22 MICHIGAN ACADEMY OF SCIENCE. 



sassafras, three elms, a hackberrv, a mulberry, a buttonwood. black wal- 

 nut, butternut, eight hickories, thirteen oaks, a chestnut, a beech, four tree 

 birches, three willows of tree size, five poplars, three pines, three spruces, 

 one hemlock, a balsam fir, one larch, one arbor-vitfe and a red cedar. 

 In all Europe there are only 85 species of trees. 



WHY HAS MICHIGAN SO MANY TREES AND GREAT BRITAIN SO FEW. 



This question now very naturally arises : Why has the Atlantic region, 

 including Michigan, so many species of trees and why has Europe so few? 

 Certainly we cannot attribute this difference to a defective soil and 

 climate of Europe, as they now exist, for Europe can grow all sorts of 

 trees now found in the temperate zone, while "(Ireat Britain alone can 

 grow double or treble the number of trees that the Atlantic States can." 



The former geological conditions of their continents help to explain all 

 this difference in the distribution of trees to the entire satisfaction of 

 scientists. 



Away back in the Tertiary Period the trees of the regions now possess- 

 ing an arctic climate were such as now thrive in a warm temperate zone 

 like that of Georgia and California. This is well illustrated by the 

 abundant fossil remains of trees. Following this, came a long time when 

 extreme cold prevailed, known as the Glacial Epoch, when snow and ice 

 for most or all of the year extended to the Ohio river. At the approach of 

 cold, the trees slowly retreated southward, as generation followed genera- 

 tion. The plants such as now thrive in southern Michigan, perhaps then 

 extended to what now forms the State of Alabama, while the arctic plants 

 reached Ohio. 



As the climate again gradually grew warmer, the trees and other plants 

 slowly migrated northward. Some arctic plants were stranded on the 

 White mountains and in Labrador, where they still remain ; others went 

 farther north. 



Plants of the cool temperate zone reached Michigan. In a similar man- 

 ner, during the Glacial Epoch the plants of Europe were driven south- 

 ward. The Alps, the Pyrenees, the Appenines, the Caucasus, still contain 

 some of these arctic plants which retreated there at the close of the 

 Glacial Epoch. Most of the plants of the warm temperate region had 

 perished and therefore were unable to retreat when the continent became 

 warmer. 



I quote the words of Dr. A. Gray, from whom other hints are taken, as 

 found in the American Journal of Science, page 1!»4, 1878. ''I conceive 

 that three things have conspired to this loss. First, Europe hardly ex- 

 tending south of latitude 40°, is all within the limits generally assigned 

 to severe glacial action. Second, its mountains trend east and west, from 

 the Pyrenees to the Carpathians and the Caucasus beyond, near its 

 southern border; and they had glaciers of their own, which must have 

 begun operations, and i)oured down the northward flanks, wliile the plains 

 were still covered with forest on the retreat from the great ice wave 

 coming from the north. Attacked both on front and rear, much of the 

 forest must have perished then and there. Third, across the line of re- 

 treat of those which may liave flanked the mountain ranges, or were 

 stationed south of them, stretched the Mediterranean, an impassable 

 barrier. 



"Greenland may be referred to, by way of comparison, as a country 



