VARIATION 421 



SO as gradually to change the entire reddish-gold upper surface 

 of the wing into black ; but certain parts first become darker, 

 and then other adjacent ones, the whole surface being blackened 

 only in the very darkest specimens. The margin and base of the 

 wing first turn black, and the change then spreads slowly towards 

 the middle, which, however, remains unaltered in most individ- 

 uals. Since we must suppose that scales of the same colour 

 arise from similar determinants, why are they affected to such an 

 unequal extent by the modifying influence of heat? 



The explanation previously used can, however, be applied in 

 this case also. 



It was shown in the chapter on reversion that although new 

 specific characters are produced by the modification of certain 

 determinants or groups of determinants, this modification never 

 affects the homologous determinants in all the ids of the eerm- 

 plasm simultaneously. It must, on the contrary, be assumed that 

 variation affects only a small majority of modified determinants 

 at first, but increases by the selection and preference of the 

 most modified individuals, until finally a predominant majority 

 of all the ids contain modified determinants. 



This evidently implies that new and old specific characters 

 are respectively represented by a small and a large majority of 

 modified determinants. If this statement be applied to the case 

 of P. phlceas, we obtain a principle by the aid of which the 

 dissimilar effect of heat on the determinants for the middle and 

 the margin of the wing respectively may be understood. It is 

 very improbable that the surface of the wing of the brown ances- 

 tral form of P. phlcEas has changed uniformly into reddish-gold, 

 and it is much more likely that a lighter spot in the middle 

 became modified first and took on a reddish-gold colour, in 

 consequence of sexual selection, and that this then gradually 

 extended towards the margin. If this were so, the reddish-gold 

 scales of the centre must be represented in the germ-plasm by a 

 greater majority of homodynamous determinants than are those 

 at the margin : and in this way we can understand why the 

 black-dusting of the wings affects the sides first, and the middle 

 last of all. This must be so if the old 'brown" determinants 

 under the influence of heat give rise to black scales more easily 

 than do the 'reddish-gold' ones. Whether this explanation be 

 correct in this particular case or not, it is nevertheless true that 

 the diversity of the extent to w/iic/i the determinants for the same 



