Chak VllI THEIE DIFFERENCES. 319 



she separated in 1848 twenty of these caterpillars, and having kept 

 the moths separate, bred from them. Of the many caterpillars 

 thus reared, "every one without exception had eyebrows, some 

 darker and more decidedly marked than the others, but all had 

 eyebrows more or less plainly visible." Black caterpillars occasion- 

 ally appear amongst those of the common kind, but in so variable a 

 manner, that, according to M. Robinet, the same race will one yeat 

 exclusively jiroduce white caterpillars, and the next year many 

 black ones ; nevertheless, I have been informed by M. A. Bossi of 

 Geneva, that, if these black caterpillars are separately bred from, 

 they rejjroduce the same col<)ui" ; but the cocoons and moths reared 

 from them do not present any difference. 



The caterpillar in Europe ordinarily moults four times before 

 passing into the cocoon stage; but there are races " a trois mues," 

 and the Trevoltini race likewise moults only thrice. It might have 

 been thought that so important a physiological difference would 

 not have arisen under domestication ; but M. Eobinet '^ states that, 

 on the one hand, ordinary caterpillars occasionally spin their 

 cocoons after only three moults, and, on the other hand, " presque 

 toutes les races a trois mues, que nous avons experimentees, ont 

 fait quatre mues a la seconde ou a la troisieme annee, ce qui 

 semble prouver qu'il a sufii de les placer dans des conditions 

 favoral)Ies pour leur rendre une faculte qu'elles avaient jDerdue sous 

 des influences moins favorables." 



Cocoons. — The caterpillar in changing into the cocoon loses about 

 50 per cent, of its weight ; biat the amount of loss differs in different 

 breeds, and this is of importance to the cultivator. The cocoon iu 

 the dilferent races presents characteristic differences; being large 

 or small ; — nearly Sj^herical with no constriction, as in the llace de 

 Loriol, or cylindrical, with either a deep or slight constriction in the 

 middle; with the two ends, or with one end alone, more or less 

 pointed. The silk varies in fineness and quality, and iu being 

 nearly white, but of two tints, or yellow. Generally the colour of 

 the silk is not strictly inherited : but in the chapter on Selection I 

 shall give a curious account how, in the course of sixty-five genera- 

 tions, the number of yellow cocoons in one breed has been reduced 

 in France from one hundred to thirty-five in the thousand. 

 According to Eobinet, the white race, called Sina, by careful 

 selection during the last seventy-five years, " est arrivee a un tel 

 etat de purete, qu'on ne voit pas un seul cocon jaune dans des 

 millions de cocons blancs." ''^ Cocoons are sometimes formed, as is 

 well known, entirely destitute of silk, which yet produce moths ; 

 unfortunately Mrs. Whitby was prevented by an accident from 

 ascertaining whether this character would prove hereditary. 



A'/7ilt stage.— 1 can find no account of any constant difference in 

 the moths of the most distinct races. Mrs. Whitby assured me 

 that there was none in tho several kinds bred by her; and I have 



'* Kobinet, ibid., p. 317. " Robinet, ibid., pp. 306-31: 



