the circulating blood asexual spores (merozoites), which, on liberation, enter the red blood-corpuscles and so renew the asexual, intracorporeal phase. This much we do know namely, that physiological strain or vital depression in the host tends to bring about conditions which break up, and that quinine and vital vigour tend to bring about conditions which favour, latency.
POSSIBILITY OF YET ANOTHER PHASE
So far the story of the life -history of these parasites seems to be complete. There are certain facts, however, which seem to indicate the possibility of yet another phase, or of etiological factors which hitherto have escaped observation. First, there are districts in India, Africa, and elsewhere that are practically uninhabited on account of the prevalence and virulence of the local malaria. If man be necessary for the completion of the life-cycle of the parasite, how explain its abundance in such circumstances—— that is to say, in the absence of man? Second, those engaged in malarious districts on works entailing disturbance of the soil, e.g. opening jungle lands, digging canals or foundations, making roads or railways, are particularly prone to contract malaria; yet such operations at first sight seem in no way calculated to foster broods of malaria-infected mosquitoes. How account for infection in such circumstances? where, and in what form, is the malaria germ to be found there?
Of the first of these difficulties two explanations may be submitted, (a) The malaria parasite may be capable of living in a variety of animal hosts, as we know to be the case with the hæmoprotozoa of birds and many other and more highly organized animal parasites; and it may be that in the malarious districts alluded to the prevalence of such an appropriate host, together with the presence of an appropriate mosquito, ensures the continuance and abundance of the parasites. Support is given to this hypothesis by Dionisi's discovery of intracorpuscular parasites in bats, closely resembling
