268 THE ENTOMOLOGIST. 



reaching from Denmark to Shetland, the Faeroes, and probably 

 Iceland, thence passing outside the Hebrides and Ireland, and 

 gradually joining the present French coast-line in the extreme 

 south-west. All this area was not, however, dry land, for it was 

 in places intersected by great rivers and lakes, the most important 

 of the latter being one lying between England and Ireland, and 

 reaching from opposite Wales to beyond the North of Ireland. 

 As I have already pointed out the climate was then genial, and 

 there was an enormous growth of forests. 



In late post-glacial times Britain became again insulated, the 

 land area being rather less than it is at present. The climate was 

 colder, with a large rainfall, snow-fields and local glaciers, and an 

 abundant growth of peat. During this time some of the southern 

 animals, which had been tempted northwards by the genial 

 climate, were killed or driven away, and the few that remained 

 decreased both in numbers and in size. After this the land was 

 re-elevated to its present position, and the climate gradually 

 became more congenial. 



Having thus briefly sketched the condition of the country 

 during the time that most of our butterflies came in, let us try 

 and discover the reasons for the absence at the present day of 

 some of them from Scotland and Ireland. I say at the present 

 day, because I think it is extremely probable that at one period 

 (say in the middle-glacial times) we had more species than we 

 have now. As, however, there is no proof for or against this, we 

 need not take it into consideration at present. 



We will begin with the species absent alike from Scotland and 

 Ireland, somewhere about nineteen in number. 



In the first place it may be remarked that there is nothing as 

 regards the food-plants of these species which would tend to 

 prevent their occurrence. They all feed on plants which are as 

 common in Scotland and Ireland as in England. The cause, 

 therefore, of their absence must be sought elsewhere. 



In the second place the question of latitude (and consequent 

 climate, for there are other factors in the climatic conditions to 

 be taken into account) must be considered. Are Scotland and 

 Ireland too far north for the species in question ? To answer 

 this we must consider the species in detail, with the following 

 result. Four of them, namely, Machaon,* Cratagi, Semiargus and 



* For brevity's sake, aud as there is no risk of confusion, I drop, in the rest of 

 this paper, the generic names of the species, — F. B. W, 



