"To this purpose some (and particularly Kircherus) are of opinion that animalcules have been the causes of malignant and pestilential fevers in epidemic times, which differ in essence and symptoms, according to the nature and venoms of those creatures. "Thus the atmosphere and air is filled both from above and beneath with innumerable millions of millions of species or corpuscles, aporrhoeas, steams, vapours, fumes, dust, little insects, &c. all which make it such a wonderful chaotic compost of things that contains the _seeds_ of good and evil to man as surpasseth the understanding (as I suppose) of even the highest order of archangels." [5] I learn from an undoubted authority that the cow when "slack of health" eats with avidity the "field parsley;" the sheep under similar circumstances seeks the ivy, and the goat the plantain. From an equally good source I have the following: that rabbits and hares, when they are what is commonly called _pot-gutted_, seek the green broom, though at a distance of _twenty miles_. [6] "My settled opinion is, that in regard every effect is necessarily such as its cause; it must needs be that every sort of venomous fevers is produced by its proper and peculiar species of virus. "And that the manner and symptoms of every such fever is not so much from the particular constitution of the sick; as from the different nature and genius of their specific venom which caused them. "And I conceive that venomous febrile matters differ not in degree of intenseness only, but in essence and _toto genere_ also; and that venomous fevers are for the most part contagious."--_Thomas Fuller, M. D. 1730._ "Another important class of organic poisons are those which when introduced in almost inappreciable quantities into the system, seem to increase in quantity; and which when communicated in the same inappreciable quantity from the individual poisoned to one who is healthy, excite the same series of febrile phenomena and local inflammation, and the same increase in the quantity of the poisonous agent."--_Med. Chir. Review._ "This unseen influence working in the body, presents very striking analogies to the modes of operation of different poisons."--_Dr. Ormerod on Continued Fever._ [7] I am aware that the vesicle does not here strictly bear the relation to the original germ, supposing one active particle alone to be sufficient for its production, that the egg does to the bird, for in the former case multitudes of active particles may have been generated from one. I have, therefore, merely used this expression to signify an aggregation of vital forces, such as may be imagined to exist in the bird.