THE ANIMAL COMMUNITY 53 



reason why plague should not have continued indefinitely to 

 threaten the lives of people in England ; but after the end of 

 the seventeenth century it practically disappeared from this 

 country. This disappearance was partly due to the better 

 conditions under which people were living, but there was also 

 another reason. The dying down of the disease coincided 

 with certain interesting events in the rat world. The common 

 rat of Europe had been up to that time the Black or Ship Rat 

 {R. rattus), which is a very effective plague-carrier owing to 

 its habit of living in houses in rather close contact with man. 

 Now, in 1727 great hordes of rats belonging to another species, 

 the Brown Rat (R. norvegicus), were seen marching westwards 

 into Russia, and swimming across the Volga. This invasion 

 was the prelude to the complete occupation of Europe by 

 brown rats.^'^ Furthermore, in most places they have driven 

 out and destroyed the original black rats (which are now 

 chiefly found on ships), and at the same time have adopted 

 habits which do not bring them into such close contact with 

 man as was the case with the black rat. The brown rat went 

 to live chiefly in the sewers which were being installed in some 

 of the European towns as a result of the onrush of civilisation, 

 so that plague cannot so easily be spread in Europe nowadays 

 by the agency of rats. These important historical events 

 among rats have probably contributed a . great deal to the 

 cessation of serious plague epidemics in man in Europe, 

 although they are not the only factors which have caused a 

 dying down of the disease. But it is probable that the small 

 outbreak of plague in Suffolk in the year 19 10 was prevented 

 from spreading widely owing to the absence of very close 

 contact between man and rats."^! We have described this 

 example of the rats at some length, since it shows how events 

 of enormous import to man may take place in the animal world, 

 without any one being aware of them. 



4. The history of malaria in Great Britain is another 

 example of the way in which we have unintentionally interfered 

 with animals and produced most surprising results. Up to 

 the end of the eighteenth century malaria was rife in the low- 

 lying parts of Scotland and England, as also was liver-rot in 



