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330 the rat is mentioned in association with the plague which broke out amongst the Philistines after they stole the Ark of the Covenant. The Greeks of Asia Minor worshipped a rat-killing Apollo who was reputed to bring and remove plague epidemics. Sambon describes a coin of Lucius Severus, struck at Pergamum in Asia Minor at the time of a great plague epidemic. On the obverse of the coin is the god of medicine, Æsculapius, having at his feet a dead rat and at his side a naked human figure in an attitude of terror or supplication. Mediæval literature likewise contains abundant proof that the association of rats with plague was well recognized even in the dark ages. Many modern observers also have remarked the great mortality among rats and other animals which generally precedes and accompanies outbreaks of plague in man. When writing about the mortality among rats during the recent Canton epidemic, Rennie remarks that the Chinese regarded this unusual and striking occurrence as a sure indication of an extension of the epidemic. From districts of the city where the plague had been raging for some, time the rats entirely disappeared, whilst they kept on dying in other quarters to which the disease afterwards spread. The rats, he says, would come out of their holes, in broad daylight even, and tumble about in a dazed condition and die. In a very short time one Chinese officer alone collected upwards of 22,000 dead rats.

Glen Liston states that in places in which plague epidemics keep recurring year after year the local rats acquire a considerable degree of immunity, and, moreover, that this immunity is transmitted hereditarily. Thus in plague-free towns in India— e.g. Madras and Dacca— plague-inoculated Mus rattus gave a mortality of 90 to 100 per cent.; in plague- stricken towns— e.g. Cawnpore and Poona— it was only 20 to 40 per cent.

Another observation, already referred to, which together with the foregoing may have important bearings on the spread of plague and the yearly recurrence of epidemics in the same place, is to the effect that