( ci ) 



the Indian breeds, the eggs have to be sent up annually to the 

 Himalayas for the necessary cold.* 



Single and double-brooded — Difficulties of transformation. 



It is sometimes thought that a fixedly single-brooded species 

 has only to move south sufficiently far to easily establish itself 

 there as a double-brooded one. But is this so 1 If you try 

 to work out, as I have done, the case of such a single-brooded 

 insect going south, I think you will find the difficulties arising 

 from want of fit between the seasons and the habit of the species 

 (to say nothing of the competition it would have to meet) such 

 as to render its task in establishing itself a lengthened and by no 

 means an easyone. Theoppositecaseof a double-brooded southern 

 species going north would, I think, be a little less difficult, 

 because a species having the inherent capacity of being double- 

 brooded can better adapt itself to circumstances, though even 

 here I think most of its attempts would be failures. t I believe, 

 however, that an altogether non-seasonal species where all the 

 pupae are of the same physiological type, regularly responsive 

 to temperature, would find the change more easy. J Supposing 

 an equatorial species having ten or twelve generations in a 

 year to flow over northwards, as it appears frequently to do, § 

 it would only have to reduce the number of these gradually 

 in natural response to the lower temperature, as it spread 

 over cooler regions. When they came down to three or four, 

 one of which would have to encounter a very mild winter, to 

 prolong this one in accordance with the lower temperature 

 of that season over three or four months, would gradu- 

 ally bring it to the South European or Mediterranean 

 continuously-brooded type. Finally, having regard to the 

 plastic character of its pupa, it might, after a long period of 

 probation and many vicissitudes, become seasonally double- 

 brooded. || 



* See "Silkworms in India," by E. C. Cotes, Indian Museum Series, 

 vol. i. I am indebted to Major Alcock, F.R.S., CLE.. Superin- 

 tendent of the Calcutta Museum, for these volumes. 



t On this-see Tutt in Ent. Record, vol. x, pp. 211, 212. 



J See Tutt, Ent. Record, vol. vii, p. 6. 



§ See Ent. Record, articles on Migration, vol. xiv. 



II In dry countries the perisd of hot weather resting, aestivation, would 

 come in as an element, as in the case of Ocnogyna beetica, mentioned by 



