42 REPORT ON THE DISEASES OP SILKWORMS IN INDIA 



Hutton about 1860. It was tested by Dr. Bonavia in Oude and did 

 not seem to give outstanding results (Watt, 1893). If both Bara- 

 polu and Nistari tend to give numbers of dark worms under ordinary 

 conditions (Mukerji, 1899) and if these are more resistant, how is 

 it that the exacting natural selection always at work in the rearers' 

 houses has not produced a dark race, the lighter worms being 

 eliminated ? I would not for a moment shut out the possibility 

 of selecting more disease-resistant races from those already, in India 

 and an inquiry might be made into this subject as a sort of side line, 

 but it seems to me that the results got will deal with so many vary- 

 ing factors and such slight differences that it will be well nigh impos- 

 sible to form a correct judgment upon them. As I have said, a 

 pretty exacting selection has been going on for generations in India, 

 and it will mean several years of careful experimenting before we 

 could claim that we had improved upon the results already achieved. 

 We must also take into consideration the fact that in dealing with 

 pebrine we are concerned, as shown above, with a parasite and a 

 host which in the past achieved a balance — the attack of the 'para- 

 site equalized by the resistance of the host — and that balance has 

 probably been disturbed not by any alteration or variation in the 

 inherited resistance of the worm or attacking power of the parasite, 

 but by an alteration in the environment due to domestication. There- 

 fore the most profitable method of assisting the caterpillar to resist 

 attack will be to remove these disturbing external conditions as far 

 as possible. Mukerji very pessimistically remarks, " if this (the 

 discovery of pebrine in wild silkworms) be a fact, it is useless attempt- 

 ing to resist the plague by improving the vigour of silkworms. '"' 

 Well, Nosema bqmbycis — or at all events an organism indistinguish- 

 able from this — has been discovered in Athercea sp. (Appendix IE). 

 It has also been found in caterpillars of the genus Gastropacha and 

 Zygcena, and I have infected a wild caterpillar (genus?) with pebrine. 

 But I would not say "it is useless attempting to resist the plague 

 by improving the vigour of silkworms," I would however tend to lay 

 very much more stress on improving external conditions than on 

 increasing the inherited resistance of the worms. 



If the attempt to improve natural immunity through selection 

 is not very promising, I do not think much progress is to be hoped 

 for from hybridizing, for there of course one must have certain 

 definite characters to Avork with— blind crossing, mere juggling 

 with the germ plasm, will never lead anywhere. Here the opinion 

 of a trained geneticist is essential. In Bengal a very considerable 

 amount of hybridizing has been attempted, on the supposition 

 that the Nistari when crossed witha better silk-yielding moth, such 

 as a European or Japanese, would supply vigour or resistance to the 

 cross— it is taken for granted that the Nistari has this character in 



