290 SCOTTISH GEOGRAPHICAL MAGAZINE. 



than are outliers of any species, since the latter attract the attention of 

 observers so much more easily. Mention has already been made of the 

 immense inlying blank of the balsam poplar, which stretches from the 

 interior of Labrador to the Ottawa valley. There is a large area 

 between James Bay and Lake Superior from which both the elm and 

 black ash are absent, although this space is within the general range 

 of both these species. This blank is probably owing to the greater 

 elevation of the area, which in this latitude must have a considerable 

 effect on the climate, rendering it less favourable than that of the belt 

 of lower ground to the northward in which these two species are still 

 sparingly scattered. The balsam fir is wanting in some tracts on the 

 upper branches of the Albany river, in which it might be expected 

 to be found. 



It is believed that closer investigation may prove the existence of 

 many vacant spaces within the general limits of various species, in which 

 none of their representatives can be found, besides such areas as swamps, 

 sand plains, etc., which are manifestly unsuited to their growth. 



CONDITIONS GOVERNING DISTRIBUTION. 



In the part of Canada under consideration, the presence or absence 

 of different trees, or the contours of the lines bounding their geographical 

 distribution, are determined by general climatic conditions, modified to 

 a greater or less degree by the following circumstances, which will be 

 briefly considered : 



1 . Distance or proximity of the sea, or of the areas which were covered 



by it in recent geological times. 



2. Changes which have taken place in the arrangement of land and 



water while the trees were spreading over the continent. 



3. General dryness or moisture of the climate affecting considerable 



areas. 



4. Extremes of heat and cold. 



5. Local heat and moisture from lakes and rivers. 



6. General elevation above the sea. 



7. Local elevations (with consequent dryness) over level, wet, or 



cold lands. 



8. Large local depressions. 



9. Diseases and insect pests. 



10. Eapid or slow natural means of dispersion. 



1. The opposite effects of the sea on the distribution of the 

 Banksian pine and the balsam poplar have been already described. In 

 the Labrador peninsula the tree-lines bend southward in approaching 

 the coast on either side, showing that the sea has an unfavourable influ- 

 ence. The blighting effect of the cold Atlantic especially is very marked, 

 as it prevents the growth of almost all timber on the coast-line, except 

 the most hardy kinds in the southern part. The tree-lines run almost 

 parallel to the ocean, the tenderer species being found farther and 

 farther inland. Some kinds of trees are found in the western part of 

 Newfoundland which do not cross to the east side of the island in the 



