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Rh except in domesticated herds and those wild creatures to which Man's domesticated productions have communicated them. The trypanosome lives in the blood of wild game and of rats without producing mischief. The hosts have become tolerant of the parasite. It is only when man brings his unselected, humanly-nurtured races of cattle and horses into contact with the parasite, that it is found to have deadly properties7. The various cattle-diseases which in Africa have done so much harm to native cattle, and have in some regions exterminated big game, have per contra been introduced by man through his importation of diseased animals of his own breeding from Europe. Most, if not all, animals in extra-human conditions, including the minuter things such as insects, shell-fish, and invisible aquatic organisms, have been brought into a condition of 'adjustment' to their parasites as well as to the other conditions in which they live: it is this most delicate and efficient balance of Nature which Man everywhere upsets. A solitary case of a ravaging epidemic constantly recurring amongst animals living in extra-human conditions, one of a strangely interesting character, is the phosphorescent disease of the sand-shrimps or sand-hoppers. This is due to a microscopic parasite, a bacterium, which infests the blood and is phosphorescent, so that the infected sand-hopper has at night the brilliancy of a glow-worm. The disease is deadly, and is common among the sand-hoppers dwelling in the sandy flats of the north coast of France, where it may readily be studied8 It has not been recorded as occurring in this country. It is not at all improbable that this disease is also in truth one which only