136 AGRICULTURE IN THE TROPICS [PT. II 



commonly carry living scale insects, often of dangerous kinds, 

 and such consignments are often compulsorily fumigated with 

 the vapour of hydrocyanic acid gas or other destructive agent 

 to kill any scale or other insects that may be adhering to them. 

 Against fungus pests this method is not successful. 



Ceylon has lately established a " Pests Ordinance," under 

 which any dangerous pest may be proclaimed, with the methods 

 recommended for treatment. The employment of these is then 

 compulsory, and recalcitrants may be fined in the police courts. 

 Inspectors are appointed for every district in which the disease 

 occurs. They instruct the headmen in methods of recognising 

 and treating it, and with them make lists of all infected places. 

 Notices are then served on the occupiers to take the necessary 

 measures, and the headmen have to see that this is done, and 

 prosecute where necessary. 



Our experience of some years with the treatment of diseases 

 in Ceylon leads us to suppose that the propagation of disease 

 in a district follows to some considerable extent the ordinary 

 laws of diffusion, and may, like diffusion, be altered by the in- 

 terposition of obstacles. Suppose a district in which there 

 exists a disease that has long been known in that locality, and 

 in which no measures for its extirpation are being taken ; then 

 it will exist there at a certain average density which will only 

 be varied, like the average density of any other plant or animal, 

 by the immediate local conditions, such as supply of food, 

 suitability of the substratum on which it grows, etc. Suppose 

 an estate in this district to extirpate it ; the estate will soon 

 be reinfected to the same average density as before from its 

 surrounding infected neighbours. If all the estates in the 

 district take measures simultaneously against the disease, its 

 average density will be greatly reduced, and by the continuance 

 of those measures may be kept at a low figure ; but if they be 

 given up, the disease will again multiply to the average density 

 allowed by the local conditions. 



When on the other hand a new disease, or one which until 

 then has been very rare in the country, commences to spread, 

 it seems usually to diverge from some one locality, and impinge 

 upon the surrounding neighbourhood, hitherto practically free 



