118 ACQUIRED CHARACTERS sec. 



thistle butterfly (Vanessa cardui) shows something of the 

 kind, although not so completely as other species. As soon 

 as it has settled on the ground, as it does in play, with wings 

 folded together, it can only be discerned if it has been seen to 

 settle, or if, as it generally does, it flaps its wings. 



It may be held, indeed, that butterflies are protected from 

 the beaks of birds which pursue them in flight by the size of 

 their wings, since a bird is more likely to bite a piece of 

 the wing than to grasp the body. 



If I mistake not, another naturalist has already expressed 

 tliis view somewhere. Some years ago I came across a 

 peculiar proof of its correctness. On a hot summer's day I 

 was on the high plateau of the Swabian Alp ; far and wide 

 no water was visible, but at one spot in the field-path there 

 ran over it the outflow of a little spring, forming a shallow, 

 clear pool in the track. Here sat hundreds of butterflies, 

 all whites and blues, closely crov/ded together, drinking 

 thirstily. On my apj)roach a number of birds (stone-chats) 

 flew from the spot, and when I came up I found a number of 

 maimed butterflies lying fluttering on the ground ; pieces had 

 been bitten from the winos of most of them — indeed tlie 

 wings were often torn to pieces before the birds succeeded 

 in getting the bodies of the butterflies, although these were 

 sitting quietly on the ground. And only because they were 

 sitting on the ground had the birds been able to get their 

 bodies. 



Thus the colour and marking of the upper side of the 

 butterfly's wing cannot be regarded as an adaptation, as a 

 protection against enemies. But there remains another 

 adaptation, that due to sexual selection : the colours and the 

 markings may be a sexual attraction. I am of opinion that 

 this is generally the case. Details of colour and marking, 

 however, as their sudden development shows, and as numerous 

 facts besides have convinced me, are due to physiological causes. 



