aria in the middle of the nineteenth century and the disease as observed in our days, there exist qualitative differences. Part of the Amsterdam records allow of differentiating between simple tertian and quartan fevers. The following diagram (fig. 2) shows the monthly number of tertian and quartan cases in 1857 and of tertian cases in 1922.
Fig. 2. Malaria at Amsterdam. Comparison of the monthly incidence in 1857 and 1922. Continuous line: tertian 1857; dotted line: quartan 1857; broken line: tertian 1922.
This diagram shows that the monthly incidence of tertian malaria in the middle of the nineteenth century was entirely different from what it is now. In those days, tertian fevers attained their highest incidence between August and December, with a maximum in September and October. In addition to this peak of so-called "autumnal fevers", there was a second peak of so-called "vernal fevers" in