from ordinary septicemia from putrid matter, Koch inoculated several mice with minimal doses of putrid blood, putrid serum, and crystalline lens, all free from bacilli. Only two out of twelve died, and in these the blood was free from bacilli. And further to test the specificity of the Bacillus anthracis, animals were inoculated with a putrid crystalline lens in which had spontaneously grown a kind of sporeforming bacillus not distinguishable from the B. anthracis, but neither it nor its spores could produce splenic fever; nor did the spores of a bacillus cultivated in hay infusion by Professor Cohn. These facts, he thinks, complete the proof of the physiological specificity of the B. anthracis as the cause of the disease. Unless I must add, as above said, no other poison accompanied the bacilli or spores, although even in that case the persistence of the poison is hard to account for if it did not reside in the spores. For matter containing these spores could be dried into dust, again wetted, and dried repeatedly, kept in putrefying liquid for a week, and, according to Pasteur, exposed to boiling heat, without losing its infective power. This, however, is contradicted by Ewart, who found that the spores do not resist boiling heat, nor oxygen at twelve atmospheres. It is difficult to suppose that amorphous protoplasmic particles like graft-germs could stand such an ordeal. Until, therefore, new facts come to light, showing an equal power of survival by graft-germs, we must for the present place the splenic fever in the category of parasitic diseases. Some of the above facts are still disputed, but it would take too long to go into details.

Another disease which may be placed in the same category is the relapsing fever. This is a contagious epidemic which has been recognised as distinct from typhus and other fevers for above thirty years, and since 1872 it is known to be universally attended with the presence of a spirillum in the blood, named, from its discoverer, the

B

Spirillum Obermeieri.* The evidence for the parasitic nature of this disease is similar, although far from being so complete, as that for the splenic fever, so I need not enter into it, but at once allow it to be placed, provisionally † at least, in the same category.

§ 8. We must here pause to point out that in admitting the last category a great leap has been made into the region of hypothesis. In the previous list the parasitic organism was an easily demonstrable morphologically distinct animal or vegetable, and therefore it was comparatively easy to connect it with the disease in the relation of cause and effect, whereas now the most we can do by simple observation is to determine the constant presence of a particular species, or even genus, in the disease, but which can and does exist in other circumstances without being accompanied with the disease. We are therefore compelled to resort to the hypothesis that, although the same in appearance and life history, the parasitic disease-causing-bacterium constitutes a variety physiologically distinct, in that it can secrete a noxious

* This bacterium is, however, not morphologically distinguishable from the Spirochete plicatilis, which is often found in stagnant water, and even in the tartar of the teeth of healthy people. Spirilla are also found in the stomachs of healthy oxen (Cohn), and I may add, from my own observation, that the germs of a spirillum are found abundantly in the Liverpool water, and are swallowed in myriads every day, while relapsing fever is unknown here at present.

+ Provisionally; for it is quite possible that when our knowledge of this disease is complete it may turn out that the parasite is merely a subordinate complication. It appears that spirilla were found in the recent famine fever of India; and it is probable they occurred in the famine fever of Ireland, in 1847. Either some internal cause altered the blood so that the common spirocheta of stagnant water and of the tartar of the teeth could live and multiply in it; or the cause of the famine coincidently acted on the spirochœta so as to produce the evolution of a specific variety, which entered the people and produced the fever. Is this last really probable? Some light may be thrown upon this by a fact observed by Mr. Dallinger and myself in our researches into the life-history of the bacteria, not yet concluded. Wishing to observe the Spirillum volutans separate from the

poison, or at any rate live and grow in the healthy body. This may be so, but it is obvious that we must have other proofs except the bare fact of disease accompanying their presence, for the cause may reside in some other poison while the bacteria are a merely subordinate complication. The strongest arguments for purely physiological specificity are derived from analogy, and were first given by Pasteur and F. Cohn, whose statements may be shortly summed up as follows:-The lactic and the acetous fermentations are chemically different processes, as also the ammoniacal urinary, and the mucous wine fermentation, and yet the organisms which produce these respectively cannot be distinguished by the microscope. The bacterial germs which produce the red, yellow, orange, and blue pigments, cannot be distinguished, yet on sowing them out the distinct colour appears. This obtains among the different genera of bacteria, and we may therefore admit purely physiological species as good species; or at least varieties or races which are persistent and breed true like morphological species. As, in fact, we see in plants,

