136 BUTTERFLIES 



of especial value when one attempts to determine tlie re- 

 sults of unusual conditions upon living things. 



In a somewhat similar way the peat bogs or sphagnum 

 swamps which occur here and there over a large part of 

 North America are of especial interest, because in a way 

 they are biological islands in which the conditions of a 

 long past age are preserved until the present. These 

 nearly always occur in a little valley surrounded on all sides 

 by hills. Here the water has collected originally into a pond 

 or lake, which has been gradually filling up through the 

 growth of peat mosses and a special set of other plants 

 that develop in such situations. One can still find many 

 stages in the process. In some bogs the surface will be 

 practically covered, although the water beneath may still 

 be so abundant that the matted moss quakes as one walks 

 over it. Sometimes such bogs are really dangerous be- 

 cause the walker may drop through to the water beneath. 

 In most of the bogs, however, the little lake is nearly filled 

 but shows the surface over a small area. 



The conditions in these peat bogs have changed little 

 since civilization began. They are relics of an earlier 

 era which have come down to us as types of conditions 

 that once existed very generally. The plant life is unique 

 and consists almost entirely of forms which are found prac- 

 tically nowhere else. There are comparatively few ani- 

 mals living in these peat bogs and all of these are likely 

 to be of especial interest. Among the insects none is 

 more remarkable than the Baltimore Checker-spot but- 

 terfly which has several peculiarities that differentiate 

 it from the other members of the group. It seems to have 

 come down to us unchanged from a far remote past and 

 to be living its tranquil life to-day in precisely the same 



