_Madagascar and New Zealand._
The two exceptions just referred to are Madagascar and New Zealand, and all the evidence goes to show that in these cases the land connection with the nearest continental area was very remote in time. The extraordinary isolation of the productions of Madagascar--almost all the most characteristic forms of mammalia, birds, and reptiles of Africa being absent from it--renders it certain that it must have been separated from that continent very early in the Tertiary, if not as far back as the latter part of the Secondary period; and this extreme antiquity is indicated by a depth of considerably more than a thousand fathoms in the Mozambique Channel, though this deep portion is less than a hundred miles wide between the Comoro Islands and the mainland.[166]
Madagascar is the only island on the globe with a fairly rich mammalian fauna which is separated from a continent by a depth greater than a thousand fathoms; and no other island presents so many peculiarities in these animals, or has preserved so many lowly organised and archaic forms. The exceptional character of its productions agrees exactly with its exceptional isolation by means of a very deep arm of the sea.
New Zealand possesses no known mammals and only a single species of batrachian; but its geological structure is perfectly continental. There is also much evidence that it does possess one mammal, although no specimens have been yet obtained.[167] Its reptiles and birds are highly peculiar and more numerous than in any truly oceanic island. Now the sea which directly separates New Zealand from Australia is more than 2000 fathoms deep, but in a north-west direction there is an extensive bank under 1000 fathoms, extending to and including Lord Howe"s Island, while north of this are other banks of the same depth, approaching towards a submarine extension of Queensland on the one hand, and New Caledonia on the other, and altogether suggestive of a land union with Australia at some very remote period. Now the peculiar relations of the New Zealand fauna and flora with those of Australia and of the tropical Pacific Islands to the northward indicate such a connection, probably during the Cretaceous period; and here, again, we have the exceptional depth of the dividing sea and the form of the ocean bottom according well with the altogether exceptional isolation of New Zealand, an isolation which has been held by some naturalists to be great enough to justify its claim to be one of the primary Zoological Regions.
_The Teachings of the Thousand-Fathom Line._
If now we accept the annexed map as showing us approximately how far beyond their present limits our continents may have extended during any portion of the Tertiary and Secondary periods, we shall obtain a foundation of inestimable value for our inquiries into those migrations of animals and plants during past ages which have resulted in their present peculiarities of distribution. We see, for instance, that the South American and African continents have always been separated by nearly as wide an ocean as at present, and that whatever similarities there may be in their productions must be due to the similar forms having been derived from a common origin in one of the great northern continents. The radical difference between the higher forms of life of the two continents accords perfectly with their permanent separation. If there had been any direct connection between them during Tertiary times, we should hardly have found the deep-seated differences between the Quadrumana of the two regions--no family even being common to both; nor the peculiar Insectivora of the one continent, and the equally peculiar Edentata of the other. The very numerous families of birds quite peculiar to one or other of these continents, many of which, by their structural isolation and varied development of generic and specific forms, indicate a high antiquity, equally suggest that there has been no near approach to a land connection during the same epoch.
Looking to the two great northern continents, we see indications of a possible connection between them both in the North Atlantic and the North Pacific oceans; and when we remember that from middle Tertiary times backward--so far as we know continuously to the earliest Palaeozoic epoch--a temperate and equable climate, with abundant woody vegetation, prevailed up to and within the arctic circle, we see what facilities may have been afforded for migration from one continent to the other, sometimes between America and Europe, sometimes between America and Asia. Admitting these highly probable connections, no bridging of the Atlantic in more southern lat.i.tudes (of which there is not a particle of evidence) will have been necessary to account for all the intermigration that has occurred between the two continents. If, on the other hand, we remember how long must have been the route, and how diverse must always have been the conditions between the more northern and the more southern portions of the American and Euro-Asiatic continents, we shall not be surprised that many widespread forms in either continent have not crossed into the other; and that while the skunks (Mephitis), the pouched rats (Saccomyidae), and the turkeys (Meleagris) are confined to America, the pigs and the hedgehogs, the true flycatchers and the pheasants are found only in the Euro-Asiatic continent. But, just as there have been periods which facilitated intermigration between America and the Old World, there have almost certainly been periods, perhaps of long duration even geologically, when these continents have been separated by seas as wide as, or even wider than, those of the present day; and thus may be explained such curious anomalies as the origination of the camel-tribe in America, and its entrance into Asia in comparatively recent Tertiary times, while the introduction of oxen and bears into America from the Euro-Asiatic continent appears to have been equally recent.[168]
We shall find on examination that this view of the general permanence of the oceanic and continental areas, with constant minor fluctuations of land and sea over the whole extent of the latter, enables us to understand, and offer a rational explanation of, most of the difficult problems of geographical distribution; and further, that our power of doing this is in direct proportion to our acquaintance with the distribution of fossil forms of life during the Tertiary period. We must, also, take due note of many other facts of almost equal importance for a due appreciation of the problems presented for solution, the most essential being, the various powers of dispersal possessed by the different groups of animals and plants, the geological antiquity of the species and genera, and the width and depth of the seas which separate the countries they, inhabit. A few ill.u.s.trations will now be given of the way in which these branches of knowledge enable us to deal with the difficulties and anomalies that present themselves.
