999 



populi, should he considered as specific features of the first-named 

 form, whereas the points in which the two species resemble each other, 

 especially on the under side, would possess the significance of generic 

 characters. This opinion, that e.g. the eye-spot of ocellata cannot 

 pretend to a higher significance than that of being a specitic peculi- 

 arity, might not only find support in its restriction to the upper 

 side, but also in the results of hybridisation, showing that the hyttrids 

 from the cross between a male ocellata and a female populi are 

 ocellata-\'\kQ in their iiabitus, though with a faint eye-spot, deprived 

 of its pupilla, whereas those from the combination of a ma.\e pojmli 

 with a female ocellata possess such a far-going similarity to popidi, 

 that they can hardly be distinguished from it, the eye-spot being 

 wholly absent. 



This view about the meaning of the differential features of r>c<?//(7to 

 seems the more attractive because a special importance for the chances 

 of survival of the animal nmy be ascribed both to the variegated 

 and marmorated design of the fore-wings and to the eye-spots, in 

 their monochrome pink field, of the hind ones. As long as the moth 

 in its attitude of rest is suspended on a willow-twig among the 

 leaves of that food-plant of its caterpillar, the hind-wings are coji- 

 cealed under the fore-wings with the exception of their nai-row 

 fore-rim, and the animal so delusively imitates by its form, colour, 

 design and proportions of a pair of dry willow leaves, that notwith- 

 standing its bulky size it can hardly be detected amongst its natural 

 surroundings, as long as it i-emains motionless. 



When however the sleeping moth gets disturbed by pushing or 

 hurting, it moves its fore-wings a little forward, thereby suddenlj' 

 displaying the eye-spots in their red surroundings, which by their 

 situation on both sides of the somewhat upheaved abdomen (this 

 part of the body at the same time making periodical jerks) cause 

 tlie illusion of a savage face with wide-opened eyes, thereby (as 

 experiments have proved) so effectually frightening birds and reptiles, 

 that they generally abstain from further attempts to devour the moth. 

 When 1 mention these long known facts from the chapter of 

 Protective Mimicry, it is because 1 think it desirable to state once 

 more, that they can never be used as an explanation of the presence 

 of markings, hues and shapes, which by their cooperation call fortii 

 the deceitful resemblance. These features owe their presence to causes 

 of quite another order of things, viz. to the variability, which itself 

 is a consequence of the coincidence of hereditary factors. When this 

 coincidence accidentally leads to an effect which in a certain direction 

 is favourable for chances of survival of the animal (oi* plant), it 



65 

 Proceedings Royal Acad. Amsterdam. Vol. XXI. 



