SECTION III.. The Dispersion of Plants and Diseases.

THE DISPERSION OF PLANTS AND DISEASES.

The dispersion of Diseases and the dispersion of Plants, exhibit analogies which might be little expected, on a superficial view of the enquiry.

We are led to believe, that the earth as a whole, was not covered with vegetation in a day, the geological history of this planet is one of development, and though at first sight this expression of opinion may appear to savour of doubt in the Mosaic record, a more extended acquaintance with the subject, favours rather and confirms Scripture history.

As the peopling of the earth has been a gradual process with the animal creation, so has it been also with the vegetable kingdom. We see at the present day, that plants by various means of transit from place to place, multiply themselves on new soils and in new climes, the same with animals. By other means we observe, or can trace, the extinction from various localities and countries, of members of both the animal and vegetable kingdom.

We learn that originally this planet had a temperature much higher than at present, and that the variation of temperature between the equator and the poles, which we now witness, did not obtain in the earlier condition of the globe. We are given to understand, and not without considerable proof, {65} if not demonstration, that the earth was a vast bog, in which rank vegetation grew, and in which the ichthyosauri and plesiosauri, must have floundered about as unwieldy and loathsome bodies. We can readily conceive a condition of atmosphere at this time to have been loaded with pestiferous vapours of an organized nature; it is entirely in accordance with all we know, that it should have been so. Allied forms of plants to those now in existence, are found in the form of fossils, by which comparisons are made, but how the transition into the present Flora took place, or at what period, it is impossible to say. That these plants should have been entirely destroyed during the revolutions of the earth by earthquakes, and their consequences; the collection of waters into the vacuities formed, and their draining off from other places by elevations of the land, is not to be dwelt on without astonishment; then again the ultimate changes of temperature on the surface of the earth, may have been another element in the history of their extinction. But if we may be allowed to imagine that there were organic germs floating in the vapours of the atmosphere, these would hardly be subject to the same influences as those which depended solely on their fixation to the soil for subsistence. The atmosphere, their native element, being influenced by the commotions from below, would be agitated; vortiginous currents would be established, hurricanes would sweep over the stagnant pool and reeking morass, {66} and the higher regions of the air might have thus given protection to these subtle germs, while almost a total extinction of the elegant ferns, the stately palm, and the towering cane was in course of procedure. Then when the strife of the earth and elements had subsided, these would descend with the gentle breezes, and again find in various spots a local habitation--

"Where blue mists, through the unmoving atmosphere, Scatter the seeds of pestilence and feed unnatural vegetation."

In the new era, when the earth took its present physiognomy, who shall say whether much of the pestiferous matter may not have been enclosed and condensed in the bowels of the earth, and when it is remembered, that earthquakes and convulsions of nature, have invariably preceded the outbreak of {67} any great pestilences, that stinking mists, coming from some unknown regions, and unusual vegetations have made their appearance in concert at these times, what I ask is more natural than to imagine, that they have been let loose during the general convulsion? It may be asked, what is to be said about that revolution of the earth, when the great Deluge spread over the whole face of the globe? It can only be replied, that this is a part of the scheme of cosmogony into which we are not called upon to enter. There are yet strenuous supporters of the partial as well as total submersion of this planet, but whether it be true that the vast torrents which appear to have swept the surface uniformly in a southern direction, were of a date coeval with the deluge, and constituted an essential portion of the phenomena, of which one was, that "the fountains of the great deep were broken up," or whether they were anterior to this catastrophe, will not at all interfere with the conjecture of a very early formation and propagation of the germs of pestilential diseases, for the commotions of a deluge were less likely to interfere with the vapours of the atmosphere, than extensive volcanic and electric disturbances. Moreover, it is rather in favour of this theory, that the {68} regions where the temperature and exhalations most nearly resemble those of the former condition of the earth, are those in which pestilential disorders most frequently arise, and where their virulence has always been most strongly marked.

After the various commotions which left the globe, with its present physiognomy of mountains, plains, valleys, rivers, lakes, and oceans; a new Flora and Fauna appeared to adorn and animate the scene of man's existence. Plants and animals were created apparently in adaptation to the numerous climes, which the seasons in the various latitudes or the elevations of the soil, were prepared to render fruitful and useful each in its own sphere. Besides this, the plants of the same latitude, in some instances, differ materially from each other; in this case it seems that the soil has much to do with this peculiarity, for it is certain that the soil and the contiguous atmosphere, have a close and intimate relation; the drought of the desert depends upon the sand, as humid atmosphere is connected with the morass. To illustrate the tendency which vegetation shews in appropriating one locality more than another, I may quote the following: "Some of the volcanic masses of the Aeolian or Lipari Islands, that have existed beyond the reach of history, are still without a blade of verdure; while others in various parts, of little more than two hundred years date, bear spontaneous vegetation, and the same is seen on two lavas of Etna near each other, for the one {69} of 1536 is still black and arid, while that of 1636, is covered with oaks, fruit trees, and vines."