_The Distribution of Marsupials._
This singular and lowly organised type of mammals const.i.tutes almost the sole representative of the cla.s.s in Australia and New Guinea, while it is entirely unknown in Asia, Africa, or Europe. It reappears in America, where several species of opossums are found; and it was long thought necessary to postulate a direct southern connection of these distant countries, in order to account for this curious fact of distribution.
When, however, we look to what is known of the geological history of the marsupials the difficulty vanishes. In the Upper Eocene deposits of Western Europe the remains of several animals closely allied to the American opossums have been found; and as, at this period, a very mild climate prevailed far up into the arctic regions, there is no difficulty in supposing that the ancestors of the group entered America from Europe or Northern Asia during early Tertiary times.
But we must go much further back for the origin of the Australian marsupials. All the chief types of the higher mammalia were in existence in the Eocene, if not in the preceding Cretaceous period, and as we find none of these in Australia, that country must have been finally separated from the Asiatic continent during the Secondary or Mesozoic period. Now during that period, in the Upper and the Lower Oolite and in the still older Trias, the jaw-bones of numerous small mammalia have been found, forming eight distinct genera, which are believed to have been either marsupials or some allied lowly forms. In North America also, in beds of the Jura.s.sic and Tria.s.sic formations, the remains of an equally great variety of these small mammalia have been discovered; and from the examination of more than sixty specimens, belonging to at least six distinct genera, Professor Marsh is of opinion that they represent a generalised type, from which the more specialised marsupials and insectivora were developed.
From the fact that very similar mammals occur both in Europe and America at corresponding periods, and in beds which represent a long succession of geological time, and that during the whole of this time no fragments of any higher forms have been discovered, it seems probable that both the northern continents (or the larger portion of their area) were then inhabited by no other mammalia than these, with perhaps other equally low types. It was, probably, not later than the Jura.s.sic age when some of these primitive marsupials were able to enter Australia, where they have since remained almost completely isolated; and, being free from the compet.i.tion of higher forms, they have developed into the great variety of types we now behold there. These occupy the place, and have to some extent acquired the form and structure of distinct orders of the higher mammals--the rodents, the insectivora, and the carnivora,--while still preserving the essential characteristics and lowly organisation of the marsupials. At a much later period--probably in late Tertiary times--the ancestors of the various species of rats and mice which now abound in Australia, and which, with the aerial bats, const.i.tute its only forms of placental mammals, entered the country from some of the adjacent islands. For this purpose a land connection was not necessary, as these small creatures might easily be conveyed among the branches or in the crevices of trees uprooted by floods and carried down to the sea, and then floated to a sh.o.r.e many miles distant. That no actual land connection with, or very close approximation to, an Asiatic island has occurred in recent times, is sufficiently proved by the fact that no squirrel, pig, civet, or other widespread mammal of the Eastern hemisphere has been able to reach the Australian continent.
_The Distribution of Tapirs._
These curious animals form one of the puzzles of geographical distribution, being now confined to two very remote regions of the globe--the Malay Peninsula and adjacent islands of Sumatra and Borneo, inhabited by one species, and tropical America, where there are three or four species, ranging from Brazil to Ecuador and Guatemala. If we considered these living forms only, we should be obliged to speculate on enormous changes of land and sea in order that these tropical animals might have pa.s.sed from one country to the other. But geological discoveries have rendered all such hypothetical changes unnecessary.
