Popular Science Monthly/Volume 17/July 1880/On the Modes of Distribution of Plants
|ON THE MODES OF DISTRIBUTION OF PLANTS.|
By JOSEPH F. JAMES.
THE study of the geographical distribution of plants over the earth is one of the most profound interest, not only to the botanist but to mankind in general. To the former it is of especial interest on account of the intimate relations existing between it and the origin of the different species of plants. Where we find an isolated example of a group of plants existing in one country, while its nearest congeners are in another perhaps thousands of miles off, we naturally feel interested in trying to discover the cause of this wide separation, and the means by which the plant has reached its present location. It is to the means of distribution that we shall devote this paper.
It is a trite remark that although there may be places identical in temperature, in soil, in humidity, and other circumstances governing the stations of plants in both North America and Europe, and in South America and Africa, still it does not necessarily follow that the species of plants in these identical localities are alike or even at all similar. Indeed, researches show it to be rarely or never the case. In almost every country, however, there seems to be a certain though sometimes a small proportion of plants which are found in other and distant parts of the world. For instance, Mr. Brown found that, out of 4,100 species of plants then known to inhabit Australia, 166 were identical with those of Europe, and that the greater proportion of these were cryptogamous plants, while those that were not were plants common to the intervening regions. There are 359 indigenous plants out of the 2,277 phænogams given in the last edition of Gray's "Manual," which are also indigenous to Europe. A number, too, of the plants of eastern North America are common to northeastern Asia, China, Japan, and India. Out of a collection of 600 plants from the river Congo in Africa, Dr. R. Brown found thirteen which also grew on the opposite coasts of Brazil and British Guiana. No less than one fifth of the algæ from the Antarctic seas, exclusive of the New Zealand and Tasmanian groups, have been identified by Dr. Hooker with British species. A few of the most remarkable cases of distribution of identical species will no doubt be of interest here. Sauvegesia erecta grows in the Antilles, in Brazil, in Madagascar, and in Java; Scirpus maritimus grows in North America, in Europe, in western India, in Senegal, at the Cape, and in Australia; Brasenia peltata grows in the United States, in Japan, in eastern India, and in Australia. Several species of the mosses (Funaria, Dicranum, and Bryum) are common to the Blue Mountains of Jamaica, the Peak of Teneriffe, and Lapland. Among 133 plants from one of the Pyrenees, Raymond found thirty-five identical with those of Melville Island in the Arctic Ocean. Potentilia anserina grows in North America from Pennsylvania to California and northward, in the northern part of the Old World, in Chili, and in New Zealand; Monotropa uniflora from Canada to Louisiana, in Oregon, in New Granada (South America), and Himalaya Mountains in India; Dichondora repens from Virginia to Chili, in New Zealand, in Tasmania, to eastern Africa, and at the Cape of Good Hope; Adiantum pedatum is found in the eastern United States and Canada, to Oregon, in Kamtchatka, Japan, and Nepaul in India; Crantzia lineata in the United States from Massachusetts to Texas, in South America from Buenos Ayres to Falkland Islands, and in New Zealand; Phleum alpinum inhabits the United States, Switzerland, and the Straits of Magellan.
These few cases will suffice to show the strange and apparently capricious distribution of plants. All these are, of course, supposed to be indigenous to the various countries given as their habitats. Now, according to the theory of natural selection and of descent with modification, we must suppose that all plants have descended from parents like themselves, and have not been specially created where they are now found. When we find, therefore, two plants of the same species, or of the same genus closely allied to each other, inhabiting the United States and Europe, or Europe and New Zealand, we must naturally suppose that at some time or other they had descended from the same kind of an ancestor, but that owing to circumstances they have become widely separated. Plants are not like animals, endowed with locomotive organs, and they must therefore have depended on the elements to transport them. To try and discover these modes of transport, then, we shall now proceed.
