I/O Transactions of the [Sess. 



and Frog, may indicate that they are of a later type than the two 

 reptilia Ave now find existing on the islands ; but more probably they 

 were sloAver migrants to Western Europe, and did not reach the west 

 of Scotland until Colonsay and Oronsay were insular. There is, we 

 think, some evidence of this in the fact that the common Yiper is 

 abundant on Jura and Islay, the nearest land to Colonsay and Oron- 

 say ; but if we find the Yiper on the former, why should we not find it 

 upon the latter % This question, we confess, is not so easily answered ; 

 but it appears to us that the probability is, that as these reptilia could 

 only reach Colonsay and Oronsay while the land connection lasted, 

 the moment that connection was severed by a depression of the in- 

 tervening land, which Avas then covered by the sea, they were com- 

 pletely isolated and shut off from the posterior migrations of reptilia 

 that either travelled Avestwards slower, and had not reached the 

 Avest of Scotland, or only came into Britain after Colonsay and 

 Oronsay, Avith the Western Isles, had become isolated at the first post- 

 glacial period of land depression. But Iioav are Ave to account for the 

 presence of the Yiper in Jura and Islay, and its absence from Colon- 

 say and Oronsay, at the present time, if, as we have every reason to 

 suppose, the first post-glacial period of land depression was a great 

 one, submerging the islands to at least 150 feet above the present 

 sea level % This is a most important and interesting question, and 

 we think it can only be ansAvered by supposing that from the time 

 of this depression Colonsay and Oronsay, from the depth of the in- 

 tervening sea, have continued insular, while Jura and Islay, at the 

 time of the second post-glacial upheaval, became once more an integral 

 part of the mainland of Scotland, Avhich enabled the Yiper, and 

 possibly other reptilia and amphibia, to have access to those islands. 

 If this is so — and we think there is good reason to believe it — the 

 flora and reptilian fauna of Colonsay and Oronsay have a peculiar in- 

 terest to the student. 



We have been asked, Why confine your paper to the flora and 

 reptilian fauna, and not rather take the mammalia, to prove the 

 theory you advance as to the migration of animals to Colonsay and 

 Oronsay % and this, no doubt, is a very pertinent question. We may 

 answer it as regards the flora, by reminding you that, with few ex- 

 ceptions, there was little probability of the plants that represent the 

 vegetation of Colonsay and Oronsay having reached those islands 

 previous to their occupation by man except by a land connection, 

 thougli possibly a few seeds might be introduced by such agencies as 

 Avind and birds. Yet, speaking generally, the flora would require to 

 reach the islands during a period of land attachment with the main- 

 land of Scotland. With regard to the reasons why we should choose 

 the reptilia in preference to the mammalia, we must ask you to bear 

 in mind that most of the forms of the latter Avere able to SAvim, and 

 that the evidence regarding the impossibility of their not being able 



