118 THE COLOURS OF ANIMALS 



that its pupae, although appearing in two forms, dark 

 and green, like those of our own Swallow-tail, also 

 resemble the latter in having no power of adjusting 

 their colours to the surroundings. 



Theories as to the manner in which the colours of such 

 pupae are determined 



These observations and experiments had been 

 made when I began to work at the subject in 1886 : 

 they appeared to prove that the power certainly exists, 

 but nothing was really known as to the manner in 

 which the adjustment is effected. Mr. T. W. Wood's 

 original suggestion, that 'the skin of the pupa is 

 photographically sensitive for a few hours only after 

 the caterpillar's skin has been shed,' was accepted 

 by most of those who had worked at the subject. 

 And yet the suggestion rested upon no shadow of 

 proof ; it depended upon a tempting but overstrained 

 analogy to the darkening of the sensitive photographic 

 plate under the action of light. But the analogy 

 was unreal, for, as Professor Meldola stated in the 

 discussion which followed Mrs. Barber's paper, ' the 

 action of light upon the sensitive skin of a pupa 

 has no analogy with its action on any known photo- 

 graphic chemical. No known substance retains per- 

 manently the colour reflected on it by adjacent 

 objects.' The supposed ' photographic sensitiveness ' 

 of chrysalides was one of those deceptively feasible sug- 