The winds undoubtedly exercise an immense influence on the distribution of plants. Many seeds are furnished with a pappus or feathery appendage, by means of which they are easily carried along by the wind. Many of these belong to the Compositæ, such as the dandelion, the thistles, hieraciums, etc. Others are provided with wings, as in the ash and the maple; still others with cottony or feathery tails, as in the anemones and clematis. Again, many are so minute as to be visible to the eye only in the form of smoke, and are so numerous as to be almost uncountable. This is especially the case with fungi, mosses, lichens, and ferns. The spores of fungi are so minute as to require a microscope to see them, and so numerous that Fries says he counted in a single specimen of Reticularia maxima no less than 10,000,000. What wonder, then, that with seeds so minute and so numerous these plants should be almost universally distributed? Out of 200 lichens, for instance, brought home to England by the Antarctic Expedition under Sir James Ross, almost every one has been ascertained to be also an inhabitant of the northern hemisphere, and most of them of Europe. It is easy enough to imagine the wind capable of transporting minute spores to immense distances over land and ocean. Many plants not possessing small seeds are carried off bodily by the wind to distant localities. Of these there is the "leap-in-the-field," or the "wind-witch," inhabiting the steppes of southern Russia. "A poor thistle-plant," says Schleiden, "it divides its strength in the formation of numerous dry slender shoots, which spread out on all sides, and are entangled with one another. . . . The domes which it forms upon the turf are often three feet high and sometimes ten to fifteen in circumference, arched over with naked, delicate, thin branches. In the autumn the stem of the plant rots off, and the globe of branches dries up into a ball light as a feather, which is then driven through the air by the autumnal winds over the steppe. Numbers of such balls often fly at once over the plain with such rapidity that no horseman can catch them; now hopping with short, quick springs along the ground, now whirling in great circles round each other, rolling onward in a spirit-like dance over the turf, now caught by an eddy, rising suddenly a hundred feet in the air; often one 'wind-witch' hooks on to another, twenty more join company, and the whole gigantic yet airy mass rolls away before the piping east wind."
Still another plant, the so-called "rose of Jericho," but really one of the Cruciferæ, has a similar method of dissemination. Professor Lindley says of it: "At the end of its life, and in consequence of drought, its texture becomes almost woody, its branches curve up into a sort of ball, the valves of its pods are closed, and the plant holds to the soil by nothing but a root without fibers. In this state the wind, always so powerful on plains of sand, tears up the dry ball and rolls it upon the desert. If in the course of its violent transmission the ball is thrown upon a pool of water, the humidity is promptly absorbed by the woody tissue, the branches unfold, and the seed-vessels open; the seeds, which, if they had been dropped upon the dry sand, would never have germinated, sow themselves naturally in the moist soil where they are sure to be developed, and the young plants will be certain of nourishment. Specimens of this curious production are sometimes brought from Palestine, and, although they may be many years old, will, if placed in water, start, as it were, from their slumbers, and assume all the appearance of plants suddenly raised from the dead." The Selaginella convoluta, one of the Lycopodiacecæ, and a native of South America, has the same strange habit; for, when the ground where it grows becomes parched and dried up, it curls itself up in a ball, loosens itself from the earth, and is then whirled along over the ground by the wind. When it reaches a place suitable for its growth it uncurls itself, takes root, and flourishes till its new home dries up, when it betakes itself in the same manner to a new locality.
The brief but violent hurricanes of the tropics, which sweep over the land, uprooting trees, overturning houses, and leaving death and desolation behind them, would contribute greatly to a wide dispersion of seeds which would otherwise be but slightly distributed. The tornadoes and cyclones which not infrequently visit the temperate parts of North America would also act a part in this work. An account given by Humboldt shows a possible means of transport over high hills or even mountains. He says "M. Boussingault and Don Mariano de Rivero saw in the middle of the day, about noon, whitish, shining bodies rise from the valley of Caracas, to the summit of the Scilla, five thousand seven hundred and fifty-five feet high, and then sink down toward the neighboring seacoast. The movements continued uninterruptedly for the space of an hour, and the objects, which were at first taken for a flock of small birds, proved to be small agglomerations of straw or blades of grass. Boussingault sent me some of the straws, which were immediately recognized by Professor Kunth for a species of Vilfa (V. tenacissima), a grass which, together with Agrostis, is very abundant in the provinces of Caracas and Cumana."
