3 12 
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 
