agency of the living white corpuscles. On the other hand, the effect of disease in lowering vital resistance is demonstrated by the experiment of Chauveau, who injected into the blood of rams, before the operation of bistournage, a quantity of putrid fluid, containing bacteria, too small to kill the animal; then, when the bacteria and germs were well distributed through the blood, the subcutaneous rupture of tissue, in which the operation consists, was performed. The part being then cut off from its supply of blood, has its power of vital resistance so far lowered that growth of bacteria, putrefaction, and septicemic infection takes place. In the healthy animal the operation is harmless, and the part cut off from the circulation gradually passes into fatty degeneration and atrophy; likewise, the injection of the same amount of putrid matter, without the operation, is harmless, but both together are fatal. By the expression vital resistance is meant merely the necessary incidental result of healthy living action, not any specific property or power, such as the imaginary vis medicatrix naturæ. § 13. We thus see that although fungi may be capable of producing disease primarily, yet the mere fact of their presence in any given disease is no proof that they are directly the cause of it, as there are other ways of accounting for their presence. Doubtless all germs are not present everywhere at the same time, so it may be difficult to account for the invariable presence of one particular form of bacteria in one disease by merely chance spores. But it is already a weak point in the proof of parasitic-germ theory that you can never inoculate with the specific bacterium, say bacillus or spirillum, without at the same time introducing some of the diseased secretion, or graft-germs, which might be the true contagium; so also, if the latter is the true cause, you can never escape inoculating the particular bacterium along with it which is best adapted to thrive in the disease secondarily. We have seen that the hypothesis of physiologically specific bacteria was put forth to account for the pathogenic power of bacteria not visibly different from the chance bacteria. Naegeli* also, while objecting to the idea of truly different species, either for disease or the fermentations, owing to the enormous time required for variation of species, adopts the theory of variation in the secretion, which, he thinks, may be brought about in a much shorter time; and thus may be accounted for the changes of epidemic diseases, and the rapid gain of virulence of septicemic disease. But it seems to me that we have no ground to assume that the varieties of yeast and pathogenic bacteria can be evolved with anything like the required rapidity. And, besides, it is not a mere variation in their secretions which must characteri se the pathogenic bacteria, but it is the adaptation of the bacterium as a parasite, i.e., its constitution must be so changed that it can live and breed and grow from the most minimal dose in the healthy body, which faculty the ordinary bacteria do not possess. Now, it is generally held that parasites are degradations by variation and natural selection from independent species, but that this process, like all change of species, is only effected in the course of years or centuries. This is quite contrary to what takes place in diseases, where, in a few days, or even hours, ordinary chance bacteria are found living and breeding in the living body, on account of a change in the body and not in the bacterium. After all, Naegeli, an adherent of the parasitic-germ theory and the physiological variation of the pathogenic bacteria, finds the theory insufficient to explain the specific nature of the infectious diseases, and is obliged to suppose, in addition, that the bacteria absorb and become charged with a morbid secretion from the diseased host; in fact, that they are also carriers *Die niedern Pilze, von C. v. Naegeli, München, 1877. of an infective or diseased stuff; or, at least, that to produce the full disease some such stuff should be transferred at the same time, whether on or in the bacteria. An analogy for the process is, he thinks, as others have done before him, given by the growth of galls on plants, viz., along with the ovum is deposited by the insect some acrid matter, which excites inflammatory and exuberant growth possessed of less vital resistance than the healthy tissue, and on which, therefore, the ovum can be nourished. Now, no doubt this corresponds to a certain extent with what happens in some of the truly parasitic diseases, e.g., the ringworm; but it opens the door to another cause of the specificity of infectious diseases, viz., morbid secretions, and even graft-germs, which may be the efficient cause, while the bacteria are merely a secondary complication. Indeed, it is particularly noticed by Darwin, that the great variety and fixed character of the different species of galls indicate the morbid specific power of the poison of the insect which produces them respectively. The admission of such a specific poison in the infectious diseases, besides the parasitic germ, vitiates the decisive character of all mere inoculation experiments, and throws us back upon the clinical history of these diseases to determine how far the different theories harmonise with the well-known facts. § 14. The following are a few of the cardinal facts of smallpox, which may be taken as the type of the specific contagious fevers : 1st. A latent or incubative stage, exactly limited to twelve days. 2nd. Almost perfect health during, and abrupt termination of, the latent stage, and invasion of the initiatory fever. 3rd. Immunity from a second attack. 4th. The inoculated disease is very much milder than the natural, but protects as completely. 5th. The latent stage much shorter in the inoculated disease, lasting only seven days. 6th. Vaccination still milder and its latent stage shorter, while it protects as well. 7th. Several vaccine pocks protect more completely than one. § 15. The first point is the absence of illness during and the abrupt termination of the latent stage. It is difficult to see how this can be reconciled with the parasitic theory, for the only mode, as yet known, of propagation of the bacteria, on which the disease is supposed to depend, is the fissiparous, or that by division into two. This produces a progression on the whole gradual, although more rapid at the later than the earlier stages; and from the time the number is sufficient to produce any perceptible effect at all the symptoms should increase gradually. It is therefore impossible to understand how in smallpox there should be no symptoms at all for twelve days, and then that the disease should suddenly burst forth in its full violence. Again, no matter how much or how little of the contagium be introduced in the inoculated disease, the length of the latent stage is the same; this also is against the theory of the multiplication of parasites, for the greater number should multiply faster and shorten the latent stage, as was indeed found by Koch in respect to splenic fever. (§7, p. 16.) It has been attempted to explain the latent stage by supposing that the parasite was in a different cycle of its life history when introduced, and the instance is given by Henle of the muscardine-a true parasitic disease of silkworms-in which it makes a difference whether the ripe or unripe filaments were used for inoculation; in the latter case the disease broke out much sooner, for the unripe filaments continue to grow at once, while the spores require time for their development. But this only serves to make the contrast between the specific contagious diseases and the parasitic disease the more glaring, for the disease which had a shorter latent stage was not milder, but, on the contrary, more rapidly fatal than the natural cases, whereas the characteristic fact about the smallpox is that the inoculated disease not only has a five days' shorter latent stage, but it is out of all proportion milder, so as to be counted among the trivial diseases. Again, notwithstanding the mildness of the inoculated disease, it protects as completely. This immunity from a second attack, after the natural as well as the inoculated disease, is itself wholly inexplicable by the parasitic theory. As is also the fact that another similar poison, viz., the vaccine, can protect as well as the smallpox poison itself, as are also a number of the details of the process, especially the fact that several vaccine pocks protect more completely than one; for with a parasite capable of reproduction to infinity, what could it matter a few dozen more or fewer parasites? The immunity from a second attack is, in fact, acknowledged to be inexplicable on the parasitic theory by the best pathologists among those who are inclined to favour it; and the attempts to explain it by less able men only make the difficulty more palpable. As, for instance, when it is supposed that the body of the host contains in health some pabulum suitable for the parasite, and where this is exhausted, not only the parasite stops breeding, but the patient is never again able to nourish the same parasite. This would be a remarkable kind of pabulum, for the loss of it does no harm to the patient, nor does the loss of a dozen other kinds of pabulum which he keeps on hand for the other specific diseases. As far as we know of bacteria, they are also not so very particular, and parasites mostly go on devouring the substance of the host till death. So, why the specific diseases, if they are parasitic, should ever stop, as they do, and that in a curiously definite and fixed way, is |