46 Mr. W. Bateson on variation in the colour 



and turn brown, and after this change the dark colour is 

 highly protective. It is also of especial importance for 

 the cocoon to be well concealed during the winter months, 

 when insect-eating animals are pressed for food, and are 

 obliged to search for it with extreme care." An experi- 

 ment with Halias prasinana is then described, in which 

 a larva which had begun to spin a brown cocoon on an 

 oak-leaf was transferred to a white box, where it sub- 

 sequently spun a ivhite cocoon. 



If it were really established that there is an intimate 

 relation of this kind between the colour of the cocoon 

 and that of the substances to which it is attached, the 

 fact would be very surprising, and perhaps unparalleled. 

 We have here to deal with a case not of a graduated 

 resemblance between the general tint of the skin of an 

 animal and that of the ground on which it lies, such as 

 is found in many forms which are provided with con- 

 tractile or moveable chromatophores (the Sole, Sepia, &c.), 

 but of a resemblance between the colour of external 

 objects and that of a secreted substance poured out upon 

 them. The existence of such a phenomenon, if proved, 

 would introduce new possibilities into physiology. 



It is, of course, believed that this power of adapting 

 the colour of the cocoon is a protection from enemies, 

 and it is suggested that as such it may have arisen and 

 been perpetuated by Natural Selection. To this view 

 there is an objection which may be widely applied in 

 like cases, but which in this one has particular force. 

 The belief that the resemblance between the cocoon and 

 adjacent objects protects the insect is based on expecta- 

 tion and not on evidence. If we ask from what enemies 

 the insect is thus protected, we are told from insecti- 

 vorous enemies ; and here the matter must rest. There 

 is as yet no direct evidence that a definite bird or mam- 

 mal, for instance, has ever been seen to open a cocoon 

 of S. carpini or E. lanestris ; still less that any such 

 animal habitually searches for these cocoons. In the 

 case of S. carpini, at least, it may be plausibly argued 

 that, so far as a priori impression goes, it is unlikely 

 that these cocoons are sought by birds, for the wall of 

 the cocoon is so tough that it must be difficult for most 

 birds to pierce it. No doubt rats and mice could gnaw 

 through them, but it is likely that these animals, which 

 are for the most part nocturnal, depend for their supply 



