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,[25] 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.[26]
{71}
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."[27]
{73}
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.[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 pass 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, 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_."[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 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.[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 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;[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.
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!"[34]
{82}
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.
{83}
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.[35]
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.
{88}
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.[36] 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.[37] 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.[38]
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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