act as a noxious or poisonous influence on the human body. And here there are various facts which justify an affirmative answer; as showing that animal matter of certain kinds, applied to absorbing surfaces, may produce the most virulent symptoms of disorder, locally or generally, according to the nature of the virus, its intensity, or the relations of the texture first affected to other parts of the system.

The evidence on this subject varies in kind and conclusiveness. That which is most familiar and direct is our actual knowledge of numerous animal poisons; the product, as well of living secretions, as of changes taking place in animal matter after death. Respecting the former class, the most important to our present purpose, it is enough to refer generally to their many varieties in all parts of the animal kingdom, and particularly among the insects. From the dangerously active venom of the Argas Persicus and the tarantula, to the more harmless poisons of the bee, the wasp, the ant, and other insects which surround us, we have every gradation and variety of these secretions, which thus act noxiously on certain textures of the human body; and the instances would be much more numerous, if including those which affect other animals.* In the cantharides we have a solitary example of their application to medicinal purposes. The class of reptiles

* In the narrative of the travels of Humboldt, Ehrenberg, and Rose, in Siberia, recently published, the last of these eminent naturalists describes a pestilence prevailing on the great steppe between the Irtish and the Ob, affecting very fatally both men and horses; the most obvious symptom that of suppurating tumours, chiefly on the exposed parts of the body. The opinion on the spot is, that the disease proceeds from the stings of insects; and there are various reasons for presuming it to be so. This is an instance before me at the moment; and many similar, of greater or less authenticity, might be drawn from different sources both of ancient and modern date, attaching disease to the casual or periodical occurrence of this cause.

affords many well known instances of such animal poisons, under the form of natural secretions, fulfilling purposes in the economy of the several animals possessing them. The virus in hydrophobia is an example of the poisonous production of disease. Recent observations have shown that the glanderous matter of the horse is capable of producing the same disorder in man; and it is probable that there are many more instances of this kind than we are yet acquainted with. Some of the secretions from diseased surfaces in the human body are known to acquire properties highly noxious, even to the frame within which they are generated; and we may perhaps attribute to the same cause the occasional poisonous quality of certain shell-fish, which commonly are innocuous as articles of food. Or it may be that the latter are instances of a virus produced after death; as we have many examples of animal matter undergoing change of this kind during the process of decomposition, when life is extinct.

If it be urged that none of these instances (of which many more might be cited) apply expressly to the question, whether or not minute or invisible insect species may become causes of disease, the answer is, that the nature of the question supposes evidence of a different kind; and that we must seek for that of analogy and presumption where more direct cannot be had. Such evidence is found in the facts just stated, and in others still to be mentioned.

One argument, though also indirect, for the hypothesis before us, is founded on our increasing knowledge of the entozoa; and of the various morbid products, as well as diseased actions, which are due to the presumed propagation of these animals within the body. Without assuming what some have supposed, that tubercles and carcinomatous formations are thence derived, we have sufficient proof through recent discoveries, that these morbid actions from parasitic animals

are much more numerous and varied than has heretofore been believed. It seems certain that future research will still further increase the number within our knowledge, and while indicating, by aid of the microscope, forms more minute than those yet discovered, will in this very minuteness suggest their more extensive agency as causes of disease.*

The same remark will apply to the ectozoa, and to the spurious worms infesting the human body as well as many other animals. They afford further proof of the extent and variety of those connexions which subsist between different forms of living organisation. It is a curious evidence of the

