CHAPTER II 



THE evidence described in the next few chapters is not 

 the result of a definite inquiry into the manner of evolu- 

 tion. It arose unexpectedly in the course of another 

 investigation, which was undertaken in order to increase 

 our knowledge of the disease called plague. This dis- 

 ease, which has caused such wide-spread distress in 

 India, is baneful alike to men and rats. It is known that 

 the bacillus, the cause of the disease, thrives within the 

 body of a warm-blooded animal. Apart from the body 

 of an animal it is short-lived, except under artificial 

 conditions. In this respect it differs from many other 

 organisms of infectious disease. Because of the peculiar 

 delicacy of the Bacillus pestis, it happens that rats and 

 mice play an important part in the maintenance and 

 dissemination of the disease. In order to complete this 

 brief description of the natural history of plague, it may 

 be mentioned that the bacillus is commonly conveyed 

 from rat to rat and in all probability from rat to man 

 by a blood-sucking insect the flea. But this is a 

 digression. 



As soon as these facts were made known, principally 

 by the work of the Plague Commission in India, it be- 

 came evident that if the number of house rats were to be 

 lessened, the disease would be reduced to some extent. 

 Consequently, a campaign against these animals was 



