organic matter; and further, by the method of subsidence in these experiments these matters are proved to be particulate and ponderable, and to contain among them living germs of a great variety of septic and alcoholic ferment-organisms and monads, differing according to the locality of the air tested. Thus a perfect cloud of noxious living matters may be wafted hither and thither in air, apparently quite pure and transparent. While we accept the fact that organic matters are contained in all air, however apparently pure, and what is also the fact that among these organic matters a great variety of protozoal germs are included, it does not follow that there are no other organic and even organised matters which may play the part of contagia. Nevertheless, as we are all liable ways, although it is not settled yet to what extent each contributes to the final result. It is, however, by far the most probable that the chemical effect of a secretion is quite subordinate in the result as a whole, and is confined to altering the dead matter so as to be more suitable for pabulum ; that, for example, a secretion from the bacteria acts on the nitrogenous matter somewhat as the gastric juice does on the food of higher animals. The bacteria, in fact, having no intestinal cavity, live as it were in their own gastric juice, and therefore require a certain stillness or stagnation in the liquid medium in order to thrive. The great bulk of the action of ferment and putrefactive organisms is thus simply that of devouring dead organic matter. How, then, do they act on the living body? Assuredly not by the chemical ferment, for that can only break up definite dead chemical compounds, and not the living matter itself; while the chemical ferment and the other products of bacterial life must act on the living matter as more or less noxious stimuli, and thus produce disease long before they could exert any notable changes on the non-living portions of living bodies as chemical agents. Hence the signs of action of saprophytes in the living body, except quite locally, are not signs of putrefaction, but signs of disease. The chemical products of the putrefactive organism thus acting as noxious stimuli are decomposed or eliminated long before they can act as purely catalytic ferments. On the other hand, the living organisms themselves, yeast and bacteria, cannot devour living matter-no pabulum is livingthey must first kill it, and then devour. That is to say, unless they are themselves first killed and devoured by the living bioplasts, which is the case in health with respect to the large majority of such organisms introduced (or their germs) into the larger animals and plants. (See § 12. Vital resistance.) There are some exceptions in which the juices of the host and its living matter are either favourable to or tolerate the sub

to be more impressed with what we see than by what we don't see, and as bacteria are readily demonstrated in a test tube with proper pabulum, while it is impossible so to exhibit, say the smallpox, in a like way, and as some contagious specific diseases are certainly attended with the growth of microphytes in the blood and other parts, it has been concluded somewhat hastily that the growth and development of these germs as parasites is the cause of specific diseases in general. In recent times the chief patrons of this theory, following Pasteur and the distinguished botanist, F. Cohn, of Breslau, have been natural historians and physicists, while in the medical profession, which is naturally more familiar with the clinical facts of disease, its adherents have hitherto been in the minority. But recently the distinguished clinical pathologist, Dr. W. Roberts, of Manchester, in his admirable Address on this subject before the British Medical Association, in 1877, has pronounced in favour of the parasitic-germ theory. And the foremost of our experimental pathologists, Dr. Burdon Sanderson, speaks, although with much hesitation, also in its favour. In the attempt to unravel the complexities in which the parasitic-germ theory of infectious diseases is involved, let us shortly trace its history, for it is by no means quite new, as some imagine. The doctrine of contagium animatum, which was even then old, was revived in the last century by Kircher, it was partially sanctioned by Linnæus, and early in this century was supported by more specific facts brought forward by Schönlein, Langenbeck, Sir Henry Holland, and others. But the most complete statement was given first by Henle, in 1840, and again in his Rationelle Pathologie, in 1853. Since then little has been added to the theoretical aspect of the question, although an enormous addition has been made to the known facts. The fundamental points of the theory are shortly these. We may safely hold that a parasite is the cause of all those symptoms which are brought on by the introduction of the parasite and disappear with the removal of it. Thus parasitic diseases may be called contagious, if the parasite is transferred from one person to another, and the parasite itself may with propriety be called the contagium. Now, as Henle says, it certainly sounds ridiculous, with our usual ideas of the nature of miasms, to speak of an eight-footed or a two-inch long contagium; but, on the other hand, is it not a mere play upon words to say of the itch that it is not contagious, since it is discovered to depend upon the transference of the Acarus scabiei? So it is quite legitimate to say that, if in any recognised contagious diseases we find the exciting cause to reside in a parasite, we have explained the contagious process in such diseases. And if the same holds good in all, we must allow that the parasitic theory explains contagious diseases in general. All depends upon the facts. The simple statement that a particular disease is coincident with the presence of foreign organisms, or even of a particular parasite, covers many fallacious inferences, for their presence may be secondary or accidental. The subject must therefore be followed out in detail.

§5. First take as the starting-point the entozoa, commonly called worms, the old-established typical parasites. Here, if the symptoms displayed by the host are to be called a disease at all, it is one certainly dependent on the presence of living beings, not produced spontaneously, nor by heterogenetic descent from the host, but which are distinct animals with, for the most part, a known life-history. The symptoms do not at all resemble those of the specific infectious diseases, and seem to depend on simple irritation and the abstraction of nutriment. But there are here already some analogies between the two, for the relationship between the parasite and the host is very close. Not only almost each species, animal and vegetable, has its proper parasite, but even different varieties or races of the same species have different and exclusive parasites, just as the specific diseases are confined to one or few species. Also, there is an indirect contagiousness, in that one stage of the life cycle of the entozoa is passed in a different host, just as in the miasmaticcontagious diseases morbid matter must pass out of the body of the patient and undergo some change outside before it can infect another person. They also resemble the infectious diseases in having a specific habitat; for of the entozoa some inhabit the stomach, some the liver, others again the large intestines, the kidneys, &c., just as smallpox attacks the skin, scarlet fever the throat, &c.

§ 6. We now pass to those affections which have been

long known as contagious and infectious diseases, and which have been more recently discovered to depend on the presence of parasites. Of these scabies, or the itch, was proved about fifty years ago to depend on the presence of the Acarus scabiei, or itch mite. Then, about thirty years ago, the various forms of ring-worm were traced to the presence of one or more species of fungi, from the transplantation of which, or the spores, the propagation of the disease depends. Within the last twenty years the list of epidemic and contagious diseases traced to animal and vegetable parasites has increased rapidly, and, if we include those of the lower animals and of plants, is now very large. It is sufficient to name as examples the potatoe disease as depending on the Peronospera infestans; the opium-blight, on the P-arborescens; the ergot of rye, on the claviceps purpurea; the grape vine disease, and a host of others among plants on well-described parasites; the fungoid diseases of silk-worms and other insects. What more immediately interests us is the tracing of internal diseases in the human species to the presence of parasites, as shown, for example, in the remarkable history of the discovery of the Trichina spiralis, or fleshworm. The existence of this worm has been recognised for some years, but its connection with specific states of disease was hardly suspected till the simultaneous occurrence of a number of fatal cases of an unknown disease was traced to its agency. In 1863, in a small town in Prussia, occurred one of the most striking instances. After a public banquet about one hundred persons were struck down with disease, which proved fatal in a large number. The symptoms were great lassitude and depression of body and mind, complete loss of appetite, sleeplessness, then fever closely resembling some of the specific fevers such as typhoid; then excruciating pains in the muscles, especially of the extremities, and contraction of the knees and elbows, which could not be