The numerous cases of sand and volcanic dust being carried enormous distances through the atmosphere sufficiently prove the importance of wind as a carrier of solid matter, but unfortunately the matter collected has not been hitherto examined with a view to determine the maximum size and weight of the particles. A few facts, however, have been kindly furnished me by Professor Judd, F.R.S. Some dust which fell at Genoa on 15th October 1885, and was believed to have been brought from the African desert, consisted of quartz, hornblende, and other minerals, and contained particles having a diameter of 1/500 inch, each weighing 1/200,000 grain. This dust had probably travelled over 600 miles. In the dust from Krakatoa, which fell at Batavia, about 100 miles distant, during the great eruption, there are many solid particles even larger than those mentioned above. Some of this dust was given me by Professor Judd, and I found in it several ovoid particles of a much larger size, being 1/50 inch long, and 1/70 wide and deep. The dust from the same eruption, which fell on board the ship Arabella, 970 miles from the volcano, also contained solid particles 1/500 inch diameter. Mr. John Murray of the Challenger Expedition writes to me that he finds in the deep sea deposits 500 and even 700 miles west of the coast of Africa, rounded particles of quartz, having a diameter of 1/250 inch, and similar particles are found at equally great distances from the south-west coasts of Australia; and he considers these to be atmospheric dust carried to that distance by the wind. Taking the sp. gr. of quartz at 2.6, these particles would weigh about 1/25,000 grain each. These interesting 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. Murray more nearly indicate the limit, but the very small portions of matter brought up by the dredge, as compared with the enormous areas of sea-bottom, over which the atmospheric dust must have been scattered, render it in the highest degree improbable that the maximum limit either of size of particles, or of distance from land has been reached.
Let us, however, assume that the quartz grains, found by Mr. Murray in the deep-sea ooze 700 miles from land, give us the extreme limit of the power of the atmosphere as a carrier of solid particles, and let us compare with these the weights of some seeds. From a small collection of the seeds of thirty species of herbaceous plants sent me from Kew, those in the above table were selected, and small portions of eight of them carefully weighed in a chemical balance.[175] By counting these portions I was able to estimate the number of seeds weighing one grain. The three very minute species, whose numbers are marked with an asterisk (*), were estimated by the comparison of their sizes with those of the smaller weighed seeds.
No| Species. |Approximate | Approximate | Remarks.
| |No. of Seeds| Dimensions. |
| |In one Grain| |
| | | in. in. in. |
1|Draba verna | 1,800 |1/60 x 1/90 x 1/160|Oval, flat.
2|Hypericum perforatum | 520 | 1/30 x 1/80 |Cylindrical.
3|Astilbe rivularis | 4,500 | 1/50 x 1/100 |Elongate, flat, tailed,
| | | | wavy.
4|Saxifraga coriophylla| 750 | 1/40 x 1/75 |Surface rough, adhere
| | | | to the dry capsules.
5|Oenothera rosea | 640 | 1/40 x 1/80 |Ovate.
6|Hypericum hirsutum | 700 | 1/30 x 1/100 |Cylindrical, rough.
7|Mimulus luteus | 2,900 | 1/60 x 1/100 |Oval, minute.
8|Penthorum sedoides | 8,000* | 1/70 x 1/150 |Flattened, very minute.
9|Sagina procumbens | 12,000* | 1/120 |Sub-triangular, flat.
10|Orchis maculata | 15,000* | --- |Margined, flat,
| | | | very minute.
11|Gentiana purpurea | 35 | 1/25 |Wavy, rough, with this
| | | | coriaceous margins.
12|Silene alpina | --- | 1/30 |Flat, with fringed
| | | | margins.
13|Adenophora communis | --- | 1/20 x 1/40 |Very thin, wavy, light.
|Quartz grains | 25,000 | 1/250 |Deep sea ... 700 miles.
|Do. |200,000 | 1/500 |Genoa ... 600 miles.