During Miocene and Pliocene times tapirs abounded over the whole of Europe and Asia, their remains having been found in the tertiary deposits of France, India, Burmah, and China. In both North and South America fossil remains of tapirs occur only in caves and deposits of Post-Pliocene age, showing that they are comparatively recent immigrants into that continent. They perhaps entered by the route of Kamchatka and Alaska, where the climate, even now so much milder and more equable than on the north-east of America, might have been warm enough in late Pliocene times to have allowed the migration of these animals. In Asia they were driven southwards by the compet.i.tion of numerous higher and more powerful forms, but have found a last resting-place in the swampy forests of the Malay region.
_What these Facts Prove._
Now these two cases, of the marsupials and the tapirs, are in the highest degree instructive, because they show us that, without any hypothetical bridging of deep oceans, and with only such changes of sea and land as are indicated by the extent of the comparatively shallow seas surrounding and connecting the existing continents, we are able to account for the anomaly of allied forms occurring only in remote and widely separated areas. These examples really const.i.tute crucial tests, because, of all cla.s.ses of animals, mammalia are least able to surmount physical barriers. They are obviously unable to pa.s.s over wide arms of the sea, while the necessity for constant supplies of food and water renders sandy deserts or snow-clad plains equally impa.s.sable. Then, again, the peculiar kinds of food on which alone many of them can subsist, and their liability to the attacks of other animals, put a further check upon their migrations. In these respects almost all other organisms have great advantages over mammals. Birds can often fly long distances, and can thus cross arms of the sea, deserts, or mountain ranges; insects not only fly, but are frequently carried great distances by gales of wind, as shown by the numerous cases of their visits to ships hundreds of miles from land. Reptiles, though slow of movement, have advantages in their greater capacity for enduring hunger or thirst, their power of resisting cold or drought in a state of torpidity, and they have also some facilities for migration across the sea by means of their eggs, which may be conveyed in crevices of timber or among ma.s.ses of floating vegetable matter. And when we come to the vegetable kingdom, the means of transport are at their maximum, numbers of seeds having special adaptations for being carried by mammalia or birds, and for floating in the water, or through the air, while many are so small and so light that there is practically no limit to the distances they may be carried by gales and hurricanes.
We may, therefore, feel quite certain that the means of distribution that have enabled the larger mammalia to reach the most remote regions from a common starting-point, will be at least as efficacious, and usually far more efficacious, with all other land animals and plants; and if in every case the existing distribution of this cla.s.s can be explained on the theory of oceanic and continental permanence, with the limited changes of sea and land already referred to, no valid objections can be taken against this theory founded on anomalies of distribution in other orders. Yet nothing is more common than for students of this or that group to a.s.sort that the theory of oceanic permanence is quite inconsistent with the distribution of its various species and genera.
Because a few Indian genera and closely allied species of birds are found in Madagascar, a land termed "Lemuria" has been supposed to have united the two countries during a comparatively recent geological epoch; while the similarity of fossil plants and reptiles, from the Permian and Miocene formations of India and South Africa, has been adduced as further evidence of this connection. But there are also genera of snakes, of insects, and of plants, common to Madagascar and South America only, which have been held to necessitate a direct land connection between these countries. These views evidently refute themselves, because any such land connections must have led to a far greater similarity in the productions of the several countries than actually exists, and would besides render altogether inexplicable the absence of all the chief types of African and Indian mammalia from Madagascar, and its marvellous individuality in every department of the organic world.[169]
_Powers of Dispersal as ill.u.s.trated by Insular Organisms._
Having arrived at the conclusion that our existing oceans have remained practically unaltered throughout the Tertiary and Secondary periods of geology, and that the distribution of the mammalia is such as might have been brought about by their known powers of dispersal, and by such changes of land and sea as have probably or certainly occurred, we are, of course, restricted to similar causes to explain the much wider and sometimes more eccentric distribution of other cla.s.ses of animals and of plants. In doing so, we have to rely partly on direct evidence of dispersal, afforded by the land organisms that have been observed far out at sea, or which have taken refuge on ships, as well as by the periodical visitants to remote islands; but very largely on indirect evidence, afforded by the frequent presence of certain groups on remote oceanic islands, which some ancestral forms must, therefore, have reached by transmission across the ocean from distant lands.
_Birds._
These vary much in their powers of flight, and their capability of traversing wide seas and oceans. Many swimming and wading birds can continue long on the wing, fly swiftly, and have, besides, the power of resting safely on the surface of the water. These would hardly be limited by any width of ocean, except for the need of food; and many of them, as the gulls, petrels, and divers, find abundance of food on the surface of the sea itself. These groups have a wide distribution _across_ the oceans; while waders--especially plovers, sandpipers, snipes, and herons--are equally cosmopolitan, travelling _along_ the coasts of all the continents, and across the narrow seas which separate them. Many of these birds seem unaffected by climate, and as the organisms on which they feed are equally abundant on arctic, temperate, and tropical sh.o.r.es, there is hardly any limit to the range even of some of the species.
