662 JOURNAL, BOMBAY NATURAL HIST. SOCIETY, Vol. XXVI. 



incorrect, since on reference to the district accounts of all the infected 

 districts it will be found that rats are only mentioned as having been a 

 pest in 1901-02. 



The frightful destruction of crops caused by these plagues can be judged 

 by the following rough estimates, which are based on my own experience 

 of the rats in North Gujarat in 1913-14. Revenue assessment (in 1906) 

 of the Ahmedabad and Kaira district— Rs. 34| lacs. Deduct 14^ lacs 

 for land such as rice lands, the crops on which are not liable to damage 

 by rats — Rs. 20 lacs. Assume that the average gross money outturn is 

 now about 50 times the assessment — Rs. 10 crores. Assume 57o of the 

 gross outturn to be destroyed by rats — result, loss of Rs. 50 lacs in two 

 districts alone. I have taken 5"/^ as a conservative estimate. From 

 personal observation I should say that in some talukas, such as Prantij, 

 the damage in 1913-14 was much higher, and in individual fields of 

 " math" as high as 507o. From this it follows that precautionary measures 

 are quite worth taking, should any be known. And there is ample time to 

 organize them. 



It would be best if preventive measures could be based on such know- 

 ledge as we possess of the life history of the field rat species, and of the 

 causes both of the rise in their numbers and their subsequent return to 

 the normal. The cause to which these rat plagues is usually attributed 

 is the diminished mortality among the young broods in the year of 

 monsoon failure through the absence of water to drown them in their 

 burrows. It is doubtful whether this drowning of young rats in normal 

 monsoons has been definitely investigated and proved. If not the cause 

 assigned is purely theoretical, and is therefore open to argument. A 

 great objection to it is that the fields worst infected in North Gujarat in 

 1913-14 were the highest fields with the sandiest soil, i.e., the fields least 

 liable to water-logging in a normal year. The ordinary monsoon rainfall 

 in such fields runs ofl' easily, and so much as does not run off soaks through 

 the light sand. The causes of natural phenomena are often not the most 

 obvious ; and a remark in the Statistical Atlas suggests another possible 

 clue. In speaking of the cessation of the 1878 plague, the compiler wrote:— 

 " It is conjectured that a parasite (a red tick found on the bodies of the 

 rats) may have aided in the work of destruction." It is possible that in 

 normal years the multiplication of the rats is kept down by the attacks of 

 this, or some other, parasite, a,nd that the failure of the monsoon is 

 prejudicial to the parasite. Ticks breed in grass, and there is a noticeable 

 diminution in the grass crop in a famine year. On the other hand they 

 do not feed on the grass, but use it merely as a jumping-off place. And 

 even in a famine year there would be herbage high enough for the ticks to 

 attach themselves to rats. A more probable cause of the diminution in 

 the number of ticks would be the diminished humidity. Animals, like 

 plants, are influenced by the humidity of the air, and some species find 

 their optimum only in a fairly humid atmosphere. The ticks which attack 

 travellers in the Kanara Forest country are an example of this. These 

 ticks find their optimum in the heavy rainfall belt on the crest of the 

 ghats, and diminish in numbers rapidly as the rainfall decreases eastward. 

 Whether the Eastern limit of ticks and the lines of equal prevalence recede 

 westward in years of light rainfall I do not know. But it seems reasonable 

 to suppose that this is the case. And in the same way the rat tick may 

 easily diminish in numbers in years when the humidity in July to October 

 is noticeably below the normal. 



Should the above theory be correct we can get little hope of preventing 

 the rat plague in 1920-21 along nature's own lines. The mischief will 

 now have been done, and artificial breeding of ticks is presumably beyond 

 the bounds of practical operations. It would be necessary therefore to find 