Let us now turn to another method of transport. As we have seen that, as a general thing, only light seeds, or those with a feathery appendage, are capable of being distributed by the wind, so we shall find that the ones dispersed by means of ocean-currents are of an entirely different character. This must necessarily be the case; for those capable of resisting the action of sea-water for a long time must be inclosed in hard shells. The Gulf Stream, that river of the ocean, is of great use in this work. By its means, seeds of Entada scandens, and other plants of the West Indies and tropical America, are annually thrown upon the shores of Ireland, Scotland, Norway, and even as far north as Spitzbergen. That many of these seeds would be capable of germinating, and of continuing to thrive if the climate were suitable, there can be no doubt. A plant of Guilandina bondue, one of the Leguminosæ, was raised from a seed cast on the west coast of Ireland. Logs of wood and bodies of Indians, which had been conveyed by ocean-currents from the West Indies, have been cast on the shores of the Azores and Madeira Islands. A Sapindus saponaria, the common soap-berry tree of the West Indies, was raised from a seed found on the south shore of one of the Bermuda Islands. The fact already noticed, of some of the plants on the coast of Brazil and British Guiana being identical with some from the banks of the lower Congo, can be accounted for on the supposition that the seeds were carried from one place to the other by an ocean-current. This becomes still more reasonable when we find that the equatorial current of the Atlantic sweeps up the west coast of Africa until after it has passed the mouth of the Congo, and then crosses the Atlantic to the coast of Brazil. We find, also, that the seeds of all these plants, common to both coasts, are incased in hard coverings, and are the ones of all others capable of resisting the action of the sea-water.
The many islands of the Pacific Ocean have undoubtedly been planted with the cocoanut-palm by ocean-currents. Growing as it does in close proximity to the shore, and thriving on salt and salt water, the nuts could be easily carried out on the ocean by the tide, and then be drifted miles away from the place of growth.
Mr. Darwin found that, out of eighty-seven kinds of seeds, sixty-four germinated after an immersion of twenty-eight days, and a few survived an immersion in salt water of one hundred and thirty-seven days. He found that ripe hazel-nuts sank immediately, but that when dried they floated for ninety days and then germinated; an asparagus plant with ripe berries floated for twenty-three days, and when dry for eighty-five days, and the seeds when planted germinated. Altogether, out of ninety-four plants, eighteen floated for above twenty-eight days, and some of the eighteen for a much longer period. Estimating the average rate of the several Atlantic currents at thirty-three miles a day, Darwin came to the conclusion that seeds of one tenth of the plants of a flora, after being dried, could be floated across a space of sea nine hundred miles wide, and would then, if driven to a favorable locality, be capable of germination. Seeds have sometimes been found lodged in trees completely protected from contact with the atmosphere. Rocks are known to be lodged in roots of trees, and these, floating on the ocean, are often drifted to islands, and the rocks taken out by the natives. Mr. Darwin thinks that seeds could often effect a lodgment in the crevices with these stones and thus be conveyed long distances.
Captain Mitchell says he passed through a mass of sea-weed, etc., twelve to fourteen miles across, when three hundred miles from the mouth of the Gambia, which, as Dr. Dickie, who noticed the fact, believed, had come from some part of America within the influence of the Gulf Stream. "Besides algæ," says Mr. Bentham, whom we quote, "the portions of this mass picked up by Captain Mitchell and examined by Dr. Dickie contained, among other substances, fruits, seeds, and 'seedling plants several inches long, all with a pair of cotyledons, roots, and terminal bud, quite fresh.'" Even if, as is suggested in the article quoted, the mass had come from some African instead of an American river, it still shows one means by which seeds could be dispersed. Wallace gives further confirmation of this fact. He says: "Rafts of islands are sometimes seen drifting a hundred miles from the mouth of the Ganges, with living trees growing on them, and the Amazon, Orinoco, Mississippi, Congo, and most great rivers produce similar rafts. Spix and Martins declare that they saw at different times, on the Amazon, monkeys, tiger-cats, and squirrels being thus carried down the stream. . . . Admiral Smyth informed Sir Charles Lyell that among the Philippine Islands after a hurricane he met with floating masses of wood with trees growing upon them, so that they were at first mistaken for islands, till it was found that they were rapidly drifting along. . . . The fact of green trees so often having been seen erect on these rafts is most important; for they would act as a sail by which the raft might be impelled in one direction for several days in succession, and thus at last reach a shore to which a current alone could never have carried it." Now, if such rafts as these were capable of conveying large animals, it would be extremely probable that they would also have on them seeds of many kinds of plants; and, as we shall see hereafter, as the animals themselves often convey unintentionally seeds sticking to their coats, they too would be vehicles for their transportation.