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 masses 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 circumstances, 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 butterflies, 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.

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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 shining 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 space of an hour. The whitish shining bodies proved to be small agglomerations of straws, or blades of grass, 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 Cumana."

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."

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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, assisting 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, passed 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 passed 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. "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 pass along." "The clouds not only bring us occasionally meteoric stones, hail, and epidemics, but also vegetable seeds."

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, grasses, 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 ships without crews were often seen in the Mediterranean, or afterwards in the North Sea, driving about, and spreading the plague wherever they went on shore."

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 ship'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 shore 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 ashore on the Hebrides. He says, "the sea and rivers waft more seed than sails." The waters of many rivers induce diarrhoea and dysentery. 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." 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 constitutions 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 "languishing 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 circumstances, 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 circumstances in this country, will occasionally send forth a most luxuriant blossom; 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.

In 1846, a large vine, the black Hambro-grape, {79} ripened its fruit out of doors, and was as fine as any green-house production; but during nine years that the vine has been under my inspection, this was the only time I have witnessed such a result.

We are apt to attribute an abundant or scarce fruit season to temperature alone, but this is an error--for we have before remarked, that though certain lands may be in the same degree of latitude, the plants which thrive well on one land, will not do so on the other: in fine, that where reason and analogy would lead one to expect a particular form of vegetation, a totally different Flora is presented to the view. These facts are indeed suggestive of new and important deductions. Is it yet explained why the town of Birmingham should be free from Cholera? There is a large manufacturing population, a great number of poor, the usual overcrowding of individuals in small chambers, a considerable amount of destitution and depravity; irregular habits of living, and unwholesome diet, and doubtless many parts of the town, which on investigation would have yielded all the elements usually considered necessary for the localization of the disease: but no--here was some repelling cause, some opposing agent to the generation and propagation of the pestilential seeds. There are no known laws by which inorganic matter could be supposed to observe such a selection, or such an antagonism. Electricity, magnetism, ozone, gases, exhibit no such elective properties that here they will destroy, and {80} there they will spare; that they can almost depopulate small villages, and scarcely find a victim in Birmingham and Bath. But if we suppose a living, and multiplying matter as the cause of disease, many local causes may conspire to arrest the development of the germs, or perhaps, even utterly destroy them.

4th. As to the time of latency, facts crowd upon us indefinitely, as elements of comparison between vegetation generally, and disease in its early stages and history. The seeds of plants are extraordinarily tenacious of life. What a mysterious arrangement of the ultimate particles of matter must there be, by which the vital force remains apparently inactive for many years, and yet when the conditions arise favourable to its manifestation, as it were by an extraordinary fiat, life appears.

Previous to the year 1715, no broom grew in the King's Park, at Stirling; but in that year a camp was formed there, and the surface of the ground consequently was broken in many places. Wherever it was broken, broom sprang up. The plant was subsequently destroyed; but in 1745 a similar growth appeared after the ground had been again broken for a like purpose. Some time afterwards the park was ploughed up, and the broom became generally spread over it. "In several places in the neighbourhood of Edinburgh," says Professor Graham, "the breaking of the surface produces an abundant crop of Fumaria parviflora, {81} although the same plant had never before been observed in the neighbourhood. It is impossible to say the lapse of time since these were buried, before they were again excited to the performance of all their vital functions." Dr. Graham also gives another proof of the vital force existing in seeds. "To the westward of Stirling there is a large peat bog, a great part of which has been flooded away by raising water from the River Teith, and discharging it into the Forth,--the under soil of clay being then cultivated. The clergyman of the parish standing by while the workmen were forming a ditch in this clay, which had been covered with fourteen feet of peat earth, saw some seeds in the clay which was thrown out of the ditch; he took some of them up and sowed them: they germinated and produced a crop of Chrysanthemum septum. What a period of years must have elapsed while the seeds were getting their covering of clay, and while this clay became buried under fourteen feet of peat earth!"

