occurred; and the immediate efficacy of the remedial measures is utterly inexplicable on the hypothesis that the disease-germs were brought from any other source than the marsh. That having found their way into it, they had grown and fructified in its congenial soil so as to produce an abundant crop, by which, rising in germ-clouds and wafted by air-currents, the inmates of the Poor-house who first received it were destructively infected, seems, in the light of our present knowledge, the obvious rationale of this most instructive case. And if accepted in one case, this rationale is applicable to many others in which the same phenomenon presented itself, of a sudden outbreak immediately following a change of wind, which caused an air-current to set from a focus of infection towards the seat of the malady.

The only difficulty in the Baltimore case is to account for the introduction of the cholera-germs into the marsh. The municipal authorities of the city had taken very active and (as the event proved) very efficient means for warding off the pestilence; and although it was very severe at New York, Philadelphia, and Washington, only a few imported cases occurred in Baltimore itself. It is, of course, quite possible that clothing or bedding soiled by the dejecta of these patients might have been sent away to the Poor-house to be washed; and on the whole I think it more probable that some human communication of this kind took place, than that the cholera-germs were brought from a remote distance by the atmosphere. But that a marsh sodden with the excreta of a large population was as prolific a breeding ground as they could meet with, accords with the experience of all who have had the largest opportunities of studying the disease in India, or during the recent outbreak of it in Egypt.

The conditions of the spread of Typhoid or Enteric fever are closely analogous to those of the diffusion of cholera; and the doctrine of disease-germs proves as satisfactorily applicable to the one case as to the other.

But because typhoid germs, when introduced into the human system, breed and multiply within it, and, when voided from the intestine, may be conveyed by the water into which they have found their way into the bodies of other persons, who then become the subjects of the disease, it by no means follows that the human body is their only breeding-ground, or that water is their only vehicle. On the contrary, those who have most carefully studied the subject are now generally agreed, that when typhoid germs have been discharged into sewers, they not only infect their contents, but so develop themselves under favouring conditions (especially warmth, stagnation, and seclusion from the air) as to give rise to an enormous increase of the contagium. And in the case of the wide diffusion of typhoid poison by milk (of which the recent epidemic in Camden Town has afforded an illustrative example), it seems far more probable that the germs introduced by the contaminated water used in washing

the milk-vessels have multiplied by self-development in the milk put into them, than that they should have originally been abundant enough to communicate the disease to so large a number of individuals as are in some instances attacked by it.

Further, that sewer-gas may be the vehicle of typhoid germs, and that they may be drawn into the body by its inhalation, is not only what all analogy would suggest as probable, but accords alike. with the judgment of our ablest pathologists in regard to the essential nature of the disease, and with the experience of our best sanitary authorities as to the mode of its propagation. That the primal seat of Enteric fever is in the blood, and that the various local affections which occur in the course of it are the results of changes set up in the circulating current, is just as clear as it is in small-pox or scarlatina, the worst forms of which may terminate fatally before any cutaneous eruption appears. And when disease-germs are inhaled into the air-cells of the lungs, they have a far more ready access to the blood spread out in the closest capillary network on their walls, than when introduced with food or drink into the alimentary canal.

I have recently had the opportunity of learning, on the spot, the full particulars of a case in which four members of one household were last year attacked with Typhoid fever-one of them narrowly escaping with her life—under circumstances which left no doubt in the mind of the very accomplished physician who had charge of the patients, that the malady originated in the opening of an old cesspool belonging to a neighbouring house, then in course of demolition. The house in which the outbreak took place is large and airy, and stands by itself in a most salubrious situation. The most careful examination failed to disclose any defect either in its drainage or its water supply; there was no typhoid in the neighbourhood; and the milk supply was unexceptionable. But the neighbouring house being old, and having been occupied by a school, its removal had been determined on to make way for a house of higher class; and as the offensive odour emanating from the uncovered cesspool was at once perceived in the next garden, and the outbreak of typhoid followed at the usual interval, the case seems one which admits of no reasonable question.