numerous putrefactive organisms in the matrix-maceration, we tried it in Cohn's nutritive fluid and several other clear media, but in vain; it died speedily in them all, and we were for a time quite baffled. However, recollecting that it was a variety of spirillum which flourished in the blood in relapsing fever, we tried fresh (sheep's) blood serum, and found that it answered perfectly, and we could watch the lineal descendants of a few spirilla for many generations in the moist chamber, under the microscope. It is easy to imagine that a small diminution of healthy vital resistance (§ 12), induced by a non-parasitic cause of fever, might allow the spirilla germs in the drinking-water or the tartar to multiply in the blood, while it might not so act with regard to other saprophytes; for Billroth noticed that the commonest putrefactive organism, viz, Bacterium termo, does not thrive well in blood. But it is very difficult to imagine that the evolution of a particular specific variety of spirillum fit to breed in the blood should happen just at that particular time. Dr. Roberts (§ 8) compares the origin of specific bacteria to the "sporting" of plants; but sports occur only once in many thousand generations; famines cannot certainly be called common; that they should coincide must be a rare event, but that they should always occur together as a mere coincidence passes credibility,

especially cultivated ones; for of two trees, otherwise indistinguishable, one will bear sweet and the other bitter almonds

-the latter being poisonous. In fact, if the parasitic theory of disease be true, this would by analogy furnish an explanation of the origin of infectious diseases; for the sweet and bitter almond, and many kinds of variety in fruits and vegetables, are produced by variation in and descent from common stocks. As an example of such variation, or "sporting," Dr. Wm. Roberts cites from Darwin the origin of nectarines:-"A peach tree, after producing thousands and thousands of peach-buds, will, as a rare event and at rare intervals, produce a bud and branch which ever after bear only nectarines; and conversely, a nectarine, at long intervals and as a rare event, will produce a branch which bears only peaches ever after." Again, in the different sorts of yeast, the formation of races by artificial selection is demonstrated by Rees. Also, just as summer rye is worthless for winter seed, although both races are descended from a common stock, so the upper yeast is useless for Bavarian beer, and almost every kind of wine or beer has its own barm. On these and similar grounds from analogy, F. Cohn constitutes a special group of physiologically specific bacteria, called the pathogenic bacteria. All of these, except two, belong to the globe, or sphero-bacteria, and already distinct names are given to those supposed to belong to the different infectious diseases, such as the micro-sphæra vaccinæ, the micrococcus diphthericus, the micrococcus septicus, &c. The same principle is extended to erysipelas, smallpox, scarlatina, cattle plague, glanders, measles, and even tuberculosis, all of which are supposed to have their physiologically specific micrococcus or sphæro-bacterium not distinguishable from ordinary saprophytes; the only exceptions being the two above diseases, splenic and relapsing fever, which depend on rod-bacteria. Oddly enough these two happen

to be the only ones in which anything like proof of the causal connection with the diseases can be admitted to exist. All the rest, without going into detailed discussion which would be too extensive, I may say at once are as yet entirely destitute of proof and altogether hypothetical. In none of them, except the two already named, has the uniform presence of any single species been demonstrated, far less has the disease been traced to the various organisms which have at different times been found in them. To unravel the complications of the relation of the bacteria and micrococci found in various infective diseases to the disease itself, let us keep in view the different categories in which such organisms can be placed in relation to the disease.

§ 9. These categories are three :-1st, true parasites which live and thrive on healthy animals and plants, and possess special organs to enable them to prey on the juices of the host (suckers: haustoria), and which cannot continue to live without the host; 2nd, fungi which thrive on dead organic substances, and can therefore be cultivated on artifi cial nutriment out of the host; but they are able also to thrive on healthy living animals and plants, and thus excite disease, and in plants consume and destroy them; 3rd, fungi which usually are saprophytes, living on dead organic matter, and unable to attack healthy uninjured tissues, but they can grow and multiply in parts or organisms whose vital powers are lowered by injury or disease (§ 11); in plants, for example, the ordinary moulds will take root and grow on any injured part, especially on fruit, and become injurious parasites. It is evident that the existence of the last category strikes a blow at the doctrine of specific organisms as the sufficient cause of corresponding infectious diseases, The presence of bacteria or other fungi in disease may thus depend on two causes, either, 1st, an alteration in the specific