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What limit can there be to the dispersion of seeds when their vital properties may remain so long unimpaired? The seeds of which we have been speaking were, no doubt many of them, washed away with the waters of the Teith, and carried by the stream into the Forth; and who shall then mark their destination; for we have seen that by such means the most distant lands are supplied with vegetation; for whence come the plants which cover the Coral Islands, unless by the air and the water, and that both contribute, has been incontestably proved. Dr. Lindley states that melon seeds have been known to grow when forty-one years old; maize thirty years, rye forty years, the sensitive plant sixty years, kidney-beans a hundred years. But seeds in general have an indefinite period, apparently, at which they can retain their power of germination; for many of the seeds which had been kept in the herbarium of Tournefort for more than a century, were found to have preserved their fertility.

It has now to be shewn that the germs of disease also retain their vital powers in a state of dormancy during a lengthened period.

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Mead has very judiciously observed, "to breed a distemper, and to give force to it when bred, are two different things." He further remarks, that the seeds of the Plague may confine themselves to a house or two during a hard frosty winter, and be preserved, and again put forth their malignant quality as soon as the warmth of the spring gives them force. It is certainly very remarkable that the Plague of London, which commenced at the latter end of the year 1664, should "lie asleep," as Mead says, from Christmas to the middle of February, and then break out in the same parish.

It has been also known that an infected bed laid by for seven years had done infinite mischief on being again brought into use. Indeed, it is quite uncertain for how long a period woollen, fur, linen, cotton, and other articles may retain infectious matter in a dormant state. It has been supposed by some that in closely packed bed and body clothes a multiplication of the germs may and does take place, nor do I see any reason why this should not be the case, for these articles contain within their structure the effluvia of the animal body, and they may possibly there find sufficient nutriment for their development. Nees von Esenbeck believed that some of the minute Cryptogamia were re-produced in the air, we are not therefore exceeding philosophical conjecture when we imagine a basis and substratum, though an unusual one, for the germs of vegetation. Exclusion from air and light, {84} however, as would be the case in packed-up clothes, would a priori give a better colour to the conjecture, as these are the usual conditions necessary for the growth of seeds.

Small Pox and Cow Pox matter, which are now proved to be the same virus, the former modified by having been through a process of growth and maturation in the cow, are both remarkable for exhibiting their active properties after having lain dormant for a considerable time. And each, though so closely allied, retaining its specific properties.

This peculiarity in the history of Small Pox virus suggests a comparison with some phenomena of vegetation, viz. that of grafting or budding. The lower Cryptogamia in their fructifications resemble rather multiplication by buds than by seeds. M. Moyen's idea is that every spore or little globule, independently of its neighbouring one, lives, absorbs, assimilates, grows, and re-produces on its own account; this is certainly the characteristic of the Torula and the Uredo, and doubtless is so of many other of the Cryptogamia, the Protococcus nivalis is another instance. Other modes of cultivation produce also great varieties of results of an unexpected kind.

Would any one, says Dr. Walker, imagine that cabbage, cauliflower, savoy, kale, brocoli, and turnip-rooted cabbage, were the same species? yet nothing is more certain than that they are only varieties produced by the cultivation of the Brassica oleracea, {85} a plant which grows wild on the sea-shores of Europe.

These varieties in vegetables have now become permanent, and though it is supposed that each is liable to return to its original condition, I am not yet certain that such is the tendency. A deterioration is not unlikely to ensue in the course of time, because the propagation by seeds must necessarily very much approach the system of intermarriage, on which Mr. Walker has so ably written and clearly shewn that as a result we may invariably expect a deterioration of the species. Dr. Darwin has also poetically described what his experience taught him.

"So grafted trees with shadowy summits rise, Spread their fair blossoms and perfume the skies, Till canker taints the vegetable blood, Mines round the bark and feeds upon the wood; So years successive from perennial roots, The wire or bulb with lessened vigour shoots, Till curled leaves or barren flowers betray A waning lineage verging to decay; Or till amended by connubial powers, Rise seedling progenies from sexual flowers."

The minute nature of the germs of disease preclude all possibility of their being submitted, as far as we know at present, to the inspection of the physiologist, but we may infer many facts from results. In the same way, though with humbler {86} ideas, as Cuvier could build up an animal from a single bone, can we by a combination of facts infer the existence of living beings and conjecture their forms. "The re-production or generation of living organized bodies is the great criterion or characteristic which distinguishes animation from mechanism." We find the virus of Small Pox, according to Mr. Ceely's experiments, developing itself as a constitutional disease upon the cow, and becoming modified into a form known as the Cow Pox; this resembles the process of cultivation by which a species is converted into a variety, this variety remains for a certain time persistent; the time is not yet known, but it is known that by degrees, as stated above, a deterioration occurs, and fertility becomes impaired, "a waning lineage verging to decay," and this has been observed as a feature in the result of vaccination. I believe Dr. Gregory was one of the first to notice this fact, and deemed it necessary to obtain fresh lymph from the cow; this has been done, and it is not improbable, if the analogy we have drawn be correct, that the slowly spreading scepticism regarding vaccination may be arrested in its progress. If we can explain the deterioration of cow pox virus on this principle we have a hold at once upon the public, and can assure them that the efficacy of the proceeding is as certain as in the time of Jenner. The people, I contend, have a right to demand of us the reason why vaccination is not so efficacious as formerly, and I {87} affirm as unhesitatingly that we are bound to give the subject our most earnest attention.