On the whole, then, the conclusion seems clear, that while the breeding ground of ordinary Malarious germs is the earth alone, and the breeding ground of the germs of the ordinary Exanthemata is the human body alone, there is an intermediate class of pestilential diseases—including cholera, typhoid, and probably yellow fever—in which (as Mr. Simon 5 tersely expressed it) certain microphytes are capable of thriving equally, though perhaps in different forms, either within or without the animal body; now fructifying in soil or waters of appropriate quality, and now the self-multiplying contagium of a bodily disease."

The doctrine that the disease-germs of cholera and typhus breed

5 Article 'Contagion' in Quain's Dictionary of Medicine.

in the human intestine only, and that they are introduced into it by water alone, obviously sets at naught a large proportion of those precautionary measures on which those who are most practically conversant with the subject lay great stress. Everything ought unquestionably to be done to preserve our domestic water-supply from contamination, as well as to secure the purity of its sources; and to disinfect, not only the intestinal dejecta of patients affected with cholera or typhoid, but everything contaminated by them. But we ought not, in doing these things, to leave others undone; and all experience justifies the emphatic warning of the Local Government Board, as to the danger of breathing AIR which is foul with effluvia from the same sorts of impurity'-a danger whose source obviously lies in the atmospheric transportation of disease-germs.

I have left myself but little space for the discussion of the second part of my subject-the bearing of the natural history view of Zymotic diseases upon the question of their origin and mutual relations. It is, doubtless, needful for the purposes of Pathological study, that these diseases should be defined as 'specific types,' just as the Naturalist defines species' of plants or animals; and as, in our preevolution days, it was held that every true species was separated from every other by constant characters genetically transmitted from parent to offspring, so has it been generally believed that the poisons, not only of small-pox, scarlatina, and measles, but of a large number of different forms of fever, as well as of other maladies propagated by contagia, are to be ranked as specifically different.

The species-making Naturalists of the past generation laid greater stress on points of minute difference than on those of general agreement, disregarded the modifying influence of environments,' and selected the strongly-characterised examples for description, neglecting the intermediate forms by which these are often gradationally connected.

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And

But in the light of the modern doctrine of Evolution, the scientific Naturalist makes it his aim to ascertain how the different races of plants and animals have come to divaricate from each other; and studies their respective variations,' as affording the best clue to the origin of their larger and more constant specific' differences. those who have most carefully studied the tribe of 'saprophytes' to which disease-germs belong, have long since come to the conclusion that there are no forms of vegetation whose range of variation' under differences of environment' is so wide; it being yet uncertain, indeed, that we know the entire life-history of any one of

them.

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Now, it has been too much the habit of Pathologists, in scientifically defining specific types of disease, to follow exactly the same course as the species-makers among Naturalists-insisting on minute differences rather than on points of agreement, and assuming that hese differences are constant. Every Practitioner of medicine, on