Besides the rafts floated down the rivers, it is very probable that those which overflow their banks periodically, as the Nile, the Ganges, Amazon, Orinoco, and Mississippi, or occasionally as many other rivers do, would transport seeds from plants growing at their sources to hundreds of miles below. Darwin gives the details of an experiment he tried, which illustrates in a remarkable manner the extent to which the mud of rivers and ponds is charged with seeds waiting for a chance to develop. He says: "I took, in February, three tablespoonfuls of mud from three different points beneath water on the edge of a little pond; this mud when dried weighed only six and three fourths ounces. I kept it covered up in my study for six months, pulling up and counting each plant as it grew; the plants were of many kinds, and were altogether five hundred and thirty-seven in number; and yet the viscid mud was all contained in a breakfast-cup!" There can be no doubt whatever that, after inundations of the land by the rivers, plants spring up in localities where they were unknown before, and the inference is just that the seeds were conveyed by the water.
Birds, too, furnish another and important means of transport. Many fruits having a seed incased in a hard shell are surrounded by a juicy pulp: such are the cherry, plum, mistletoe-berry, hawthorn, etc. All these are eaten by birds which, assimilating the pulp, cast the stones in their excrement. The parasitic mistletoe has no way of being disseminated but by the birds; these, swallowing the berries, use the pulp and cast the stone on branches of trees with bark suitable for their growth, where they take root and flourish. That the vitality of many seeds is not at all impaired by this process of passing through the stomachs of birds has been incontestably proved. Indeed, it is said that, when the farmers of some parts of England are desirous of making a hedge of hawthorn (Cratægus oxyocantha) grow in a short time, they feed the haws to their turkeys; the stones are rejected in the excrement, and when collected and planted a whole year is gained in the growth of the plant. It is also known by experiments that seeds in the crops of birds are not always injured; for the crop does not secrete gastric juice, and, as it is often not until twelve or eighteen hours after the act of swallowing that the food passes into the stomach, birds which are capable of rapid and prolonged flight could pass over a large tract of land or of sea. Passenger pigeons have been killed in the neighborhood of New York with their crops still full of rice collected by them in the rice-fields of Georgia and Carolina. As it is positively asserted that they will decompose food in less than twelve hours, they must have traveled three or four hundred miles in less than six hours. This is by no means an extravagant estimate, but rather under the mark. Falcons are reckoned the swiftest of all birds. It is recorded that one, sent from the Canaries to Spain, returned to the Peak of Teneriffe in six hours, a distance of about seven hundred and eighty miles. Seeds of wheat, oats, millet, Canary hemp, clover, and beet, germinated after being twelve to twenty-one hours in the stomach of birds of prey; and two seeds of beet germinated after having been thus retained for two days and fourteen hours. Seeds taken out of the crop of a pigeon, which had floated on artificial sea-water for thirty days, nearly all germinated. Hawks are always on the lookout for weary birds, those which have made long journeys; and pigeons and ducks coming from over the sea, as they are often known to do, would be easily caught and devoured by these birds. The bodies are devoured, and the contents of the crop perhaps scattered in a locality favorable to the development of any seeds which might be contained therein. Darwin forced seeds of various kinds into the stomachs of dead fish which were then given to eagles, storks, and pelicans. These birds, after an interval of many hours, passed the seeds in their excrement, or else rejected them in pellets, and several of them were then capable of germination. Some kinds, however, were invariably killed by the process.
