( Ixxx ) 



grouped under the headings chmate and food, direct action 

 of the environment and so forth, may be of the nature of 

 physiological adaptations correlated with external significant 

 or non-significant characters. Certainly where colour as the 

 result of pigmentation comes into question, such a view 

 seems unavoidable. Of all the characters which appeal to 

 the eye of the systematist, colour, when due to pigment, is 

 one of the most obvious products of physiological activity. 

 In harmony with this view is the fact that where colour 

 variability exists— and there is no character more obviously 

 variable — it is generally pigment colours that are concerned, 

 and not those due to physical structure such as striation or 

 lamination. Moreover, the observations of Mr. Gowland 

 Hopkins, to which I referred last year, and which have now 

 been published in extenso in the " Philosophical Transactions " 

 of the Eoyal Society,* show how an excretory product, uric 

 acid, can be utilised in the production of colour and pattern. 

 I say "utilised " advisedly, because uric acid itself is colourless, 

 but can. give rise to coloured products by chemical trans- 

 formation. In the case of mimetic Pierids, some of which 

 have been examined by Mr. Hopkins, the coloured derivatives 

 of uric acid are of direct use because they contribute to the 

 disguise. Therefore, it may be said that natural selection 

 has here made use of physiological variability by picking out 

 individuals whose uric acid transforming powers were capable 

 of being exerted in a particular manner, i.e., those individuals 

 having an advantage in the way of approximating to the 

 model in colour and pattern. And what is true of uric acid 

 must be true of all other physiological products, whether they 

 are utilised for the production of colour and pattern or for 

 any other purpose. Thus the secretions employed for 

 defensive purposes, such as the formic acid of the larva of 

 Dicranura vinula,f the butyric acid of CnrahidiB,l etc., may 

 fairly be ranked with those characters which, like adaptive 

 colouring, have an obvious use. If this be admitted we claim 



* Vol. 183, 189.5, B., pp. 061-082. 



t Poultou, Trans. Ent. Soc, 188(5, p. 15": "The Colours of Animals," 

 p. 274. 



X I'clouze, Jahresbericht iiher die Fortschritle der Chemie, 1856, p. 71G. ' 