*Almost at the moment of transcribing this, I observe the notice of the discovery of minute worms (polystoma sanguicola) in the expectorated blood of phthisical cases (Andral, Delle Chiaje, and others had before discovered cephalocysts in the venous blood of man, and even of some of the invertebrate animals); and also of animalcules (vibriones) in syphilitic pus. The latter fact, however, appears from one of the papers already cited, in the Amoenitates Academicæ, to have been ascertained nearly a century ago. In other morbid secretions other animalcules have been seen; and, according to M. Donné, the author of some of these researches, with constant relation to the acid or alkaline nature of the discharge. The occasional presence of a minute worm (trichinia spiralis) in the muscular substance, has been ascertained in this country; and still more recently another entozoon (the cysticercus cellulosa) has been found by Dr. Knox and others in the same texture. That excellent observer, Mr. Owen, to whom we owe the best classification of the human entozoa, has been led by observation to suppose that the cataract of the eye is produced by a species of worm; a supposition at variance, however, with the results to which Sir D. Brewster has been led by his refined and beautiful researches on the crystalline lens in its healthy and diseased states. It is well known that minute animals (filaria oculi) have been seen in movement in the aqueous humour of the eye of the horse. Still more recently they have been observed in milk. Different animals, according to Gruithausen, exist in pus and mucus. Some of these statements need confirmation, and, above all, the identity of the animal in similar conditions of disease; but, if obtaining such proof, they manifestly become very important to all future inquiries in pathology.

progress of such research, that while only eleven species of intestinal worms are recorded in the 12th edition of the Systema Naturæ of Linnæus, nearly 1000 species are described by Rudolfi in his Entozoorum Synopsis, and others have since been discovered.

It is a remark of Ehrenberg, and seemingly founded in reason, that, looking to the extreme fecundity of some of the entozoa, there is more cause for wonder at the limitation of their effects by the actions of the living bodies they inhabit, than at any morbid effects they appear actually to produce. The recent speculations of this eminent naturalist derive a sanction for their boldness from his remarkable discovery of the fossil infusoria, as well as from his researches among the entozoa, in their various species, and on different parts of the globe; in ascertaining the highly organised structure, astonishing minuteness, and fecundity of which, he has obtained arguments for the belief that they form a direct cause of many of the diseases affecting man. In connexion with his view, that the minute ova of parasitic animals may be taken up by the absorbents and deposited in particular textures of the body, he hazards the opinion that scrofula, in its various forms, may be thus produced; -a supposition not wholly new, nor incompatible with the fact of its being an hereditary disorder; but certainly requiring more evidence than has yet been given, and perhaps incompatible with the relation of size between the ova of entozoa and the capillaries or lymphatics of the animal textures.

However this may be, the whole subject of parasitic animals and plants, and of the mutual relations of each class, is replete with curious matter of research; and now first pursued with an earnestness proportionate to its importance. It provides us with argument and analogy from every part of organic existence, in attestation of the fact, that the life of one being

is in innumerable cases supported by the life of another; and that there are express relations of dependence of this kind established throughout creation, scarcely less definite and remarkable than those by which the functions of individual life are carried on; - relations which, though often compatible with health, are in numerous instances the cause of those morbid actions and changes which constitute disease.* If admitting that certain diseases are thus produced by one species of animal life (whether truly parasitic or otherwise) acting on another, we must adopt the conclusion, as probable at least, that the symptoms of such disorders must be derived in part from the progressive changes incident to the affecting cause, partly from the vital or other actions of the recipient; and also, that under both these conditions there will be a tendency to regular form, series, and duration of the morbid phenomena. And we are still further carried forward to the notion of infection as appertaining to such diseases, — the element of life, thus introduced, affording a more intelligible

*It is a singular proof of the extent of the fact stated above, that entomologists have ascertained more than forty genera of insects to be infested by parasitic worms (filaria), and that it is often possible to determine the species of the insect by that of the parasite living upon it. I see a recent notice (1837) of a cryptogamic plant growing on the body of the common fly; the converse (if indeed it be correctly stated) of the numerous cases in which animals live upon plants, and completing all the possible mutual relations of parasitic growth in animal and vegetable life. The discovery of monades within the bodies of certain of the entozoa is another circumstance not less worthy of notice. These facts are very interesting in themselves; and further remarkable as showing the exactness of modern observation, and the advance it has made into the most minute operations of nature. Ehrenberg has used a bold, but not unjustifiable figure, in speaking of the "milky way of the smallest organisation" we may add, to human research, like that of the heavens, only through artificial instruments; but yielding to these, in the one case as in the other, the most marvellous proofs of the infiniteness of creative power.

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