48 COLOUR-PATTERNS. [introd. 



wings of G. rhamni when boiled yield a soluble yellow, which 

 according to Hopkins (Proc. Chem. Soc, reported Nature, Dec. 31, 

 1891) is a derivative of mycomelic acid, allied to uric acid. This 

 substance turns orange with reagents. The wings of G. rhamni 

 turn orange-red when exposed to wet potassium cyanide {Proc. 

 Ent. Soc, 1871, p. xviii) as may be easily seen. 



When these facts, meagre though they are, are considered 

 together with the evidence of variability, the suggestion is very 

 strong that the discontinuity between these several charac- 

 teristic colours is of a chemical nature, and that the transitions 

 from one shade of yellow to another, or from yellow to orange or 

 red is a phenomenon comparable with the changes of litmus and 

 some other vegetable blues from blue to red or of turmeric from 

 yellow to brown. If such a view of these phenomena were to be 

 accepted, it would, I think, be simpler to regard the constancy of 

 the tints of the several species and the rarity of the intermediate 

 varieties as a direct manifestation of the chemical stability or 

 instability of the colouring matters, rather than as the con- 

 sequences of environmental Selection for some special fitness as 

 to whose nature we can make no guess. For we do know the 

 phenomenon of chemical discontinuity, whatever may be its ulti- 

 mate causes, but of these hypothetical fitnesses we know nothing, 

 not even whether they exist or no. 



II. Colour-patterns. Thus far I have spoken only of dis- 

 continuous variations in colours themselves, but there are no less 

 remarkable instances of discontinuous variations in the distri- 

 bution of colours in particoloured forms. By a combination of 

 these modes, variations of great magnitude may occur. 



One of the most obvious cases of this phenomenon is that of 

 the Cat. In European towns cats are of many colours, but they 

 nevertheless foil very readily into certain classes. The chief of 

 these are black, tabby, silver-grey and silver-brindled, sandy, tor- 

 toiseshell, black and white, and white. Of course no two cats 

 have identical colouring, but the individual variations group very 

 easily round these centres, and intermediate forms which cannot at 

 once be referred to any of these groups are immediately recognized 

 as something out of the common and strange. Yet it is almost 

 certain that cats of all shades breed freely together, and there is 

 no reason to suppose that the discontinuity between the colour- 

 groups is in any way determined by Natural Selection. 



Another example may be seen in the Dog-whelk {Purpura 

 I a pill us). This animal occurs on nearly the whole British coast, 

 wherever there are rocks or even clay hard enough to form 

 definite crevices. Like most littoral animals, the Dog-whelks of 

 each locality differ more or less from those of other localities, and 

 these differences may be differences of size, texture of shell, 

 degree of calcification, amount of " frilling," kc. The peculiarities 