Now concerning the re-production of Cow Pox matter, and assuming it to resemble that of the lower Cryptogamia, we can easily understand how degeneration in a course of years should ensue, for we find that though the Small Pox is a constitutional disease, that produced by vaccine lymph is a local affection, so that it bears the relation that grafting does to vegetation, and it is not improbable that such a modification takes place in the germs by passing through or becoming generated in the blood of the cow, that they entirely lose their original and characteristic form of reproduction: the seeds of the disease were originally capable of vegetating, if I may be allowed to use the term, by diffusion through the atmosphere; they now, however, have lost that property, and require to be grafted to exhibit any manifestation of vitality.

How often will the seeds of a cultivated fruit grow? If you bud it upon another plant, you obtain a being exactly like the parent, but this, as we have seen, deteriorates in a course of years, we have also seen that the virus deteriorates; but not to stretch this point to an unseemly length, I cannot avoid expressing my conviction, that these are elements of comparison, possessing an interest and a practical utility of no small value.

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I have before said, that the reproduction in the Cryptogamia, rather resembles budding than seeding. If we observe the Torula, or take the process of all formation, generally it will be found to accord more exactly with the budding than the seeding process, and this peculiarity is not confined to vegetation, it is also a marked feature in the reproduction of infusoria, sponges, polypes, &c.

"New buds surround the microscopic plant."

The reproduction of plants and animals appears to be of two kinds, solitary and sexual; the former occurs in the formation of the buds of trees, and the bulbs of tulips.

The microscopic productions of spontaneous vitality propagate by solitary generation only.

We have but reached the threshold of this vast and interesting subject, the experiments which suggest themselves to the mind while reflecting upon it, would alone occupy a whole life of leisure, and I can but feel how forcibly Mr. Sewell's words apply to us: "The grand field of investigation lies immediately before us, we are trampling every hour upon things which to the ignorant seem nothing but dirt, but to the curious are precious as gold."

It is difficult, perhaps, to bring many instances, in which the germs of disease have lain dormant for a lengthened period, because many may take exception to them, from the fact, that sporadic cases of {89} most epidemic and infectious diseases, are rarely absent from any country in which those diseases have become indigenous, and these cases may be said to be the foci whence originates the epidemic constitution of the air; this, however, would not invalidate the supposition, because one of two inferences must be drawn, either that the germs of disease always exist in a dormant state, requiring circumstances and conditions only for their development, or that the germs are imported from some distant locality, where the disease has occurred, and finding a nidus there, grow and multiply. Whichever notion we take, however, matters very little to the fact of the dormancy of the germs, for in both, a certain period elapses between their transmission and their propagation. It may fairly be presumed, that sometimes one method may apply {90} and sometimes the other, perhaps both during general epidemic conditions of the atmosphere.

The Oidium vitis attacked the vines partially last year, and I believe generally spared other forms of vegetation; but this year in my vicinity, cucumbers, melons, and vegetable marrows, are all suffering more or less under the disease. How shall we say, whether are the seeds of last year the cause of the general diffusion at the present time, or were there a sufficient number of old and dormant seeds, universally diffused, and only waiting opportunities for multiplying themselves? We are here on the horns of a dilemma; and spontaneous generation, from which one naturally shrinks, can alone extricate us, if we do not admit diffusion and dormancy. I think I may, without undue assumption, affirm that a period of latency of indefinite duration, applies as cogently to the germs of disease as to those of plants.

There is yet one other point in connection with this subject, and that is the apparent extinction of some diseases, at any rate their non-appearance in certain localities, which had been at one time congenial to them, and in which they flourished. We have seen, in illustrating the dormancy of seeds, that the broom must have been a common plant at {91} some considerable period back, in the King's Park at Stirling, or on that site.

Then again, the appearance of Fumaria parviflora in the vicinity of Edinburgh, in several places where the ground is broken, is sufficiently convincing that this plant must once have been a common form of vegetation there; and as it had never before been observed in the neighbourhood, there must have been a combination of peculiar circumstances capable of rendering germination impossible, otherwise a continued multiplication, as in other forms of vegetation, would have followed of necessity.