Besides this method there is still another. This is by means of dirt or dried mud adhering to the legs and feet of birds. Still drawing on that cyclopædia of learning, Darwin's "Origin of Species," we read: "Although the beaks and feet of birds are generally clean, earth sometimes adheres to them. In one case I removed sixty-one grains, and in another case twenty-two grains, of dry argillaceous earth from the foot of a partridge, and in the earth there was a pebble as large as the seed of a vetch. Here is a better case: the leg of a woodcock was sent me by a friend, with a little cake of dry earth attached to the shank weighing only nine grains, and this contained a seed of the toad-rush (Juncus bufonis), which germinated and flowered. . . . Professor Newton sent me the leg of a red-legged partridge (Caccabis rufa), which had been wounded and could not fly, with a ball of earth adhering to it weighing six and a half ounces. The earth had been kept for three years, but, when broken, watered, and placed under a bell-glass, no less than eighty-two plants sprang from it; these consisted of twelve monocotyledons, including the common oat and at least one kind of grass, and of seventy dicotyledons which consisted, judging from the young leaves, of at least three distinct species. With such facts before us, can we doubt that the many birds which are annually blown by gales across great spaces of ocean, and which annually migrate—for instance, the millions of quail across the Mediterranean—must occasionally transport a few seeds in dirt adhering to their feet or beaks?"
So, too, animals perform a part in this grand work. Many seeds are furnished with hooks or prickles of various kinds, which enable them to cling to the hair and wool of animals. Take Lyell's illustration of the hunted deer as an instance of how this work could be performed: "A deer has strayed from the herd when browsing on some rich pasture, when suddenly he is alarmed by the approach of his foe. He instantly takes to flight, darting through many a thicket and swimming across many a river and lake. The seeds of the herbs and shrubs which have adhered to his smoking flanks, and even many a thorny spray which has been torn off and fixed itself in his hairy coat, are brushed off again in other thickets and copses. Even on the spot where the victim is devoured, many of the seeds which he has swallowed immediately before the chase may be left on the ground uninjured and ready to spring up in a new soil."
As Lyell remarks in this quotation, many of the seeds which animals swallow may pass through the stomach and still retain vitality enough to sprout after being left on the ground. Instances of this can be seen in almost every barnyard, where the grains of corn and oats dropped in the excrement of cows and horses sprout, if not picked up by the barnyard fowls. Farmers know well, too, that a field manured with fresh manure is likely to produce not a few weeds along with its legitimate crop.
Even such insignificant forms of life as insects may and do perform a part in the transportation of seeds. From a small packet of locusts' dung received from South Africa, Mr. Darwin extracted and raised seeds of seven grass-plants, which belonged to two species of two genera. These locusts are sometimes found as far as three hundred and seventy miles from land; and an account is given of a cloud which hovered round the Island of Madeira for three days, and then disappeared without alighting. Such a cloud as this would undoubtedly be capable of introducing the seeds of foreign plants into insulated localities. The immense number of grasshoppers which have devastated the plains of Kansas and Nebraska would in the same way be the means of introducing seeds of foreign plants.
There is still another method which has been at times used by Nature for the distribution of plants, and that is by means of the alternation of hot and cold epochs, commonly known as glacial periods. Now, it has been demonstrated beyond all doubt that at one period of the earth's history the Arctic regions were much warmer than they are at present; this is proved by the occurrence in the geological formations of these high northern latitudes of plants in a fossilized state, which were utterly incapable of existing in any latitude where the climate was colder than it is now in our temperate regions. Reasoning from analogy and our knowledge of the present distribution of Arctic plants, it would not be improbable that the plants inhabiting the lands of the pole were the same on all longitudes of the Arctic Circle. Let us, then, suppose the glacial period to commence in these warm lands. Each plant, following to a greater or less degree the longitudinal line on which it grew, would be slowly but steadily driven by the increasing cold to take refuge in warmer and more southern stations. The cold, in the course of years following them slowly up, would compel them to keep continuing their journey southward until such time as the maximum of cold had been reached. Then, if, as it is reasonable to suppose, many of these plants had migrated on the longitudinal line upon which they had lived directly southward, we would find that the plants, which at the Arctic Circle, or beyond, had lived in close proximity to each other, would be separated when they reached the temperate zone by hundreds of miles.
