4 1 2 Studies in tlie Tlieory of Descent. 



dimorphism occurs in certain pupae, but only in 

 those which are openly exposed and which are 

 therefore visible to their foes. I am only ac- 

 quainted with such cases among the pupae of 

 butterflies, and it is likewise only among these 

 that I have found any considerable amount of 

 variability. 



Facts of this kind indicate that Nature does 

 not uselessly sport with forms, but that at any 

 rate changes of this sort result from external 

 influences. The greater frequency of variability 

 among larvae and its comparative rarity in imagines 

 is also undoubtedly in favour of this view. 



It has already been shown that species with 

 variable larvae and constant imagines are extremely 

 common, but that those with constant larvae and 

 variable imagines are very rare. This confirms 

 the conclusions, already drawn above, first, that 

 the variability of the imago cannot owe its exist- 

 ence to the variability of the larvae, and secondly, 

 that the causes which produce variability affect 

 the larval condition more commonly than that of 

 the imago. 



Where can these causes be otherwise sought 

 than in the external conditions of life, which are 

 so widely different in the two stages, and which 

 are much more variable for the larva than for the 

 imago ? 



Let us take the species of one genus, e.g. those 

 of Deilephila. The imagines of our European 