Land-birds are much more restricted in their range, owing to their usually limited powers of flight, their inability to rest on the surface of the sea or to obtain food from it, and their greater specialisation, which renders them less able to maintain themselves in the new countries they may occasionally reach. Many of them are adapted to live only in woods, or in marshes, or in deserts; they need particular kinds of food or a limited range of temperature; and they are adapted to cope only with the special enemies or the particular group of compet.i.tors among which they have been developed. Such birds as these may pa.s.s again and again to a new country, but are never able to establish themselves in it; and it is this organic barrier, as it is termed, rather than any physical barrier, which, in many cases, determines the presence of a species in one area and its absence from another. We must always remember, therefore, that, although the presence of a species in a remote oceanic island clearly proves that its ancestors must at one time have found their way there, the absence of a species does not prove the contrary, since it also may have reached the island, but have been unable to maintain itself, owing to the inorganic or organic conditions not being suitable to it. This general principle applies to all cla.s.ses of organisms, and there are many striking ill.u.s.trations of it. In the Azores there are eighteen species of land-birds which are permanent residents, but there are also several others which reach the islands almost every year after great storms, but have never been able to establish themselves. In Bermuda the facts are still more striking, since there are only ten species of resident birds, while no less than twenty other species of land-birds and more than a hundred species of waders and aquatics are frequent visitors, often in great numbers, but are never able to establish themselves. On the same principle we account for the fact that, of the many continental insects and birds that have been let loose, or have escaped from confinement, in this country, hardly one has been able to maintain itself, and the same phenomenon is still more striking in the case of plants. Of the thousands of hardy plants which grow easily in our gardens, very few have ever run wild, and when the experiment is purposely tried it invariably fails. Thus A.
de Candolle informs us that several botanists of Paris, Geneva, and especially of Montpellier, have sown the seeds of many hundreds of species of exotic hardy plants, in what appeared to be the most favourable situations, but that in hardly a single case has any one of them become naturalised.[170] Still more, then, in plants than in animals the absence of a species does not prove that it has never reached the locality, but merely that it has not been able to maintain itself in compet.i.tion with the native productions. In other cases, as we have seen, facts of an exactly opposite nature occur. The rat, the pig, and the rabbit, the water-cress, the clover, and many other plants, when introduced into New Zealand, nourish exceedingly, and even exterminate their native compet.i.tors; so that in these cases we may feel sure that the species in question did not exist in New Zealand simply because they had been unable to reach that country by their natural means of dispersal. I will now give a few cases, in addition to those recorded in my previous works, of birds and insects which have been observed far from any land.
_Birds and Insects at Sea._
Captain D. Fullarton of the ship _Timaru_ recorded in his log the occurrence of a great number of small land-birds about the ship on 15th March 1886, when in Lat. 48 31" N., Long. 8 16" W. He says: "A great many small land-birds about us; put about sixty into a coop, evidently tired out." And two days later, 17th March, "Over fifty of the birds cooped on 15th died, though fed. Sparrows, finches, water-wagtails, two small birds, name unknown, one kind like a linnet, and a large bird like a starling. In all there have been on board over seventy birds, besides some that hovered about us for some time and then fell into the sea exhausted." Easterly winds and severe weather were experienced at the time.[171] The spot where this remarkable flight of birds was met with is about 160 miles due west of Brest, and this is the least distance the birds must have been carried. It is interesting to note that the position of the ship is nearly in the line from the English and French coasts to the Azores, where, after great storms, so many bird stragglers arrive annually. These birds were probably blown out to sea during their spring migration along the south coast of England to Wales and Ireland.
