Part 3 (2/2)
In comparing the diffusion of plants, and the diffusion of diseases, the different modes by which this generally has been effected may be considered under heads, that the comparison may be more readily traced.
_First_, seeds are diffused by the atmosphere, either by the prevalence of certain currents, which are produced by known laws, in which case, no difficulty occurs in the explanations; or in a more imperceptible manner, as by those more uncertain atmospheric currents of a partial nature, which, though they seem to have laws governing them, are not yet understood.
_Second_, seeds are transported by water across oceans, &c. when they can be floated on any material by which they are preserved, as by wrecks and ma.s.ses of wood, which have been washed down the rivers.
_Third_, they are conveyed by man to all parts of the globe.
_Fourth_, a period of latency is observed to apply to them, that is, they require certain essential conditions before germination occurs; so that even in some localities, a plant may not have been known to exist in a particular neighbourhood, but by a train of circ.u.mstances, it may make its appearance, and again be a centre of development.
1st. I shall not here wander into the speculation, {70} whether plants had originally one birth-place, as a centre from which they spread by various agencies, as supposed by Linnaeus, nor into any enquiry beyond those facts, which may fairly come within our own comprehension, and within our own means of demonstration.
Many seeds are provided with means adapting them for floating in the atmosphere, these are by pappi, or winglets and hairs, but it cannot be doubted that the agency of atmospheric currents, is productive of considerable effects in the dispersion of lighter seeds, such as those of mosses, fungi, and lichens--lichens have been discovered in Brittany, which are peculiar to Jamaica, and Monsieur De Candolle concludes, that their seeds had been carried thence by the south-westerly winds, which prevail during a great part of the year on this portion of the French coast.
But Humboldt's testimony on the subject of winds is most satisfactory, for he says, ”Small singing birds, and even b.u.t.terflies, are found at sea, at great distances from the coast (as I have several times had opportunities of observing in the Pacific), being carried there by the force of the wind, when storms come off the land.” It is generally believed, from abundance of proofs, that the trade winds, and other continuous currents, are means by which plants are conveyed from one country to another.[26]
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As to the partial currents, Humboldt further says, ”The heated crust of the earth occasions an ascending vertical current of air by which light bodies are borne upwards. M. Boussingault, and Don Mariano De Rivero, in ascending the summit of the Silla, one of the gneiss mountains of Caraccas, saw in the middle of the day, about noon, whitish s.h.i.+ning bodies rise from the valley to the summit of the mountain, 5755 feet high, and then sink down towards the neighbouring sea coast. These movements continued uninterruptedly for the s.p.a.ce of an hour. The whitish s.h.i.+ning bodies proved to be small agglomerations of straws, or blades of gra.s.s, which were recognized by Professor Kunth, for a species of vilfa, a genus, which together with agrostis, is very abundant in the provinces of Caraccas and c.u.mana.”
On the plague of locusts we read, that ”the Lord brought an east wind upon the land, all that day and all that night, and when it was morning the east wind brought the locusts.”
On the Black Death we read, ”There were many locusts which had been blown into the sea by a hurricane, and a dense and awful fog was seen in the heavens, rising in the east, and descending upon Italy.”
Of the Plague of 542, Gibbon says, ”The winds might diffuse that subtle venom, but unless the atmosphere be previously disposed for its reception, the plague would soon expire in the cold or {72} temperate regions of the north. The disease alternately languished and revived, but it was not till a calamitous period of fifty-two years, that mankind recovered their health, or the air resumed its pure and salubrious quality.”
In the history of the Sweating Sickness, of which there were five distinct visitations, we find ample allusions to the atmosphere, and the mode in which the disease was conveyed by this medium.
I quote again from Hecker: ”It seemed that _the banks of the Severn_ were the _focus of the malady_, and that from hence, a true impestation of the atmosphere, was diffused in every direction. Whithersoever the winds wafted the stinking mists, the inhabitants became infested with the sweating sickness. _These poisonous clouds of mists were observed moving from place to place_, with the disease in their train, affecting one town after another, and morning and evening spreading their nauseating insufferable stench. At greater distances, these clouds being dispersed by the wind, became gradually attenuated yet their dispersion set no bounds to the pestilence, and it was as if they had imparted to the lower strata of the atmosphere, _a kind of ferment which went on engendering itself even without the presence of the thick misty vapour_, and being received into men's lungs, produced the frightful disease everywhere.”[27]
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Mr. K. B. Martin, harbour-master of Ramsgate, in a communication to Lord Carlisle on the Cholera of last autumn, says, ”At midnight of the 31st August (1849), the Samson (steam-tug) proceeded to the Goodwin Sands, where the crew were employed under the Trinity agent, a.s.sisting in work carried on there by that corporation. While there, at 3 A.M. 1st September, _a hot humid haze, with a bog-like smell_, pa.s.sed over them; and the greater number of the men there employed instantly felt a nausea. They were in two parties. One man at work on the sand was obliged to be carried to the boat; and before they reached the steam vessel at anchor, the cramps and spasm had supervened upon the vomitings; but here they found two of the party on board similarly affected. Here then is a very marked case without any known predisposing local cause. Doubtless it was atmospheric, and in the hot blast of pestilence which pa.s.sed over them.”
