1867.] DRUMMOND -DISTRIBUTION OF PLANTS. 169 



almost skirts the southern and western parts of Lake Michigan. 

 Now, it is generally known that the north-eastern parts of North 

 America have a temperature lower than that of the central plains 

 and wooded countries in similar latitudes, and that the lines of 

 mean temperature rise very considerably as they cross the conti- 

 nent from the New England States and Canada westward. The 

 reason for this lies in the much greater mass of land on the western 

 half of the continent extending far into the Arctic Sea, the large 

 areas of polar land on the eastern side separated by extensive 

 bodies of water from the mainland, and the Great Lakes — all (if 

 which tend, on principles Ion-' since, stated by Lycll, Humboldt, 

 Dana, and others, to produce a lower temperature in the north- 

 eastern sections of the continent. Other influences, arising from 

 proximity to the sea, from the Labrador current, and the general 

 configuration of the coast, also lend their aid. Now, a plant from 

 the warmer temperate zone, in migrating northward, would not 

 range far up those valleys having a north-eastward bearing from 

 the gradually lower temperature mot with there, and yet, favoured 

 by the course of the valleys and the warmer climate, would be 

 found in much higher latitudes farther inland. Further, the Ap- 

 palachian chain of mountains must form to some extent a barrier 

 to eastward distribution. It is also a noteworthy circumstance, 

 when taken in connection with the lower temperature in proceed- 

 ing northward, that at least the larger river valleys of eastern New 

 York and the New England States have a general southern direc- 

 tion. In this way, it seems to me, the apparently anomalous 

 north-westward range of many American plants can be fully ac- 

 counted for. To some of the causes mentioned, added to the con- 

 figuration of the coast lines of Lakes Superior, Michigan, Huron, 

 and Erie, must be also ascribed the presence of the few south 

 temperate plants which occur around Lake Superior. The lower 

 temperature and the broken character of the country must alone 

 prevent many other species from also finding homes there. 



In the districts which border Lake Erie there is a not unex- 

 pected intermingling of northern temperate with more southern 

 forms. The most casual observer will not fail to account for this. 

 Separated on the one side by the River Niagara from the western 

 part of the State of New York, the district extends westwardly 

 along Lake Erie, widening gradually in its course, consequent on 

 the form of the lake, until it almost touches upon a not inconsid- 

 erable part of Michigan. We would be quite prepared to meet 