During the autumnal migration, however, great flocks of birds--especially starlings, thrushes, and fieldfares--have been observed every year flying out to sea from the west coast of Ireland, almost the whole of which must perish. At the Nash Lighthouse, in the Bristol Channel on the coast of Glamorganshire, an enormous number of small birds were observed on 3d September, including nightjars, buntings, white-throats, willow-wrens, cuckoos, house-sparrows, robins, wheatears, and blackbirds. These had probably crossed from Somersetshire, and had they been caught by a storm the larger portion of them must have been blown out to sea.[172]
These facts enable us to account sufficiently well for the birds of oceanic islands, the number and variety of which are seen to be proportionate to their facilities for reaching the island and maintaining themselves in it. Thus, though more birds yearly reach Bermuda than the Azores, the number of residents in the latter islands is much larger, due to the greater extent of the islands, their number, and their more varied surface. In the Galapagos the land-birds are still more numerous, due in part to their larger area and greater proximity to the continent, but chiefly to the absence of storms, so that the birds which originally reached the islands have remained long isolated and have developed into many closely allied species adapted to the special conditions. All the species of the Galapagos but one are peculiar to the islands, while the Azores possess only one peculiar species, and Bermuda none--a fact which is clearly due to the continual immigration of fresh individuals keeping up the purity of the breed by intercrossing. In the Sandwich Islands, which are extremely isolated, being more than 2000 miles from any continent or large island, we have a condition of things similar to what prevails in the Galapagos, the land-birds, eighteen in number, being all peculiar, and belonging, except one, to peculiar genera. These birds have probably all descended from three or four original types which reached the islands at some remote period, probably by means of intervening islets that have since disappeared. In St.
Helena we have a degree of permanent isolation which has prevented any land-birds from reaching the island; for although its distance from the continent, 1100 miles, is not so great as in the case of the Sandwich Islands, it is situated in an ocean almost entirely dest.i.tute of small islands, while its position within the tropics renders it free from violent storms. Neither is there, on the nearest part of the coast of Africa, a perpetual stream of migrating birds like that which supplies the innumerable stragglers which every year reach Bermuda and the Azores.
_Insects._
Winged insects have been mainly dispersed in the same way as birds, by their power of flight, aided by violent or long-continued winds. Being so small, and of such low specific gravity, they are occasionally carried to still greater distances; and thus no islands, however remote, are altogether without them. The eggs of insects, being often deposited in borings or in crevices of timber, may have been conveyed long distances by floating trees, as may the larvae of those species which feed on wood. Several cases have been published of insects coming on board ships at great distances from land; and Darwin records having caught a large gra.s.shopper when the ship was 370 miles from the coast of Africa, whence the insect had probably come.
In the _Entomologists" Monthly Magazine_ for June 1885, Mr. MacLachlan has recorded the occurrence of a swarm of moths in the Atlantic ocean, from the log of the ship _Pleione_. The vessel was homeward bound from New Zealand, and in Lat. 6 47" N., Long. 32 50" W., hundreds of moths appeared about the ship, settling in numbers on the spars and rigging.
The wind for four days previously had been very light from north, north-west, or north-east, and sometimes calm. The north-east trade wind occasionally extends to the ship"s position at that time of year. The captain adds that "frequently, in that part of the ocean, he has had moths and b.u.t.terflies come on board." The position is 960 miles south-west of the Cape Verde Islands, and about 440 north-east of the South American coast. The specimen preserved is Deiopeia pulch.e.l.la, a very common species in dry localities in the Eastern tropics, and rarely found in Britain, but, Mr. MacLachlan thinks, not found in South America. They must have come, therefore, from the Cape Verde Islands, or from some parts of the African coast, and must have traversed about a thousand miles of ocean with the a.s.sistance, no doubt, of a strong north-east trade wind for a great part of the distance. In the British Museum collection there is a specimen of the same moth caught at sea during the voyage of the _Rattlesnake_, in Lat. 6 N., Long. 22-1/2 W., being between the former position and Sierra Leone, thus rendering it probable that the moths came from that part of the African coast, in which case the swarm encountered by the _Pleione_ must have travelled more than 1200 miles.
A similar case was recorded by Mr. F.A. Lucas in the American periodical _Science_ of 8th April 1887. He states that in 1870 he met with numerous moths of many species while at sea in the South Atlantic (Lat. 25 S., Long. 24 W.), about 1000 miles from the coast of Brazil. As this position is just beyond the south-east trades, the insects may have been brought from the land by a westerly gale. In the _Zoologist_ (1864, p.
8920) is the record of a small longicorn beetle which flew on board a ship 500 miles off the west coast of Africa. Numerous other cases are recorded of insects at less distances from land, and, taken in connection with those already given, they are sufficient to show that great numbers must be continually carried out to sea, and that occasionally they are able to reach enormous distances. But the reproductive powers of insects are so great that all we require, in order to stock a remote island, is that some few specimens shall reach it even once in a century, or once in a thousand years.
