof that they are so earned for hundreds or thousands of miles across the sea, owing to the difficulty of detectino- sincrle objects which are so small and incon pi cuons. It is probabl~ however, that the wind as an agent of di.y r~al is really more effective than any of those we have hitherto con:=;idere<.l, because a very large number of plants have . cc<ls which are very sm<tll and light, and arc often of snch a form a to facilitate acriuJ carriage for enormous chstances. It is evident that such seeds are especially liable to be transported by violent winds, because they become ripe in autumn at the time wh ·u storms arc most prevalent, while they either lie npon the snrfacc of the ground, or arc disposed in dry capsul es on the plant reu.dy to be blown awa.y. If inorganic particles comparable in weight, size, or form with such seeds arc carried for great chstances, we may be sure that seeds will aJso be occasionally carried in the same way. It will, therefon', be necessary to give a few examples of wind-carriage of small objects. On 27th July 1875 a remarkable shower of small pieces of hay occurred at Monkstown, near Dublin. They appeared floating slowly down from a great height, as if falling from a dark cloud which huno- overhead. The pieces picked up " ere wet., and varied from single blades of grass to tufts wci gh in~ one or two ounces. A similar shower occurred a few <bys earlier in Denbighshire, and was observed to travel in a direction contrary to that of the wind in the lower atmospherc.L There is no evidence of the distance from which the hay was 1 Nature (18i5), vol. xii. pp. 279, 298. XII GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 363 ~)l·ought,_l~nt as it had been carried to a great height, it ,ras I~ a pos1~10n to be conveyed to almost any distance by n, vwlent wmd, had such occurred at the time. Mineml Matter ca?·'riecl by the JVincl. The numerous cases of sand and volcanic dust beino- carried enor_mous distances through the atmosphere sufficiently prove the Importance of wind as a carrier of solicl mntter, but unfortunately the matter collected has not been hitherto examined with a view to determine the maximum size and weicrht of the particles. A few facts, however, have been kindly bfurnished me by Profes. or Judd, F.R.S. Some dust which fell at Genoa on 15th October 1 85, and was believed to have been brought from the A_frican de. crt, consisted of quartz, hornb~ endc, and other _mmerals, and contained particles having a dtametcr of 5 -&0 mch, each weighing :roo~ooo grain. This dust had probably travelled over 600 miles. In the du t from Kr~katoa, which fell at Batavia, about 100 miles distant, durmg the great eruption, there are many solid particles even larger than those mentioned above. Some of this dust was given me by Profe. sor Judd, and I found in it several ovoid particles of a much larger size, being 5 1 0 inch long, and "'~~ wide and deep. The dust from the same eruption, which fell on board the ship Ambella, 970 miles from the volcano also contained solid particles 5 -&0 inch tliameter. Mr. J oh~ Murray of the Challenger. Expedition writes to me that he finds in the deep sea deposit 500 and even 700 miles west of the coast of Africa., rounded particles of quartz, havino· a diameter of 'I:51 o m• c h , an d sm. u'1 ar particles are found °a t equally great d1st~nces from the south-west coasts of Australia; and he ~ons1ders these to be atmospheric dust carried to that distance 1 y the wind. Taking the sp. gr. of quartz at 2·6, th~se par_ticles would weigh about z b.-&oo grain each. These mterestmg facts can, however, by no means be taken as indicating the extreme limits of the power of wind in carrying solid particles. During the Krakatoa eruption no gale of special violence occurred, and the region is one of comparative calms. The grains of quartz found by Mr. Mmray more nearly indicate the limit, but the very small portions of matter brought up by the dredge, as com- |
