12 REPORT ON THE DISEASES OF SILKWORMS IN INDIA 



at about this time. To the credit of N. G. Mukerji be it noted that 

 he did not believe pebrine to be a recent introduction, for he says 

 " that the plague has spread throughout the world during the last 

 50 years does not seem to be due to the spread of the germs connected 

 with it from one country to another." He evidently recognized that 

 the germ of the disease was in India all the time, but thought that 

 only about 1 875 it began to cause disease. He blames domestication 

 for rendering the silkworm more susceptible to the disease, but as 

 silkworms were domesticated in the East many centuries ago I should 

 have thought that the disease might possibly have made its appear- 

 ance prior to 1875, if it is admitted that the causal organism was 

 in the worms all the time. 



I believe we are justified in stating the matter in this way. 

 Many insects are infected with a parasite belonging to the order 

 Microsporidia. Like the huge majority of parasites these organisms 

 do not damage their hosts under ordinary conditions. If, however, 

 conditions are changed these parasites are liable to cause injury to 

 the insect attacked. In the wild state or under natural and normal 

 conditions there is a balance maintained between all living organ- 

 isms, and so between host and parasite. Upset the ordinary 

 conditions in any way, and in every likelihood that delicately 

 adjusted balance will be destroyed and disaster will result, usually 

 to the host. Silkworms, in the wild state, like so many other 

 insects, harboured a parasite, Nosema bombycis. When they were 

 domesticated they brought their parasite with them into men's 

 houses. There the conditions were somewhat different from out in 

 the wild, though probably at first the worms were reared under very 

 much more natural conditions than they are at present. Thus by 

 domestication their balance of life was interfered with. Doubtless 

 the worm became less strong, less resistant : the conditions under 

 which they lived tended to make the infection of the worm by the 

 parasite easier. In this way a normal harmless parasite became a 

 disease-causing parasite. As domestication was probably gradual — 

 indeed in China to-day silkworms are sometimes reared in the 

 open — no very serious results of the disease were noted as long as 

 rearing was done on a very small scale. As soon as larger rearings 

 were attempted — as soon as rearers in any way tried to force the 

 worms — then disaster followed. 



The only thoroughly studied epidemic of pebrine — that in Europe 

 between 1850 and 1870 — shows clearly that the most probable cause 

 of the plague was the large rearings which were attempted about 

 1850. Thus while the average French' crop in the years 1841-45 

 was 17,000,000 kilogrammes, in the following five years it rose to 

 21,000,000 and in 1853 it was actually 26,000,000. In attempting 