In the general journey southward, the plants of the mountains would descend to the plains and mingle with those of the far north. Then the climate commences to moderate; and, as the mountains of ice and snow retire to their original homes in the north, many of the plants would keep company with the cold and return, but many others, encountering mountains in their paths, would find the climate cold enough for their growth, and would be left there in isolation while their nearest relatives would be separated from them by hundreds of miles of country.
Now would come the cold period of the southern hemisphere and drive the plants inhabiting the country there northward, and these again would retire south or up the mountains of the tropics when the cold moderated and the warm season again commenced. Then another season of glacial cold begins in the north. The plants which had before been left on the mountains would stand a good chance of being driven by the increasing cold to the plains, and still farther south, even perhaps across the equator into the southern hemisphere, and when the cold again decreased would retire to the fastnesses of the mountains.
If we accept this view of the influence of glacial periods on the vegetable kingdom, we shall see that many of the apparently anomalous cases of distribution mentioned in the first part of this article are explained. We can easily see why some of the inhabitants of the temperate and arctic zones of the northern are represented in corresponding zones in the southern hemisphere; it is easily explained why identical species of mosses are found in Lapland, on mountains of Jamaica, and the Peak of Teneriffe; why plants of the Pyrenees are identical with those of the Arctic regions; why species are found on the White Mountains of New Hampshire and in Greenland, but not in the intervening region, and why species are found in northern Europe and America, in Chili and New Zealand. This theory of alternate hot and cold periods is as yet the only one by means of which these cases can be explained.
Such, then, are some of the natural methods for the distribution of plants; the air, the water, beasts, birds, and fishes, as we have shown, all perform their several offices; but there is still another method of transport of which nothing has been said, and this is the part which man plays in the grand work. This is by no means insignificant, and can be shown in many ways. Out of the 2,582 species given in Gray's "Manual of Botany", there are 305 introduced species, and, of these, 278, all but 27, were imported from Europe. Nothing shows more strikingly man's influence than this fact, which is further corroborated by the assertion that the greater part of the plants naturalized at the Cape of Good Hope and in Australia are of European origin. With the great increase of facilities for travel, on land and on sea, with the extension of commerce to all quarters of the globe, and with the settlement and consequent clearing off of formerly unoccupied lands, we find both the fauna and flora of many countries greatly modified. There can be no more striking example of this influence of mankind then that shown in the Island of St. Helena. "When St. Helena was discovered, about the year 1506," says Lyell, "it was entirely covered with forests, the trees drooping over the tremendous precipices that overhang the sea. Now, says Dr. Hooker, all is changed; fully five sixths of the island is entirely barren, and by far the greater part of the vegetation which exists, whether herbs, shrubs, or trees, consists of introduced European, American, African, and Australian plants, which propagated themselves with such rapidity that the native plants could not compete with them. These exotic species, together with the goats, which, being carried to the island, destroyed the forests by devouring all the young plants, are supposed to have utterly annihilated about one hundred peculiar and indigenous species, all record of which is lost to science, except those of which specimens were collected by the late Dr. Burchell, and are now in the herbarium at Kew."
Dr. Burchell himself sowed on a point of this island, in 1845, seeds of Chenopodium ambrosioides, and it multiplied so rapidly that, in four years, it became one of the commonest weeds on the island, and has maintained its ground ever since. Erigeron Canadense, introduced from America into Europe, has become there one of the commonest weeds, and is now naturalized all over the country, Datura stramonium, now known over almost all Europe and North America, was introduced from the East Indies by the gypsies, who used the seeds as a medicine. Œnothera biennis, introduced from America into Europe by the French, on account of its esculent roots, in 1674, has since spread so that it now grows wild in almost every country in Europe, in the hedges and about the villages. Our agave and Opuntia vulgaris have both been so extensively naturalized in the south of Europe, and they form so conspicuous a feature in the landscape that they have been noticed by many travelers and recorded as indigenous. Anacharis Canadensis (water-weed) was introduced into England in 1841, and spread so rapidly that it has become a nuisance by impeding navigation in rivers and canals, in spite of efforts made to eradicate it. Strange to say, nothing of the kind is complained of here in its native country. In the district of Canterbury, New Zealand, Mr. Locke Travers, writing in 1863, says Polygonum aviculare (common dock) and sow-thistle (Sonchus) grow luxuriantly; the water-cress increases in the still waters and rivers so rapidly as to threaten to choke them up entirely, and to put the inhabitants to the expense of £300 annually to keep open a single stream. Stems of this cress have been measured twelve feet long and three fourths of an inch in diameter. "In some mountain districts the white clover is displacing the native grasses; and foreign trees, such as poplars and willows, and gum-trees of Australia, are growing rapidly."
