1 66 PROBLEMS OF GENETICS 



but I suspect that real and complete continuity, in the sense 

 thus defined, will only be found where the character of the local 

 populations depends directly on the conditions of life, and shows 

 an immediate response to changes in them apart from that post- 

 poned response which we suppose to be achieved by selection. 

 Obviously the character must be one, like size for instance, 

 capable of sensibly complete gradation. 



The only example I have met with of the phenomenon of 

 anything like a complete intergradation between local types 

 really distinct in kind is that provided by the butterfly Pararge 

 egeria. It is well known to entomologists that this insect exists 

 in two very different types, a northern one, the "Speckled Wood " 

 of England, in which the spots are a pale whitish yellow, and a 

 southern type having the full fulvous colour that we know as 

 characteristic of megaera, the "Gatekeeper." It appears that 

 Linnaeus gave the name egeria to the southern type, 1 and our 

 own is now called egerides. Broadly speaking, so far as Great 

 Britain, France, and the Spanish Peninsula are concerned, the 

 tawny-coloured egeria occupies Spain and western France up 

 to the latitude of Poitiers and the pale yellow egerides extends from 

 Scotland, where it has a scanty distribution, through southern 

 England, where in suitable localities it is common, and the north 

 of France to Paris. 2 The two types when placed side by side 

 are strikingly different from each other, and are an excellent 

 illustration of what is meant by climatic variation. The insect 

 is not a great traveller and probably scarcely ever wanders far 

 from its home. It should therefore be possible by collecting 

 from north to south to find out how the transition is effected, 

 whether suddenly or gradually. This at various times I have 

 endeavoured to do, but I am still without exact information as 

 to the population in certain critical areas. In addition to the 

 information derived from specimens which I have collected or 

 seen in the collections of others there is a good account of the 

 general distribution in Europe given by the Speyers, 3 who evi- 



1 Often referred to by older writers as Meone, Esper's name. 



2 There are also two distinct island forms, unlike the European, Xiphia of 

 Madeira, and a smaller variety, Xiphioides of Canary. See especially, Baker, 

 G. T., Trans. Ent. Soc. London, 1891, p. 292. 



3 Speyer, Adolf, and August. Verbreitung der Schmetterlinge, 1858, I, p. 217. 



