176 NATURAL SCIENCE. March, 



(imperfectly ?) in acids. They never gave the murexide reaction. 

 From Urech's description there can be little doubt that these pigments 

 belong to the widely spread group of the melanins, which are chiefly 

 characterised by their colour and their insolubility ; their chemical 

 relationships are unknown. 



Looking now at the colours of butterflies in general, we may note 

 that the characteristic pigments are derivatives of the uric acid group 

 and melanins. Lipochrome pigments seem to be entirely absent in 

 the adult. This is a very interesting fact, not only because lipochromes 

 are so widely spread and so common as colour-producing agents, but 

 because, as we have already seen, they are usually so important in 

 the coloration of the larvae. Again, there are many interesting 

 analogies in the coloration of birds and insects, and yet in birds 

 lipochromes are characteristically the pigments of the plumage, and 

 waste products are not known to occur as colouring agents. Such 

 facts as these must be explicable in terms of the physiology of the 

 individual, and must, therefore, be allowed for in the construction of 

 theories as to the origin of colour. 



So far we have been concerned only with observed facts, but this 

 paper would be incomplete without some reference to the conclusions 

 which have been drawn from these facts. It has been repeatedly 

 noticed that in the development of the individual a certain succession 

 of colours can be seen. Thus in Vanessa urticce towards the end of 

 pupation, the wing-scales are white, and the development of the 

 coloured elements takes place in the following order : — yellow, orange, 

 red, red-brown, dark brown, and finally black. Urech, Eimer, and 

 others believe that this ontogenetic succession corresponds to the 

 phylogenetic order of the development of the characteristic colours. 

 Urech, indeed, compares the succession of colours to various organic 

 series where successive steps in substitution are associated . with 

 colour progression. He regards greenish-yellow or yellow as the 

 simplest pigment, and says that, as the molecular weight increases, 

 there is a colour progression through orange, red, violet, to blue and 

 finally green. The cause of this increase in molecular weight he 

 holds to be the influence of external conditions, such as better food 

 and warmer climate ; as to the nature of the effect and the question 

 of inheritance, nothing is said. 1 He considers that the pigments arise 

 from uric acid and from the allied nuclein bases (xanthin, hypoxanthin, 

 adenin, guanin). Their proximate origin is from leucocytes, and 

 their ultimate origin from nuclein, which undergoes spontaneous 

 decomposition into the nuclein bases, albumin, and phosphoric acid. 

 In connection with this suggestion, Urech notices the frequent 

 occurrence in the scales of colourless substances which are associated 

 with pigment, and, according to him, are derivatives of uric acid. 



1 Since this was written, a suggestive, if somewhat vague and mystical, paper 

 has been published by Simroth (Bivl. Centralbl., xvi., pp. 35-51; Jan., 1896), who 

 works out in detail a similar view as to the evolution of colour. 