If now we compare the seeds with the quartz grains, we find that several are from twice to three times the weight of the grains found by Mr. Murray, and others five times, eight times, and fifteen times as heavy; but they are proportionately very much larger, and, being usually irregular in shape or compressed, they expose a very much larger surface to the air. The surface is often rough, and several have dilated margins or tailed appendages, increasing friction and rendering the uniform rate of falling through still air immensely less than in the case of the smooth, rounded, solid quartz grains. With these advantages it is a moderate estimate that seeds ten times the weight of the quartz grains could be carried quite as far through the air by a violent gale and under the most favourable conditions. These limits will include five of the seeds here given, as well as hundreds of others which do not exceed them in weight; and to these we may add some larger seeds which have other favourable characteristics, as is the case with numbers 11-13, which, though very much larger than the rest, are so formed as in all probability to be still more easily carried great distances by a gale of wind. It appears, therefore, to be absolutely certain that every autumnal gale capable of conveying solid mineral particles to great distances, must also carry numbers of small seeds at least as far; and if this is so, the wind alone will form one of the most effective agents in the dispersal of plants.
Hitherto this mode of conveyance, as applying to the transmission of seeds for great distances across the ocean, has been rejected by botanists, for two reasons. In the first place, there is said to be no direct evidence of such conveyance; and, secondly, the peculiar plants of remote oceanic islands do not appear to have seeds specially adapted for aerial transmission. I will consider briefly each of these objections.
Objection to the Theory of Wind-Dispersal.
To obtain direct evidence of the transmission of such minute and perishable objects, which do not exist in great quantities, and are probably carried to the greatest distances but rarely and as single specimens, is extremely difficult. A bird or insect can be seen if it comes on board ship, but who would ever detect the seeds of Mimulus or Orchis even if a score of them fell on a ship's deck? Yet if but one such seed per century were carried to an oceanic island, that island might become rapidly overrun by the plant, if the conditions were 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 Innsbruck examined the snow on the surface of glaciers, and assiduously 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 mountains, plateaux, or lowlands. Hence he concluded that the prevalent opinion that seeds may be carried through the air for very great distances "is not supported by fact."[176] 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, first and in the greatest abundance, those of the immediately 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 yards 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 fact that it could not be found be taken as a proof that it was not there? Besides, a glacier is altogether in a bad position to receive such remote wanderers, since it is generally surrounded by lofty mountains, often range behind range, which would intercept the few air-borne seeds that might have been carried 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. We 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 of the wind observed during twelve hours at the Ben Nevis observatory, while the velocity sometimes rises to 120 miles an hour. A twelve hours' gale might, therefore, carry light seeds a thousand miles as easily and certainly as it could carry quartz-grains of much greater specific gravity, rotundity, and smoothness, 500 or even 100 miles; and it is difficult even to imagine a sufficient reason why they should not be so carried—perhaps very rarely and under exceptionally favourable conditions,—but this is all that is required.
As regards the second objection, it has been observed that orchideae, which have often exceedingly small and light seeds, are remarkably absent from oceanic islands. This, 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 Islands, proves that they must have once reached these localities either by the agency of birds or by transmission through the air; and the facts I have given above render the latter mode at least as probable as the former. Sir Joseph Hooker remarks on the composite plant of Kerguelen Island (Cotula plumosa) being found also on Lord Auckland and MacQuarrie Islands, and yet having no pappus, while other species of the genus possess it. This is certainly remarkable, and proves that the plant must have, or once have had, some other means of dispersal across wide oceans.[177] 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 remarks on the limited area occupied by most species of Compositae, notwithstanding their facilities for dispersal by means of their feathered seeds; but it has been already shown that limitations of area are almost always due to the competition of allied forms, facilities for dispersal being only one of many factors in determining the wide range of species. It is, however, a specially important factor in the case of the inhabitants of remote oceanic islands, since, whether they are peculiar species or not, they or their remote ancestors must at some time or other have reached their present position by natural means.
I have already shown elsewhere, that the flora of the Azores strikingly supports the view of the species having been introduced by aerial transmission only, that is, by the agency of birds and the wind, because all plants that could not possibly have been carried by these means are absent.[178] In the same way we may account for the extreme rarity of Leguminosae in all oceanic islands. Mr. Hemsley, in his Report on Insular Floras, says that they "are wanting in a large number of oceanic islands where there is no true littoral flora," as St. Helena, Juan Fernandez, and all the islands of the South Atlantic and South Indian Oceans. Even in the tropical islands, such as Mauritius and Bourbon, there are no endemic species, and very few in the Galapagos and the remoter Pacific Islands. All these facts are quite in accordance with the absence of facilities for transmission through the air, either by birds or the wind, owing to the comparatively large size and weight of the seeds; and an additional proof is thus afforded of the extreme rarity of the successful floating of seeds for great distances across the ocean.[179]
Explanation of North Temperate Plants in the Southern Hemisphere.