.TUNDRA AND CONIFEROUS FORESTS 69 



with the complete disappearance of the land ice, as far as Alaska. 

 Since the morainal strip of boreal vegetation was, in the Middle West 

 at least, always narrow, frequently interrupted, and probably com- 

 pletely absent about the Des Moines lobe, and composed chiefly of a 

 xerophytic selection of species, the western end of the forest belt was 

 composed, during its advance to its present location, of a comparatively 

 meager flora. Each mile of advance to the northward saw the widen- 

 ing of the belt and the junction of portions previously isolated, and 

 consequently permitted a westward migration of species from the 

 eastern states, which were little affected by the xerothermic climate. 

 This westward migration has by no means been completed and is 

 probably even now in progress through the forests of Quebec and 

 Ontario, but the boreal flora of the upper Lake region is still poor in 

 comparison with that of Quebec, New Brunswick, and the New Eng- 

 land states. The northwestern extension across Manitoba to Alaska 

 shows a progressive diminution in the number of species concerned, 

 as graphically shown by Transeau's 38 map. 



When these forests reached the eastern foothills of the Kocky Moun- 

 tains in Alberta, they were again in contact with the closely related 

 species of the Pacific Conifer Province, from which they had long been 

 separated. It will be interesting to learn from future studies what 

 exchange of species now took place ; whether widely distributed species 

 common to both regions, such as Populus tremuloides, had been in 

 both since their segregation ; whether the numerous Pacific plants of 

 the St. Lawrence valley are preglacial members of the northeastern 

 flora, or migrated eastward at this comparatively recent stage. It is 

 not too much to expect that all of these matters are possible of solution. 



In this migration of boreal plants, numerous relic colonies were 

 left behind. Many of them have since disappeared before the encroach- 

 ment of surrounding vegetation or the changes in climate, but thousands 

 still exist. As is generally the case with relic colonies, they occupy 

 for the most part extreme habitats, either xerophytic rock hills and 

 sand dunes, clothed with Pinus Strobus and Pinus Banksiana, or bogs, 

 characterized by Larix laricina and Thuja occidentalis. It is especially 

 noteworthy that in Ohio, where the influence of xerothermic conditions 

 was reduced by the eastern position, the hydrophytic colonies persist 

 farther south than the xerophytic. In Illinois the condition is re- 

 versed, and neither of the two hydrophytic trees is reported from Iowa. 

 Tamarack bogs are found in Illinois only along the borders of deep 

 lakes in the immediate vicinity of Lake Michigan, while Pinus Strobus 



38 Transeau, Edgar N. Forest centers of eastern America. Amer. Nat. 39 : 875- 

 889. 1905. 



