150 The Scottish Naturalist. 



bare, — as, for example, a moor from which the turf has been 

 pared, a drained lake, or a slope uncovered by a landslip on 

 the hills. Perhaps the latter will show us something of what 

 may, in part, have actually happened at the time of which I 

 treat. Examining such a place, we will notice how, in course of 

 time, one set of plants, and frequently those that are rarest in the 

 immediate vicinity, begin to dot the surface of the unoccupied 

 ground. In a year or two they are joined and jostled, as it were, 

 by others before whom they gradually disappear, and then per- 

 haps the second set are joined by others before which some of 

 them too vanish. So it is easy to imagine how the arctic and 

 arctic-alpine plants, which seem less fitted than others to live in 

 a crowd, would first occupy the German Ocean plain, gradually 

 cross it and invade Britain, spread over perhaps a great part of 

 the country, be pursued and crowded by other plants, and be 

 finally driven up the mountains, where the conditions of life 

 would place them more on an equality with their pursuers (not 

 all of which could live on the mountains), and where they could 

 hold their own. 



But to return to our insects. Whenever plants had become 

 established, herbivorous insects would follow, each pursuing the 

 food plant or plants of its larva. The first insects would likely 

 be those attached to cryptogamic plants, such as mosses or 

 lichens ; and it is possible that if the Cramlnis and Scoparice. in the 

 list given above have larvae which feed on such plants — at pres- 

 ent their food-plants being unknown — they were the first to 

 cross the Germanic plain. Of the higher plants, those which 

 are not so dependent as others upon insect agency for the fer- 

 tilisation of their flowers would possibly be first in possession of 

 the ground, and the insects which use them as food would thus 

 get a start in the race, and by their presence assist in the spread 

 of the plants depending on their assistance for fertilisation. 



Thus Erebia Epiphron is a grass-feeder, and grasses being wind- 

 fertilised, are independent of insect agency. The Erebia^ conse- 

 quently, would find food htloxt Pachnobia, the Auartts, &c., which 

 are attached to the Ericaceous plants, which are usually fertilised 

 by insect agency. 



These are the known or probable food-plants of the insects 

 treated of in this paper : — 



Erebia Epiphron — Grasses. 



Zyga^na exulans — Low plants (Trifolium? Empetrum? &c). 



Pachnobia hyperborea — Vaccinium myitillus. 



