CANADIAN FORESTRY ASSOCIATION 117 



In Siberia the assemblage of species and their gradation according to climate, 

 resembles what we have in Canada. The Ural and Caucasian Mountains and 

 the Caspian and Black Seas, may have acted as barriers in preventing the free 

 passage of the forests from Asia into Europe, as the open ocean, with its adverse 

 current, prevented the trees of northern America reaching the same region. These 

 considerations may account for the small number of species in Europe, as compared 

 with either America or Asia, while the British Islands, having been separated from 

 the rest of Europe, would account for its still smaller number of indigenous trees, 

 numbering less than a dozen species. 



From the foregoing sketch, it will be seen that the history of the North Ameri- 

 can forests is a very long one, and that there appears to be no doubt the large num- 

 ber of species and their present distribution are due to the advantageous conditions 

 which prevailed so long in the broad polar regions of the continent in former geologi- 

 cal periods, followed by the wide-spread glacial conditions of a later time. 



It has been already stated that the present courses of the northern limits of 

 various trees in Canada, do not follow the parallels of latitude, even approximately, 

 but show many curious eccentricities. Some of those may be accounted for by 

 changes in the levels of certain parts of the continent, and consequently in the 

 positions and contours of former coast-lines which set bounds to the areas over 

 which the trees could spread themselves. One species may grow best near the 

 salt water, while another appears to shun the sea, either on account of the excessive 

 moisture in the air or the blighting influence of its cold winds at certain seasons, or 

 because the tree prefers higher ground. The Banksian pine and the balsam poplar 

 afford good examples of these opposite tendencies. The former is found on high 

 ground and away from the sea, its present general distribution having resulted 

 from the contours of the coasts of a by-gone period, when the continent was at 

 a lower level. On the other hand, some, at least, of the lower ground occupied by 

 the balsam poplar, has been left dry by a later elevation of the land, which conse- 

 quently lies nearer the present coasts. The distribution of some species may have 

 been affected by the changing of ocean currents, due to elevation of the land, as 

 in the case already cited in regard to the white cedar in the maritime provinces. 

 Some of the positions of the tree lines have been governed or influenced by the 

 contours of coasts which existed when the continent was more elevated than at 

 the present day, while other tree limits indicate approximately the situations of 

 coasts during a period of depression. On the east side of James Bay some tree 

 lines run parallel to the coast, as far as the present head of the bay, but continue on 

 southward for some distance further and then 'form a loop round the low ground 

 where the head of the bay once stood. 



Towards the close of the glacial epoch, the distribution of the advance guard 

 of the forests returning northward would be modified by the centres of ice-disper- 

 sion or by regions still covered with ice, although the land on one or both sides 

 might be free from it. The varying altitudes in any latitude would affect the 

 locating of different species to a greater degree than it would in the same latitudes 

 at the present day, owing to the greater general severity of the climate in the tem- 

 perate zone at that time. Even at the present day, there are isolated areas in the 

 northern woods from which a species occurring all around such areas may be absent, 

 on account of their slightly greater elevation. In going northward, species which 

 have been wanting over a whole country side may recur upon large tracts further 

 on, which lie at somewhat lower levels. This happens in passing from the. high 

 ground north of the great lakes to the lower levels towards James Bay. 



The outer limits of many species do not appear to have been influenced so 



