LIFE IN LONDON 401 



ductory portion of this paper in the first edition of my " Con- 

 tributions to the Theory of Natural Selection " in 1870, but 

 in later editions it was omitted, as being- rather too technical 

 for general readers, and not easily followed without the 

 coloured plates. I will therefore give a short outline of its 

 purport here. 



I may state for the information of non-entomological 

 readers that the Papilionidse form one of the most extensive 

 families of butterflies, and from their large size, elegant forms 

 and splendid colours were considered by all the older 

 writers to be the princes of the whole lepidopterous order. 

 They are usually known by the English term " Swallow- 

 tailed butterflies," because the only British species, as well as 

 a great many of the tropical forms, have the hind wings 

 tailed. They are pretty uniformly distributed over all the 

 warmer regions, but are especially abundant in the tropical 

 forests, of which they form one of the greatest ornaments. 

 In coloration they are wonderfully varied. The ground 

 colour is very frequently black, on which appear bands, spots, 

 or large patches of brilliant colours — pale or golden yellow, 

 rich crimsons or gorgeous metallic blues and greens, which 

 colours sometimes spread over nearly the whole wing surface. 

 Some are thickly speckled with golden green dots and 

 adorned with large patches of intense metallic green or azure 

 blue, others are simply black and white in a great variety 

 of patterns, many very striking and beautiful, while others 

 again have crimson or golden patches, which when viewed at 

 certain angles change to quite different opalescent hues, unsur- 

 passed by the rarest gems. 



But it is not this grand development of size and colour 

 that constitutes the attraction of these insects to the student 

 of evolution, but the fact that they exhibit, in a remarkable 

 degree, almost every kind of variation, as well as some of the 

 most beautiful examples of polymorphism and of mimicry. 

 Besides these features, the family presents us with examples of 

 differences of size, form, and colour, characteristic of certain 

 localities, which are among the most singular and mysterious 

 phenomena known to naturalists. A short statement of the 



