66 SCIENCE PROGRESS. 



races occupying distinct areas. We might suppose that 

 the former case is in reality comparable with familiar ex- 

 amples where there is an apparently constant dimorphism 

 of one sex, or with the reciprocal dimorphism of many 

 flowers, in which it is believed that the dimorphism is a 

 permanent attribute of the species as much as the differen- 

 tiation of parts in the body of an individual. The case of 

 betularia shows how the one may pass into the other. 



Before leaving the subject I will give one more ex- 

 ample, to which I have been paying some attention, because 

 it is especially one where co-operation is needed. I take 

 this opportunity of thanking M. Rene Oberthur for kindly 

 sending me specimens, and for other assistance rendered 

 me in regard to this species. The Speckled Wood butter- 

 fly of England is now well known to be a geographical 

 variant on the form inhabiting France and Spain generally, 

 the Mediterranean littoral and a great part of the rest of 

 Europe. In the South the pale yellow of our own insect is 

 replaced by a bright fulvous yellow. The southern form is 

 now called by the strict rules of nomenclature Pararge 

 egeria, while the British form is the var. egeriades. Omit- 

 ting other complications, our problem is raised in the 

 simplest form by the facts of the distribution of the 

 varieties in the French and Spanish continent. At Gib- 

 raltar egeria flies alone. Practically similar egeria flies alone 

 in the Basses Pyrenees (as also at Avignon and Tarascon) 

 and in all the country northwards as far as Poitiers. About 

 sixty miles north of this point, at Tours in the Loire 

 Valley, country not noticeably different, there is a very 

 different form which is almost exactly intermediate between 

 the two chief varieties. This intermediate form is spread 

 over Brittany and Normandy. Those from the neighbour- 

 hood of Paris are of the same intermediate form, or per- 

 haps a little paler. As far as my own observation goes, 

 there are essentially three forms, southern, northern and in- 

 termediate. From between Poitiers and Tours I have no 

 specimens, and it would be interesting to know whether in 

 that region the southern form shades off into the inter- 

 mediate. Similarly it would be interesting to have col- 