Many more instances might be quoted, to shew that the germs of disease, as well as of plants, are borne on the wings of the wind from place to place {74} in one country, and from one country to another, the distance being no obstacle, however great that may be.[28] ”Dust and sands,” says Sharon Turner, ”heavier than many seeds, are borne by the winds and clouds for several hundred miles across the atmosphere, falling on the earth and seas as they pa.s.s along.” ”The clouds not only bring us occasionally meteoric stones, hail, and _epidemics_, but also vegetable seeds.”[29]
2nd. The transportation of seeds of plants by water requires very little notice; every one is familiar with the mode in which coral islands, which gradually rise out of the sea, become covered with vegetation. ”If new lands are formed, the organic forces are ever ready to cover the naked rock with life.--Lichens form the first covering of the barren {75} rocks, where afterwards lofty forest trees wave their airy summits. The successive growth of mosses, gra.s.ses, herbaceous plants and shrubs or bushes, occupies the intervening period of long but undetermined duration.”
The following may be cited as an instance of the transportation of disease by water. ”Cyprus lost almost all its inhabitants, and s.h.i.+ps without crews were often seen in the Mediterranean, or afterwards in the North Sea, driving about, _and spreading the plague wherever they went on sh.o.r.e_.”[30]
It requires no argument to enforce the conviction that cottons, woollens, furs, skins, &c. will retain the matter of infection for almost an indefinite period; instances of the kind have been already given; it is therefore easy to understand that portions of wrecks and s.h.i.+p's goods would be a frequent though unsuspected source of infection. Dr. Halley mentions a case, in which a bale of cotton was put on sh.o.r.e at Bermuda by stealth; it lay above a month without prejudice, where it was hid, but when opened and distributed among the inhabitants, it produced such a contagion that the living scarce sufficed to bury the dead. Dr. Walker found seeds dropt accidentally into the sea in the West Indies cast ash.o.r.e on the Hebrides.
He says, ”the sea and rivers waft more seed than sails.” The waters of many rivers induce diarrhoea and dysentery.[31] Well water also in many {76} places has a similar effect, especially if any surface drainage happens to find its way into the well.
3rd. The part performed by man himself in the communication of disease to his fellow creatures, is perhaps the most fruitful source of the extensive spread of epidemic and contagious diseases.
In the time of Moses, restrictions were laid on those who had the plague of the leprosy to avoid contagion; the dictum for one so affected was, ”he shall dwell alone; without the camp shall his habitation be.”[32] All the ancient authors believed in the {77} infectious nature of pestilential fevers, and some other diseases; but, according. to Mr. Adams, they held that no specific virus was the cause, and merely a contamination of the surrounding air by effluvia from the sick. Thucydides, Hippocrates, Procopius, Galen, Plutarch, all recognized the property of communicability from one individual to another of the plague; and Hecker, on the epidemics of the middle ages, abounds with instances in support of contagion. As regards small-pox and measles, Rhazes observes particularly the connection that exists between the condition of the air and the severity or mildness of these diseases, remarking that small-pox seldom happens to old men, except in pestilential, putrid, and malignant const.i.tutions of the air in which this disease is usually prevalent.
The history of the introduction of Scarlet Fever, Hooping Cough, Lues, and other diseases into the various countries of the globe, is sufficiently convincing that men carry about with them the seeds of disease; that while these attach themselves to the persons and clothing of those who introduce them into new climes, and flourish independently of cultivation, yet the exotics which they foster with so much care, often disappoint their most sanguine expectations; and these ”languis.h.i.+ng in our {78} hothouses can give but a very faint idea of the majestic vegetation of the tropical zone.” Art in this procedure fails to accomplish here, what nature but too sadly, under some circ.u.mstances, effects most readily. The germs of some diseases though of an exotic character, under congenial influences of various kinds, appear to flourish with native vigour: is it not so, also, with some forms of vegetation? The aloe, a native of Mexico, which lives, but does not thrive well, or reproduce under ordinary circ.u.mstances in this country, will occasionally send forth a most luxuriant blossom;[33] so rare is this, that some say it occurs every 50 or 100 years, but no law seems to be established on this point, any more than the statement that we may expect pestilential diseases at certain intervals. But that there are intervals of _uncertain_ duration when the aloe will blossom, when the grapes will ripen, and a general productiveness of exotics will occur, is as certain as that seasons will occur when contagion will be rife, and a most unusual multiplication of disease prevail. This is not an imaginary or speculative notion,--all observers of seasons and diseases within the last twenty years, may fully verify the statement.
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