[36] For the whole subject, see _Lancet_, 9th September 1899, paper by Surgeon-Major Birt and Surgeon-Captain Lamb. Two other cases of accidental inoculation occurred at Netley. Another point is noted by Sir Patrick Manson, in his recent Lane Lectures (Constable, 1905). "For some time back," he says, "a commission of experts, working under the direction of the Royal Society, has been studying this disease in Malta. The commission has accumulated much detailed information; but the most important observation it has published is the fact that a large percentage of the goats in Malta are infected with _Micrococcus melitensis_, and that the milk of the infected goats contains the bacterium. May not this account for the great prevalence of Mediterranean fever there and in other places having perhaps a similar milk-supply?" X THE MOSQUITO: MALARIA, YELLOW FEVER, FILARIASIS Within the last few years, it has been proved that the mosquito is an intermediate host, between man and man, of malaria, yellow fever, and filariasis (elephantiasis).[37] Just as the grosser parasites, the tapeworms, must alternate between man and certain animals, and cannot otherwise go through their own life-changes and reproduce their kind, so the micro-parasites that are the cause of malaria alternate between man and the mosquito, having the mosquito as an intermediate host. These organisms, once they get into the mosquito, pick out certain structures, and there carry out a definite cyclical phase of their lives, whereby their progeny make their way into the stylets of the mosquito, and so get back to man, who is their "definite host." Thus, malaria is not, strictly speaking, a disease of man; it is one phase in man of micro-organisms that have another phase in mosquitoes. So also with filariasis; the filariæ in man, their ova, and their embryo-worms, are one phase of filariasis; and the embryo-worms in certain structures of the mosquito are another phase. The _plasmodium malariæ_ and the _filaria_ are instances of a law of animal life that holds good also of plant life:-- "All plants and animals possess parasites, and thousands of different species of parasites have been closely studied by science; we therefore know much about their general ways of life. As a rule, a particular species of parasite can live only in the particular species of animal in which, by the evolution of ages, it has acquired the power of living. It is therefore not enough for the parasites of an individual animal--say a man--to be able to multiply within that individual, but they must also make arrangements, so to speak, for their progeny to enter into and infect other individuals of the same species. They cannot live for ever in one individual; they must spread in some way or other to other individuals.