DISTRIBUTION OF THE BRITISH BUTTERLLIES. 267 



From this statement it will be seen that all the sixty-four 

 species enumerated above are found in England, but that twenty- 

 eight (not including three of doubtful or sporadic occurrence) do 

 not inhabit Scotland, and twenty-five, or perhaps twenty-six, have 

 not been detected in Ireland. 



It remains, therefore, to discover (if we can) the reasons why 

 certain species are absent from Scotland and Ireland, why certain 

 others which inhabit Scotland do not occur in Ireland, and why 

 some of the Irish species are not also Scottish. Having discussed 

 these points we will then pass on to the question of the order in 

 which the various species arrived in Britain. 



In trying to find reasons for the absence from, or presence in, 

 a country of any particular species, there are several things which 

 must be first taken into account, as, for example, the accessibility 

 of the country, the presence in it of the food-plant, the climatic 

 conditions, &c. 



It seems unnecessary to remark that a species will spread 

 much more easily over a comparatively level land- surface than 

 across a sea, or even over a mountain range. As far, therefore, as 

 mere accessibility is concerned, the butterfly fauna of Scotland 

 ought to be more extensive than that of Ireland, for in the one 

 case there is still a continuous land -surface, in the other a 

 formidable passage across the sea must be encountered. That 

 this was not always the case is now well known ; but before 

 entering on the subject we propose discussing, it will be well to 

 give an outline of part of the geological history of the British 

 Islands in post-glacial times.* 



Immediately after the Glacial Period (during which great ice- 

 sheets covered the most of North and North-western Europe, 

 including the British Islands) the area of Great Britain and 

 Ireland was much the same as it is now, though South Scandinavia 

 was submerged. At this time the climate was genial, and the 

 plants and animals which had been driven south by the cold 

 began to come north again. After this, or in middle post-glacial 

 times, the land rose, and our islands were joined to continental 

 Europe. This land connection was very extensive, all the sea- 

 bottom which is not now more than one hundred fathoms from 

 the surface being then dry land. This would give us an area 



* For a fuller account the reader is referred to Dr. James Geikie's ' Great 

 Ice-Age' and ' Pre-Historic Europe ; ' also ' Scottish Naturalist,' v. 202. 