the other hand, who has had opportunities of observing the same diseases in different localities, at different seasons, and in different epidemics, well knows how greatly their characters vary; hybrid forms' and 'sub-varieties' presenting themselves from time to time, which receive passing notice and then die out. Thus, although no eruptive fevers are more clearly differentiated, when occurring in their characteristic forms, than Measles and Scarlatina, yet cases every now and then occur, in which their symptoms are so mingled as to puzzle the most experienced doctors. I even remember such a hybrid disease to have been epidemic some thirty years ago in the East of London; and as Sydenham, one of the most sagacious medical observers that ever lived, did not separate the two, I cannot but think it probable that this' hybrid' was the disease prevalent in his time. Again, the Small-pox epidemic of 1871 and subsequent years has been characterised by the re-appearance of the 'malignant' type of that disease, which had not previously shown itself in Europe, except in a few isolated cases, during the present century. The whole course of that 'hæmorrhagic' type, when presented in its most characteristic form (in which death occurs before the appearance of the eruption), is so entirely different from that of ordinary smallpox, whether 'confluent' or 'discrete,' that the two diseases might be well accounted specifically different, if it were not certain that they originate in the same contagium. So, again, some of those who have had largest experience of the severest forms of Malarious disease, are satisfied of the unity of causation that underlies variety of manifestation. Thus, says Dr. Haspel, the author of a very able work on the 'Diseases of Algeria' (Paris, 1850), 'fevers, dysentery, and diseases of the liver constitute an indivisible whole under the dominion of a single cause; and those who deny this truth are either misled by theoretical prejudices, or will not make use of their eyes.' It is a significant fact, rightly insisted on by Dr. Maclean, that exactly in proportion as we have banished malaria from the soil of the British islands, so have we got rid not only of ague, but of dysentery and of suppurative inflammation of the liver, as endemic disI have already adverted to changes in the type of fever from 'non-infective' to infective,' of which there seems to me adequate evidence; and I might adduce a number of other instances-such as the difficulty that often occurs in India in discriminating between. Cholera and Enteric fever-in support of my position, that even the best-marked types of Zymotic disease are not distinguishable by constant and invariable characters, but that, just as higher plants are modified by cultivation, so may the germs of these diseases develop themselves in a great variety of modes, giving rise to very different maladies, according to the conditions, whether local or individual, under which their development takes place."

eases.

6

• A very curious example of this kind, which came under the observation of Professor Huxley, when serving as assistant-surgeon in H.M.S.Rattlesnake,' was

6

But the same analogy carries us further, and suggests that the peculiar morbific activity possessed by each specific type of diseasegerm may be derived from the operation of particular environments' on ordinary saprophytes through a long succession of generations, just as among plants of higher types. And this view is borne out by the remarkable influence of artificial 'culture' upon some of those which have been most carefully studied in this manner. It is a fact of great significance, that the malignant Bacillus anthracis of 'charbon' does not differ morphologically in any important character from the innocent Bacillus subtilis of hay infusions; and although it has not yet been certainly shown that any method of treatment can give to the latter the potency of the former, yet it seems not improbable that such will prove to be the case. With Dr. William Roberts, 'I see no more difficulty in believing that the Bacillus anthracis is a "sport" from the Bacillus subtilis, than in believing, as all botanists tell us, that the bitter almond is a "sport" from the sweet almond-the one a bland, innocuous fruit, and the other containing the elements of a deadly poison.'

So, as it seems to me, there is nothing inconsistent with our recognition of Cholera and Typhoid as specific types of disease, in the admission that under some possible conditions they may originate de novo from saprophytic germs not ordinarily capable of engendering such maladies in the human system.

Among my earliest professional recollections, going back to the year 1829, is that of the occurrence of a very remarkable outbreak of a severe malady in a school at Clapham, of a type then quite unknown to practitioners in this country, but which an old Indian doctor, who was asked to see the patients, declared to have the characters of the cholera of India, which was then (as subsequently appeared) on its way towards us, but whose advent no one at that time regarded as probable. Having lately referred to the Medical Gazette of August 22 in that year, I have found this recollection. fully confirmed by the record of the Fatal Cholera at Clapham' published at the time; and cannot hesitate in the belief that if the outbreak (affecting twenty out of twenty-two boys at the school, and the two children of the master, of whom one died after only eleven hours' illness) had occurred during a cholera epidemic, the patients would have been regarded as suffering under that disease. A few days previously, a cesspool had been opened to let off from the playground stagnant water accumulated by the recent heavy rains, and its contents had been distributed over the garden adjoining the boys' playground. Whether true Asiatic cholera or not, this sudden simultaneous outbreak can scarcely be regarded as a mere result of putrescent emanations; it had every character of a specific disease implanted by germs; and the probability seems strong that these related by Sir James Paget in his lecture on 'Specific Diseases,' at the end of the first volume of his Surgical Pathology.