[9] This has been established in the case of the _Trypanosoma Brucei_, a minute parasite living in the blood of big game in south-east Africa, amongst which it is disseminated by a blood-sucking fly, the _Glossina morsitans_ or Tsetze fly. The parasite appears to do little or no harm to the native big game, but causes a deadly disease both in the horses and cattle introduced by Europeans and in the more anciently introduced native cattle (of Indian origin). Similar cases are found where a disease germ (such as that of measles) produces but a small degree of sickness and mortality in a population long associated with it, but is deadly to a human community to which it is a new-comer. Thus Europeans have introduced measles with deadly results in the South Sea Islands. A similar kind of difficulty, of which many might be cited, is brought about by man’s importations and exportations of useful plants. He thus brought the Phylloxera to Europe, not realizing before hand that this little parasitic bug, though harmless to the American vine, which puts out new shoots on its roots when the insect injures the old ones, is absolutely deadly to the European vine, which has not acquired the simple but all-important mode of growth by which the American vine is rendered safe. Thus, too, he took the coffee-plant to Ceylon, and found his plantations suddenly devastated by a minute mould, the _Himileia vastatrix_, which had lived very innocently before that in the Cingalese forests, but was ready to burst into rapacious and destructive activity when the new unadjusted coffee-trees were imported by man and presented in carefully crowded plantations to its unrestrained infection. [10] The phosphorescent disease of the sand-hopper (_Talitrus_) is described by Giard and Billet in a paper entitled ‘Observations sur la maladie phosphorescente des Talitres et autres Crustacés,’ in the memoirs of the Société de Biologie, Oct. 19, 1889. Billet subsequently gave a further account of this organism, and named it _Bacillus Giardi_--after Professor Giard of Paris. (Bulletins scientifiques de la France et de la Belgique, xxi. 1898, p. 144). It appears that the parasite is transmitted from one individual to another in coition. The specimens studied by Giard and Billet were obtained at Wimereux near Boulogne. I found the disease very abundant at Ouistreham near Caen in the summer of 1900. I have not observed it nor heard of its occurrence on the English coast. Sea-water commonly contains a free-living phosphorescent bacterium which can be cultivated in flasks of liquid food so as to give rich growths which glow like a lamp when the flask is agitated so as to expose the contents to oxidation. This bacterium is not, however, the cause of the ‘phosphorescence’ of the sea often seen on our coasts. That is due in most cases to a much larger organism, as big as a small pin’s head, and known as _Noctiluca miliaris_.