DISTRIBUTION OF THE BRITISH BUTTERFLIES. 275 



be for some groups, it is, I venture to think, only partly appli- 

 cable to the Lepidoptera. 



It will be remembered that at the time when the British 

 Islands were receiving the bulk of their animals and plants, and 

 when there was land connection between the Continent and the 

 various islands, that a large lake of considerable breadth extended 

 from Wales to beyond the North of Ireland. From this lake a 

 large river flowed south, and being joined probably by the Seine 

 and other French rivers, emptied itself into the Atlantic. That 

 this lake and river formed an effectual barrier to the passage of 

 many animals and plants, and necessitated their passing round 

 the head of the former, and so reaching, by way of Scotland, the 

 North of Ireland, may well be believed. But it seems quite 

 possible that many Lepidoptera were able to fly across the river. 

 If such was the case, there is no need of believing that the 

 butterflies common to England and Ireland, but not now found 

 in Scotland, passed through Scotland on their way to Ireland. 

 In fact, some of them need not necessarily have passed through 

 the present area of England either. Hence the occurrence of 

 certain species in Ireland does not present sure evidence of their 

 arrival in Britain before the communication with Ireland was 

 cut off. 



Are there any other grounds on which we can base the 

 relative antiquity in Britain ? In the first place (as suggested at 

 the beginning of this paper), the relations of the species to its 

 food-plant; and, in the second, the range or distribution at home 

 and abroad, may afford some evidence. In some cases these two 

 factors may be considered together. Flowering plants (to which 

 division all the food-plants of the British butterflies belong) may 

 be classed as self-fertilised or anemophilous (or wind fertilised), 

 and entomophilous, or requiring for the most part insect-agency 

 for fertilisation. It is reasonable to suppose that the first of these 

 — the self-fertilised and anemophilous plants — would establish 

 themselves before the entomophilous plants (the reader will 

 remember that the ice-age had cleared Britain of every living 

 thing, animal and vegetable), since the plants would precede the 

 insects that fed on them, and the self-fertilised or anemophilous 

 species would have an advantage at first over the entomophilous 

 ones. Consequently, the insects which fed on the self- fertilised 

 and anemophilous plants would in like manner have at first 



