what is usually healthy country. There are a few well-authenticated instances of countries (Mauritius, Réunion) which, although previously exempt, subsequently became endemically malarial; and there are many instances of countries previously malarial which afterwards, especially under the influence of cultivation and drainage, became salubrious.
These circumstances evidently have reference to the distribution of species of malarial mosquitoes. Like other insects, mosquitoes occasionally, under specially favourable conditions, increase enormously in numbers and spread out in every direction. New species, which may belong to the malaria-bearing kinds, may be introduced into places where they formerly did not exist, just as Culex fatigans has been introduced into Australia. I believe a mosquito capable of subserving the malaria parasite was introduced in this way in the early 'sixties into Mauritius, an island whose fauna and flora had been hitherto very peculiar and special. In this way we can account for the outbreak of the great epidemic of malaria that swept over that formerly healthy island, and also for its present endemic insalubrity. A similar misfortune may very well happen to the non-malarious South Pacific islands in the near future.
The exemption of certain islands from malaria, even though in the midst of an archipelago of malaria-haunted islands, is a very remarkable circumstance. Barbados is an instance in point. Low has confirmed the popular belief that malaria is unknown there as an indigenous disease, and points to the absence of Anophelinæ as the explanation. But how explain the absence of Anophelinæ, seeing that Culex abounds and all the conditions favouring mosquito life are present? The disappearance of malaria from Britain is another remarkable, and perhaps not fully explained, fact. Anophelinæ still abound in many places, yet the endemic malaria has vanished. It may be that the general use of quinine and the improved domestic hygiene have to be credited with this. Still more remarkable is the circumstance that there are villages and districts in
