234 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



pended upon a tempting but overstrained analogy to 

 the darkening of the sensitive photographic plate un- 

 der 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 photographic chemical. No 

 known substance retains permanently the color re- 

 flected on it by adjacent objects.' The supposed 'pho- 

 tographic sensitiveness ' of chrysalides was one of those 

 deceptively feasible suggestions which are not tested 

 because of their apparent probability. It would have 

 been very easy to transfer a freshly formed pupa from 

 one color to another which is known to produce an 

 opposite effect upon it ; and yet if this simple experi- 

 ment had been made the theory would have collapsed, 

 for the pupa would have been found to resemble the 

 first color and not the second. Furthermore, Mr. 

 Wood's suggestion raised the difficulty that chrysa- 

 lides which had become exposed in the course of a 

 dark night would have no opportunity of resembling 

 the surrounding surfaces, for the pupal colors deepen 

 very quickly into their permanent condition. 



"Having thus defined the time of susceptibility, 

 the next question was to ascertain the organ or part of 

 the larva which is sensitive. At first it appeared 

 likely that the larvae might be influenced through their 

 eyes (ocelli), of which they have six on each side of 

 the head. Hence, in many experiments the eyes of 

 some of the larvae were covered with an innocuous 

 black opaque varnish, and they, together with an equal 

 number of normal larvae from the same company, were 

 placed in gilt or white surroundings. The pupae from 

 both sets of larvae were, however, always equally light- 



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