158 



THE EVOLUTION THEORY 



up. In the case of the ' emperor moth ' (Satumia carpini) this is 

 effected by means of a circle of stiff bristles of silk on the inside 

 (Fig. 33), the points of which bend outwards like those of a weir- 

 basket (r) ; from the inside the emerging moth can easily push aside 

 the bristles, while the threatening enemy from without is scared off 

 by the stiff points of the bristles. 



Such a cocoon is comparable to a work of art in which every 

 part harmonizes with the rest, and all together are adapted as well as 

 possible to their purpose. And yet it is all accomplished without the 

 caterpillar having the remotest conception of what it is aiming at 

 when it winds the endless silken thread about itself in the artistic 

 and precisely prescribed coils. Nor has it any time for trying 

 experiments or for learning ; it must make all the complex bendings 

 . and turnings of the head which 



B spins the thread, and of the 



anterior part of the body which 

 guides the thread, quite exactly 

 and correctly the first time if 

 a good cocoon is to be produced. 

 Here every possibility of inter- 

 preting this instinct as ' an 

 inherited habit ' is excluded, for 

 each caterpillar becomes a pupa 

 only once ; and it is just as 

 impossible to suppose that it can 

 be directed by intelligence, since 

 it can neither know that it 

 is about to become a pupa, nor 

 that, in the pupa stage, it will be in danger from enemies which will 

 attempt to force their way into the cocoon, nor that the hedge of 

 bristles will protect it from such enemies. Our only clue to an inter- 

 pretation is in the slow process by which minute useful variations in 

 the primitive instinct of spinning are accumulated through selection ; 

 and it is wonderful to see how exactly these spinning powers are 

 adapted to the particular life-conditions of individual species. 



Thus there are several of the Saturnides whose enormous cater- 

 pillars live on large-leaved trees, and these make use of the large 

 leaves to form a shelter for the pupa stage, spinning them together so 

 that the cocoon is for the most part surrounded by leaf. But as the 

 leaf might easily fall off with the weight of the pupa, they make 

 the leaf-stalk fast to the twig from which it grows by binding the 

 two firmly together with a broad, strong, closely-apposed silken band. 



Fig. 33. Cocoon of the Emperor Moth 

 {Satumia carpini), after Rosel. A, enclosed 

 pupa. B, emerging moth, r, hedge of bristles. 

 fl, wings. 



