Canadian Forests 115 



became covered with an immense glacier which reached 

 south, in many places well beyond the international 

 boundary ; and we also know that in time this ice dis- 

 appeared, and that much of the territory occupied by it 

 is again covered by forest. These changes did not come 

 suddenly. This ice field withdrew gradually, leaving a 

 lake along its southern border. As the ice receded still 

 farther, this lake filled up with moss, forming peat bogs. 

 Gradually these became dry enough to support other plants. 

 First the bushes of the heath family took possession, 

 Labrador tea, swamp blueberry and sheep laurel perhaps 

 most in evidence, with cranberries creeping in the moss. 

 Then came the trees. The spruces and tamarack were 

 first, because they grow when the ground is too wet for other 

 species. These were closely followed by the white birch, 

 aspen, balsam poplar, jackpine and balsam fir, and in that 

 order. Then might come the white and red pines, hemlock 

 and cedar. Trees follow this order partly because of the 

 condition of the soil, partly because of the influence of 

 climate, and partly because of the power they have to dis- 

 tribute their seeds and thus gain possession of new territory. 

 Where the white pine and hemlock are found, there will 

 be various kinds of hardwoods as well. The number of 

 species now rapidly increases, and the struggle of tree 

 against tree for light, air and moisture becomes severe. 

 Gradually the pines are overcome, and the forest becomes 

 almost exclusively hardwood, the conifers holding their 

 own only in certain vantage points, such as swamps, hills 

 or sandy plains, where conditions favor them. Thus has 



