154 The Scottish Naturalist. 



folia, and (in Switzerland) on ,5". aizo'dn. Saxifraga aizoides must 

 have been a very widely-spread species in this country before it 

 was crowded out by other plants, as, though it is chiefly confined 

 to the mountains, it yet occurs in various low-lying localities to 

 which it cannot have been brought by water from the hills. It 

 also descends the rivers. A curious fact in its distribution is 

 that, though common in Ireland, it has not been found in Wales. 

 We may therefore suppose that it entered England on the east, 

 and reached Ireland by way of Scotland. The Zelleria doubtless 

 followed it, and, though as yet only noticed (in Britain) in a few 

 localities in Scotland, is probably of much wider distribution 

 than is imagined. I am led to suppose this by finding it in every 

 suitable locality in which I have looked for it. Against this 

 theory of its line of migration must be set the fact that Saxifraga 

 oppositifolia, on which it also feeds, does occur in Wales. The 

 plant par excellence to which it is attached is, however, S. aizoides. 

 The SiJuanwierdamia had better be considered with the remain- 

 ing insects. The latter, so far as their food-plants are known, 

 feed on ericaceous plants — species which mostly afi"ect a peaty 

 soil, and which, perhaps, would scarcely cross to Britain, or at least 

 spread widely there, till a suitable soil had been prepared for them.^ 

 Some of these plants will scarcely flourish except when there is a 

 large amount of peat in the soil; but others, though mostly found 

 on such soils, will grow well enough where there is no peat, and 

 probably only occupy the peat because they find on it less of a 

 struggle for existence — or, in other words, they can flourish on it, 

 while other plants which, on a different soil would overgrow 

 them, cannot. Be this as it may, this kind of soil results from 

 the previous growth and decay of other plants, more especially 

 such as love a damp situation or climate. Now we have good 

 reasons for believing that after the close of the glacial epoch 

 great alternations of wet and dry periods of climate took place. 

 During the wet periods moisture loving and marsh-plants would 

 find conditions most favourable for their existence and spread, 

 and consequent on their growth and decay peat and peaty soil 

 would be formed. During the dry periods the reverse would 

 happen, and the vegetation would consist of plants that prefer a 



1 I need scarcely remind my readers that wherever the ice-sheet had spread 

 (and that was over the greater part of the country), the peaty soil which had 

 accumulated in the pre-glacial or inter-glacial periods was, except in a few 

 sheltered places, swept away by the grinding ice, and replaced or overlaid 

 by the bouldcr-clay or till. 



