158 INSECTS AND HEREDITY 



A still more convincing example is to be found in the 

 origin and maintenance of the instincts involved in the 

 construction of a freely exposed yet admirably concealed 

 cocoon on bark. Think of the natural cracks just filled 

 up, of tunnels closed flush with the surfaces around, of 

 the resemblance to excrescences or ridges which appear 

 perfectly natural upon bark. Considering not only the 

 forms but the colours and texture of the external surface, 

 we recognize at once that such structures are the product 

 of a highly perfected group of instincts. At first sight 

 indeed the case seems to prove too much ; for it may 

 be thought that such cocoons are so completely hidden 

 as to defy the sharpness of any enemy however acute, 

 and believers in Natural Selection may properly be asked 

 to bring evidence of the existence of a struggle in which 

 the high elaboration of the instincts in question is a 

 defence. There is no difficulty in meeting the challenge, 

 for specially directed observation at once reveals the 

 existence of a keen struggle in which the concealment 

 of the cocoon is the criterion of life or death. 



My attention was first directed to this particular aspect 

 of the struggle for existence in insects, on April 12, 1893, 

 when I found on the bark oiPopulus nigra near Yoxford, 

 Suffolk, a cocoon of Dicranura bifida (the ' Poplar Kitten 

 Moth ') which had been opened by some enemy, and the 

 pupa removed. The observation is, I believe, a common 

 one, in fact Commander Walker and Mr. Holland inform 

 me that it is usually difficult to find cocoons of this 

 species which have not been thus attacked. Never- 

 theless, for the sake of those who have not had the 

 experience, I think it is worth while to re-describe the 

 evidence which certainly justifies us in inferring that 'an 

 enemy hath done this '. 



' The edges of the opening were still brown and fresh, 

 as was the interior of the cocoon ; and the larval skin 

 remained fresh and untouched inside. The opening was 



them all off, that we had much trouble to find a dozen. We caught the 

 birds in the act, and although they had so nearly finished their feast they 

 were very unwilling to go, and loudly objected to our disturbing them.'— 

 Entomologist's Monthly Magazine, 1890, p. a 16. 



