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benign parasites rarely give rise to pernicious attacks, the malignant parasites frequently do.

The benign parasites are of two kinds: one, the quartan parasite, having a cycle of seventy-two hours, causing a fever recurring every three days—quartan fever; the other, the tertian parasite, with a cycle of forty-eight hours, causing a fever recurring every two days—tertian fever.

The malignant parasite has three forms, perhaps more:^[1] a pigmented parasite, the subtertian,^[2] of forty-eight, or approximately forty-eight hours' cycle; a pigmented parasite—pigmented quotidian—of approximately twenty-four hours' cycle; and an unpigmented parasite—unpigmented quotidian—also approximately of twenty-four hours' cycle.^[3]

We may arrange them thus:—

Benign

$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$

Quartan Tertian

$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$ Do not form crescents.

Malignant	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	Subtertian: Form crescents.
		Quotidian—pigmented Quotidian—unpigmented	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$ Supposed to form crescents.

Clinical classification.—Formerly, classification being based entirely on clinical phenomena, malarial diseases were divided into quotidian, ter-

1. Many authorities refuse to recognize more than one species of malignant parasite, the differences in length of cycle and pigmentation being regarded merely as variations depending on circumstances and not as specific differences.
2. This parasite has received several names, none of them quite appropriate. Thus the Italians call it "æstivo-autumnal," a term appropriate enough in Italy, where the infection is acquired only during the summer and autumn months, but manifestly inappropriate in the tropics, where it may be acquired at any season. Koch calls it the "tropical parasite," a name equally unsuitable, seeing that the range of the parasite embraces countries far beyond the tropic belt. The term subtertian I have adopted, following Sambon's suggestion. It implies no error either as regards clinical habit, seasonal or geographical range, and it has the additional recommendation of approximating to the name hemitertian, applied by Hippocrates and the ancients to the class of fevers it gives rise to.
3. Several additional species or varieties of malaria parasites have been described lately, e.g. the Plasmodium tenue of Stephens, the Plasmodium falciparum quotidianum of Craig, the Plasmodium vivax minuta of Emin; but as to whether these are valid species or merely varieties it is as yet impossible to pronounce.