The Scottish Naturalist. 205 



in this way. Again and again the trees examined by me showed 

 that they had fallen from natural decay. And in many cases, 

 especially upon the tops and slopes of hills, the peat mosses con- 

 tained no trace of trees, or at most, only a few stunted trunks and 

 brushwood. No one could deny that the overthrow of large trees 

 upon the low ground and plains, from whatever cause, would ob- 

 struct the drainage, form marshes, and so give rise to the growth 

 of peat. But here were many cases where no such stoppage of 

 the drainage could have taken place — where only thin scrub grew, 

 and where the steepness of the slopes must have sufficed to pre- 

 vent any arrest of the surface drainage. Thus, even if we allowed 

 that all the great forest trees of the low-lying districts had been 

 destroyed by man, and the overlying bogs to have originated 

 through the obstruction of the drainage caused by the fallen 

 trunks, we had still to account for the existence of widespread and 

 thick peat-mosses on hill-tops and considerable slopes. Consider- 

 ations such as these led me to conclude that the presence of the 

 overlying peat-mosses looked at in a broad way, was the result of 

 changed climatic conditions, which, while less favourable to the 

 growth of forest trees, encouraged the extension of bog-forming 

 plants. And from the occurrence of successive layers of trees in 

 not a few bogs, I suspected that there might have been in post- 

 glacial times alternations of genial and ungenial conditions. But 

 when I wrote my first paper on the subject, the data bearing on 

 this particular point were too few or too little known to justify me 

 in formulating this conclusion. Later investigations in the valleys 

 of the Forth and Tay, however, considerably cleared up the 

 matter : and I was preparing this new material for publication, 

 when Axel Blytt, the well-known Norwegian botanist, issued his ad- 

 mirable essay on the immigration of the Norwegian flora. From 

 this paper I learned that Blytt had independently come to the 

 same general conclusions, from a study of the geographical distri- 

 bution of the existing Norwegian flora. 



The genial post-glacial epoch which nourished the great trees of 

 which I have been speaking, was succeeded, then, by an epoch of 

 wet and ungenial conditions. It is remarkable that at the same 

 time the British area became insulated, and the sea advanced upon 

 the land to a greater extent than is now the case. To this epoch 

 belongs the formation of the 45-50 ft. sea-beach and of the major 

 portion of the Carse-clays, etc., of the Tay and Forth. That 



