314 Mr. E. B. Poulton. Essential Nature of the [Apr. 23, 



this, that such pigments afford the material which is moulded by- 

 some subtler influence into a likeness to a special part of the environ- 

 ment. Of the nature of this influence we know nothing at present, 

 except that the whole investigation points towards a nervous circle 

 whose efferent effects are seen in the regulation of the passage of pig- 

 ments through the digestive tract into the blood, and finally the tissues, 

 and in the colour of a certain amount of true larval pigment, while 

 the afferent part of the circuit must originate in some surface capable 

 of responding to delicate shades of difference in the colour of the part 

 of the environment imitated. This interpretation is rendered un- 

 usually difficult by three facts : the gradual working of the process, 

 often incomplete in a single life ; the excessively complex and diverse 

 result, and the special character of the stimulus (for it is only the 

 part of the environment imitated which produces any effect, e.g., the 

 undersides of the leaves in the case of S. Ocellatus, and yet the 

 environment, of course, includes both surfaces). In parallel cases 

 (Amphibia, fish, &c), as far as any parallelism exists, the stimulus 

 acts upon the eye, and this may be true of larvae also, but it is useless 

 to speculate on the subject until further data have been acquired by 

 experimental research. 



Variations in the colour of the derived pigments in the blood occur 

 apparently spontaneously and uselessly in the opaque P. Bucephalus, 

 for there is a great difference in the yellowness (xanthophyll) of the 

 blood of this pupa and in the amount of chlorophyll. Thus it is 

 possible that the variation began in this way, and was afterwards 

 rendered efficacious by co-ordination with the environment. But 

 there are difficulties in the way of any suggestion founded upon 

 observations on this species, for the existence of green blood seems 

 to indicate a different method of colouration in former times, and if 

 so, the variations in colour may be themselves remnants of a past 

 susceptibility to the influence of environment. 