_Insects at great Alt.i.tudes._
Equally important is the proof we possess that insects are often carried to great alt.i.tudes by upward currents of air. Humboldt noticed them up to heights of 15,000 and 18,000 feet in South America, and Mr. Albert Muller has collected many interesting cases of the same character in Europe.[173] A moth (Plusia gamma) has been found on the summit of Mont Blanc; small hymenoptera and moths have been seen on the Pyrenees at a height of 11,000 feet, while numerous flies and beetles, some of considerable size, have been caught on the glaciers and snow-fields of various parts of the Alps. Upward currents of air, whirlwinds and tornadoes, occur in all parts of the world, and large numbers of insects are thus carried up into the higher regions of the atmosphere, where they are liable to be caught by strong winds, and thus conveyed enormous distances over seas or continents. With such powerful means of dispersal the distribution of insects over the entire globe, and their presence in the most remote oceanic islands, offer no difficulties.
_The Dispersal of Plants._
The dispersal of seeds is effected in a greater variety of ways than are available in the case of any animals. Some fruits or seed-vessels, and some seeds, will float for many weeks, and after immersion in salt water for that period the seeds will often germinate. Extreme cases are the double cocoa-nut of the Seych.e.l.les, which has been found on the coast of Sumatra, about 3000 miles distant; the fruits of the Sapindus saponaria (soap-berry), which has been brought to Bermuda by the Gulf Stream from the West Indies, and has grown after a journey in the sea of about 1500 miles; and the West Indian bean, Entada scandens, which reached the Azores from the West Indies, a distance of full 3000 miles, and afterwards germinated at Kew. By these means we can account for the similarity in the sh.o.r.e flora of the Malay Archipelago and most of the islands of the Pacific; and from an examination of the fruits and seeds, collected among drift during the voyage of the _Challenger_, Mr. Hemsley has compiled a list of 121 species which are probably widely dispersed by this means.
A still larger number of species owe their dispersal to birds in several distinct ways. An immense number of fruits in all parts of the world are devoured by birds, and have been attractively coloured (as we have seen), in order to be so devoured, because the seeds pa.s.s through the birds" bodies and germinate where they fall. We have seen how frequently birds are forced by gales of wind across a wide expanse of ocean, and thus seeds must be occasionally carried. It is a very suggestive fact, that all the trees and shrubs in the Azores bear berries or small fruits which are eaten by birds; while all those which bear larger fruits, or are eaten chiefly by mammals--such as oaks, beeches, hazels, crabs, etc.--are entirely wanting. Game-birds and waders often have portions of mud attached to their feet, and Mr. Darwin has proved by experiment that such mud frequently contains seeds. One partridge had such a quant.i.ty of mud attached to its foot as to contain seeds from which eighty-two plants germinated; this proves that a very small portion of mud may serve to convey seeds, and such an occurrence repeated even at long intervals may greatly aid in stocking remote islands with vegetation.
Many seeds also adhere to the feathers of birds, and thus, again, may be conveyed as far as birds are ever carried. Dr. Guppy found a small hard seed in the gizzard of a Cape Petrel, taken about 550 miles east of Tristan da Cunha.
_Dispersal of Seeds by the Wind._
In the preceding cases we have been able to obtain direct evidence of transportal; but although we know that many seeds are specially adapted to be dispersed by the wind, we cannot obtain direct proof that they are so carried for hundreds or thousands of miles across the sea, owing to the difficulty of detecting single objects which are so small and inconspicuous. It is probable, however, that the wind as an agent of dispersal is really more effective than any of those we have hitherto considered, because a very large number of plants have seeds which are very small and light, and are often of such a form as to facilitate aerial carriage for enormous distances. It is evident that such seeds are especially liable to be transported by violent winds, because they become ripe in autumn at the time when storms are most prevalent, while they either lie upon the surface of the ground, or are disposed in dry capsules on the plant ready to be blown away. If inorganic particles comparable in weight, size, or form with such seeds are carried for great distances, we may be sure that seeds will also be occasionally carried in the same way. It will, therefore, 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 hung overhead. The pieces picked up were wet, and varied from single blades of gra.s.s to tufts weighing one or two ounces. A similar shower occurred a few days earlier in Denbighshire, and was observed to travel in a direction contrary to that of the wind in the lower atmosphere.[174] There is no evidence of the distance from which the hay was brought, but as it had been carried to a great height, it was in a position to be conveyed to almost any distance by a violent wind, had such occurred at the time.
_Mineral Matter carried by the Wind._
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.