All these plants have been introduced into the different countries by man's agency. Numbers of seeds are no doubt conveyed in the raw-hides taken from one country to another. De Candolle says he found numbers of exotic plants growing in the vicinity of a place in the south of France where the hides brought by ships were washed and cleaned. Ballast-heaps near large seaport towns are favorite places of resort for resident botanists, who there often reap rich harvests of introduced plants. To show their number, witness the long lists given by Mr. Isaac Martindale of those found near Philadelphia, and of Mr. Charles Mohr, of Alabama, of those found at Pensacola, New Orleans, and other places. Wars, too, are the means of spreading plants. It is said that great numbers of new plants have been found in France, in the vicinity of places where the Germans had brought forage for their horses and stacked it. Of course, many plants introduced in this way do not thrive longer than a year or two, but some of them no doubt take up their residence permanently. Man's propensity to seek out attractive plants in far off countries, and to transplant them to his home to make his garden attractive, has been the means of the naturalization of many species. These have ample opportunity to escape into the surrounding country, and with favorable conditions to spread extensively in all directions. Seeds of many weeds are mixed with the wheat and other grains which man carries with him wherever he goes, and plants wherever he may happen to settle. Railroads are efficient agents in the work. Seeds are lodged on the platforms of the cars, are carried along by the wind created by the passing trains, and in many other ways are distributed along the track. There is an instance of the work of the railroad in the east end of Cincinnati, near Fulton, on the Little Miami Railroad. For the last two or three years there have been growing great numbers of Euphorbia marginata, a plant which is a native of the plains of Kansas, and which is slowly but surely working its way toward the East by means of the railroads. Eastern plants, which a number of years ago were wanted in exchange with the West, are now naturalized in the West, and vice versa.
- Lyell, "Principles of Geology," ii., p. 387.
- Lyell, ibid., ii., p. 394.
- Lyell, ibid., ii., p. 359.
- Jussieu, "Elements of Botany," p. 718.
- Gray's "Manual of Botany," p. 55.
- Humboldt's "Travels," i., p. 115.
- Jussieu, loc. cit., p. 712.
- These five and many others are noticed in an article by Professor Asa Gray, in "Silliman's Journal," second series, vol. xxiii., p. 381, et seq.
- Humboldt, loc. cit., i., p. 423.
- Quoted from Lindley by Lyell, "Principles of Geology," vol. ii., p. 390.
- "Aspects of Nature," p. 247.
- Jones, "Naturalist in the Bermudas," p. 190.
- Darwin, "Origin of Species," chapter xii, p. 324.
- Ibid., p. 325.
- Ibid., p. 326.
- Ibid., p. 326.
- Address by Mr. Bentham before the Linnæan Society. "Nature," vol. vi., p. 131.
- Wallace "Geographical Distribution of Animals," vol. i., pp. 14, 15.
- "Origin of Species," pp. 345, 346.
- Lyell, "Principles of Geology," vol. ii., p. 398.
- Figuier, "Reptiles and Birds," p. 193.
- Darwin, loc. cit., p. 327.
- Darwin, p. 326.
- Darwin, ibid., p. 327.
- P. 328.
- "Principles of Geology," vol. ii., p. 397.
- "Origin of Species," p. 327
- Lyell, "Principles of Geology," vol. ii., p. 402.
- Lyell, ibid., vol. ii., p. 457.
- Lyell, "Principles of Geology," ii., p. 402.
- Jussieu, "Elementary Botany," p. 717.
- Willd., "Botany," p. 419.
- Willd., ibid., p. 420.
- Jussieu, ibid., p. 717.
- Lyell, loc. cit., ii., pp. 401, 402.
- Lyell, loc. cit., ii., p. 458.