Page:Tropical Diseases.djvu/31

i] summary statement of the principal reasons for this belief:—

1. The occurrence of the parasites in the blood is practically always, sooner or later, associated with the clinical phenomena of malarial infection.

2. Malarial fever throughout, or at one time or another during its course, is invariably associated with the presence of one or other of these parasites in the blood.

3. The phases of a malarial fever bear a definite relation to the phases of the life-cycle of the particular parasites present in the blood.

4. That absolutely characteristic feature of malarial disease, malarial pigmentation of viscera, is fully accounted for by the pigment-forming property of the parasites.

5. Intravenous or subcutaneous injection of blood from a case of malarial infection that is, of blood containing the parasite is generally, after an incubation period of eight to twelve days, followed by an attack of malarial fever, and by the appearance in the blood of the person injected of the same species of malaria parasite.

6. The administration of quinine, which brings about the cessation of the clinical symptoms of acute malarial infection, rapidly causes most phases of the parasite to disappear from the blood.

7. If, after they have imbibed malarial blood, certain species of mosquitoes be dissected at serial intervals, the evolution of the malaria parasite can be followed in their tissues until, finally, the germs of the parasite can be tracked into the cells and secretion of the salivary glands of the insect.

8. If, after a week or thereabouts, a similarly-fed mosquito bite a hitherto uninfected man, in many instances, after a few days, that man will exhibit the clinical phenomena of malarial infection and the characteristic parasite in his blood.

9. A non-immune, if effectually protected against mosquito bite, will not contract malarial disease, however long he may live in highly malarious localities.

The proof of the causal relationship of the para-