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can hardly have existed from the foundation of the world; they must each have had their period of development, and what has developed through one period of time, may be developed again under similar circumstances. This may require a combination of circumstances acting simultaneously. Each circumstance separately may be in frequent action, but the necessary combination may present itself very rarely. A certain temperature and a special pabulum—a product perhaps of a certain stage of putrefaction—may be the first requisites to develope the pathogenic power of the bacterium; it may then require a special condition of the living animal organism, a lowered vitality, or an enfeebled state of health—the result perhaps or the same conditions of air, soil and temperature &c. which have called forth the power of the parasite—before it can find an entrance; and possibly after that its transmission through different species of animals may be necessary before it acquires destructive virulence, and is able, like the bacillus anthracis, to attack sound and healthy tissue. Instances are not rare of vegetable cells possessing poisonous properties at one time and not at another. Nageli has called attention to one of the most striking which is thus described by de Bary:—"The bitter almond tree is poisonous from the amount of amygdalin it contains, though it is not very dangerous to human beings; the sweet almond contains no amygdalin and is not poisonous. The sweet almond tree does not differ specifically from the bitter; a tree with bitter seeds may be produced from a sweet seed; bitter and sweet seeds may even be borne on the same tree in flowers and fruits not morphologically distinguishable from each other." What the origin or cause of this difference is, has not yet been discovered, and no explanation can as yet be offered;
