312 COLOUR IN NATURE chap. 



of the " negative " produced, and all the developments 

 of modern photography are rendered possible. Now, 

 if it is possible to obtain inorganic substances which 

 are so extraordinarily sensitive to light, it is surely 

 not impossible that organic substances, in their 

 ordinary position within the organism, may display 

 a similar sensitiveness, and therefore that pigment 

 production may be the result of exposure to light. 

 Further, as every one knows, one of the great objects 

 of recent photographers has been to discover a 

 method of photographing in colours — that is, of find- 

 ing substances which react in such a manner to 

 different rays of light as to themselves build up 

 compounds having the same colour as the inci- 

 dent light. According to Herr Otto Wiener, certain 

 compounds of silver chloride will do this ; and he 

 suggests that organic substances may possess the 

 same property, and that thus " protective " coloration 

 may be accounted for. A caterpillar may be like 

 its environment, because its skin photographs that 

 environment by means of the sensitive compounds of 

 its own tissues. So far, therefore, Simroth's theory 

 is largely based upon Wiener's suggestion, though he 

 carries it much further. 



Again, Simroth's suggestion as to a relation 

 between the colour of a pigment and its chemical 

 composition has been made on a smaller scale by 

 Urech, whose researches on the pigments of butter- 

 flies we have already quoted. Urech, in comment- 

 ing on the fact that in the butterflies of the genus 

 Vanessa the wings are at first white and the colours 

 then develop in the order of the spectrum (yellow, 

 orange, red, brown, black), suggests that there is a 



