130 A Colony of Butterflies. [ March, 
vated lands. To the northward it is 1000 miles to Hopedale, 
Labrador, and here again very similar butterflies are found living 
in that northern region. 
This is a strange distribution for a butterfly, and so the question 
comes up as to the manner in which it was brought about. By 
comparing what has been found out with regard to past condi- 
tions of the earth andthe present state of things, a solution of the 
question has been offered. This solution gives us the ice period in 
North America as the agent which has induced the present dis- 
tribution of the genus to which the White Mountain butterfly 
belongs. And the colonization of the butterfly on our New 
England mountains would have been effected in this wise. 
Before the ice period commenced in New England, it had gath- 
ered in the extreme north of the continent. ‘The ice gradually 
and very slowly advanced year by year to the southward. Al- 
ways more snow fell than was melted, and this snow stayed sum- 
mer and winter, and accumulated more and more. It consoli- 
dated into nevé and glacial ice. Forming on the highest lands, 
the ice-rivers filled the ravines and joined upon the plains the 
main body of ice which was pressing southward from the pole. 
Summer and winter still alternated, but, as is the case now at the 
extreme north, the summers were short and the winters long. 
The advancing ice destroyed. or drove before it the insects and 
animals of the warmer climates, which it chilled by its approach. 
But it was kind to its own children, It brought down with it its 
Oenets butterflies and its reindeer. Before its feet it spread food 
for both of these, year by year, always pushing food and animals 
to the south. At the probable rate of less than a mile in a hun- 
dred years, it brought them at last into Virginia, from the far- 
thest north; not the Virginia of to-day, but Virginia changed 
into an Arctic scene. 
At length the climate changed. The point of farthest advance 
reached, the ice began to retrace its steps. And it called its own 
back with it, alluring them by their food, scattered ever farther 
and farther to the north. At some time the lengthening summers 
and shortening winters brought the main ice sheet back into 
New England. From Southern New York to Connecticut, to 
Massachusetts, to Vermont, to New Hampshire, it retreated all 
the way. It was as the retreat of an army with all its baggage 
and equipments, and in perfect order. Year by year it ¢ 
upon its plants, its butterflies, its animals, and they followed in 
its royal train. It had overridden all obstacles, all lives and 
