The Scottish Naturalist. 155 



drier soil and climate — as, for example, many of our forest trees. 

 That such alternations of climate took place we may learn from 

 a study of deep sections of peat, where — with greater or less dis- 

 tinctness — the succession of wet periods, characterised by marsh 

 plants, and of dry periods, characterised by forest trees, may be 

 seen. Of course there was no sudden change from one period 

 to another, but a gradual one from wet to dry, and vice versa. 

 In those intermediate periods — neither very wet nor very dry — 

 the plants that affected an intermediate condition would grow 

 and spread, and the insects that feed on such plants would be 

 sure to follow them. 



Betula 7ia7ia is one of the plants that like a moderately damp 

 situation, such as is afforded by a spongy sphagnum -covered 

 morass. The ericaceous plants (Arctostaphylos, Vaccinium^ and 

 Callmid) prefer drier ground, and though lovers of a peat soil are 

 not dependent on it. .They are, however, species that probably 

 spread during the wet or intermediate periods. Except the Arc- 

 tostaphylos, these plants are of wide distribution in this country, 

 and hence the range of the insects is not restricted by that of 

 the food-plants, even supposing that they were (or are) always 

 confined to these plants, which there are grounds for believing 

 not to be entirely the case. 



Judging, however, from the present distribution in Britain 

 of these insects (namely, their being confined to the northern 

 half of Scotland), it seems quite possible that they were never in- 

 habitants of England, but that spreading along the shore (ex- 

 tending, as has been pointed out, between the Continent and 

 Scotland) of the North Sea they reached Scotland, and ascended 

 to the mountains when these were sufficiently free of snow and 

 ice, and when the low grounds could be no longer held. In 

 this manner the Zetlands were reached by Anarta nielanopa. Of 

 course, on the other hand, it is equally possible that they did in- 

 habit England, but did not succeed in maintaining their position. 



The reason already suggested for the non-persistence of those 

 alpine species in lowland localities, once necessarily occupied by 

 them, and which reason may be briefly defined by the now clas- 

 sical expression, " survival of the fittest," would be brought about 

 by various agencies, more or less obscure or imperceptible to us 

 in their action, but not the less efficacious. None of the Lepi- 

 doptera in question seem to be less protected from the attacks 

 of enemies (such as birds, carnivorous or parasitic insects, &c.) 

 than the majority of other species, though such enemies are 



