18 Transactions of the [Sess. 



flora similar to the latter, especially if currents run between the 

 two, a cessation of the currents may lead in time to the flora 

 becoming specifically, though not generically, distinct. 



As the work of upheaval or depression cannot be going on every- 

 where at the same time, it follows that if we find the character of 

 one habitat changing, the plants which grew on it may retire to 

 others which are not changing, provided that means are present 

 for the efiectual transmission of seeds. This migration would be 

 most effectually accomplished among seeds with a pappus, while 

 pond-weeds whose seeds sink to the bottom might have some 

 difficulty in spreading. The principle of migration, if the word is 

 suitable for plants, finds a fitting analogy in paleontology. Be- 

 tween the Chalk and the Eocene there is an enormous break both 

 in geological time and fossil contents. This is correctly explained 

 by supposing that the mammals living at the time of the Weald 

 migrated elsewhere, upon the great depression of land-surface pre- 

 vious to the deposition of the cretaceous rocks, and returned during 

 Eocene times, previous to which a great process of upheaval took 

 place. Australia at the present day has several forms akin to 

 those of Mesozoic times, as the bivalve Trigonia, the Port Jackson 

 Shark, and the Burramunda [Ceratodus Fosteri), all of them gener- 

 ically akin to species long extinct in our islands, which must have 

 betaken themselves thither when they could no longer keep up the 

 struggle for existence here. 



It is a fact well known to botanists that some alpines are not 

 restricted as to their range of altitude. On the west coast of Scot- 

 land they descend to sea-level. In mountainous districts they are 

 also found almost on a level with the sea-shore, especially if streams 

 running down from higher grounds are present. This fact may 

 either be owing to the excessive rainfall of the west coast, or from 

 the fact that the soil at the sea-shore is the same as that within 

 alpine limits, as all the rocks of the Highlands are metamorphosed 

 Lower Silurians, except the patches of Cambrian rocks in the ex- 

 treme north. From which it is evident that a very considerable 

 depression would not cause some of our alpines to become extinct 

 in certain parts of Scotland, where the moisture and soil are the 

 same as that on mountain-summits. We do not, however, find 

 alpines growing on the sea-coast in the east of Scotland, although 

 the seeds must be carried to sea-levels in some places ; and it 

 would be a curious matter to inquire into, and to ascertain whether, 

 if we had a rainfall on the west coast equivalent to that on the 

 east, and a soil resulting from the disintegration of carboniferous 

 rocks instead of Lower Silurians, we should still find alpines de- 

 scending to the sea-shore. Very probably they would not. 



