THE COLONIZATION OF NEW ENGLAND. 269 



the greater part of it was derived from the 

 south. 



We are thus led from a consideration of the 

 structure and relationship of the present butterfly 

 inhabitants of New England and the neighboring 

 region to the same conclusion to which the recent 

 geological history of the country would force us. 

 The great glacial sheet which covered the land 

 was no fit residence for butterflies ; but many 

 sorts, companions of the cold, which it had driven 

 southward in its advance from the pole, hugged 

 its southern boundaries in the Middle States, and 

 when this huge continental glacier commenced 

 slowly to retreat, they too were driven back to 

 their ancient sporting grounds, by the too fierce 

 heat of the summer's sun and the parching of 

 their food, the plants which love the snow. 

 Others followed at a more cautious distance, and 

 of these insects perhaps a fourth of our fauna is 

 composed. That this is a true history appears 

 from many facts, but from none more curiously 

 than the presence, on the summit of Mount Wash- 

 ington, of a fragile butterfly [Fig. 192], whose 

 home is there limited to a few acres of ground, 

 but which occurs again two thousand miles away, 

 on the alpine summits of the Rocky Mountains, 

 and is represented at half that distance, in the 

 north, by another butterfly, doubtless descended 

 from the same ancestor at no very remote epoch. 