But besides these instances, how many are passing under our own eyes of the disappearance of plants under the influence of cultivation, and the generation of the noxious fumes arising from different and innumerable manufactories. In the vicinity of large cities and manufacturing towns, how rarely do we see healthy vegetation; shrubs and animals drag on a sickly and almost unprolific existence, and their term of natural life is much shortened.

And if we compare diseases with this peculiar feature of vegetation, how very close do we find the analogies. The Sweating Sickness which appeared in the latter part of the fifteenth century, and at certain intervals multiplied and extended itself at first only in this country, but ultimately more or less over the continent of Europe, has {92} never since the year 1551 shewn any symptom of productiveness, indeed for all we know the disease may be extinct; on the other hand, it is impossible to say whether or not circumstances may arise, under which it may commence again, to put forth its energies and again desolate the land.

Since 1665, the Bubo-plague has not found a congenial soil in this country, or if the seeds be here, which is more than probable, the necessary conditions to excite them to activity do not exist.

It cannot be imagined that with all the merchandize which comes into this country from the Mediterranean, but that an abundance of the germs of the disease are annually brought into our ports, and disseminated throughout the land. The law by which we have seen that they possess a power of vitality and reproduction, holds now as it did in former times;--the properties of matter never alter, but the conditions under which they exist may be so modified, as to influence their properties, and the usual course of their operations. It is therefore to {93} an alteration or modification of conditions that we are to look for the exemption, during the last two centuries, from an invasion of the Plague. To say what those conditions may be in their totality is difficult, perhaps impossible. We may generalize on the subject, and imagine the reason discovered, but all those causes which were said to have conspired to favour the spread and contamination with Plague, were as distinctly specified and attributed, as the cause of our late infliction with Epidemic Cholera. Why then did we have the Cholera and not the Plague? To what particular element was it--in the mode of living, of destitution, of filth and want of drainage--can it be ascribed that we suffer under one disease, and not under the other?

We have made some few observations and comparisons on the mode of dispersion of plants and diseases,--but there is yet one more point which invites notice. Not only do seasons vary in their effects on vegetation in a remarkable and unexplained manner, but there are many localities to which some special form of vegetation attaches, and which appear to have a power of exclusion of other forms; and as yet I have not been able to trace the connexion, nor can I discover it in the writings of botanists and travellers, who would be most likely to have sought an explanation of so interesting and curious a fact. Dr. Prichard has on this subject some very apposite illustrations. "Still further southward, the austral temperated zone completely {94} changes the physiognomy of vegetation, and the Isle of Norfolk has, in common with New Holland, the Auracania found also in the harbour of Balade, and with New Zealand, the Phormium tenax. It is however remarkable, that this vast island, composed of two lands, separated by a channel, though so near New Holland, and lying under the same latitude, differs from it so completely, that they display no resemblance in their vegetation. Yet New Zealand, so rich in genera peculiar to its soil, and little known, has some Indian plants: such as Pepper, the Olea, and a reniform Fern, which is said to exist in the Isle of Maurice."

I must quote one more passage from Dr. Prichard's excellent work. "We have one instance of an island at no great distance from a continent, having a peculiar vegetation. Mr. R. Brown has remarked, that there is not even a single indigenous species characterising the vegetation of St. Helena, that has been found either on the banks of the Congo, or on any other part of the Western coast of Africa. Does the diversity of marine and atmospheric currents more completely separate this island from the continent, than its situation would imply; or are the nature of soil and other local circumstances, the cause of so marked a diversity? The last supposition seems the most probable; because not only the species of plants, but likewise the genera in St. Helena, are different from those of the African coast." {95}

We are not without instances of diseases, observing this peculiarity which attaches to plants; but their specific characters have hardly been sufficiently considered in reference to climate and situation, together with diet and local influences, to afford us accurate data for comparison. It has, however, been remarked, in every country where Epidemics have prevailed, that some districts or tracts of country, though supposed to possess all the qualities favourable to the development of the diseases, have nevertheless been entirely or nearly free from them. The following passage on the course of the Cholera gives an example of this peculiarity. "Whenever the malady deviated, so to speak, from its normal direction, and passed towards the west, it seemed incapable of propagating itself; and died away spontaneously, even in places which appeared to be well fitted for its reception.--The rich fertile and densely peopled countries to the right of the Dneiper, enjoyed an equal freedom from attack, which can only be explained by the fact that they were situated beyond the line of the disease." With this I close the subject of the diffusion of plants and diseases, though it would require a volume of itself, to record all that has been noticed. I have endeavoured to select such instances as shall mark distinctly the features which point to comparison without overloading the enquiry.

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