112 Canadian Record of Science. 
the junction of the Ohio and Mississippi rivers. There at 
least fourteen species are found. If we now move north- 
ward, we find that a line passing through central New 
York, northern Pennsylvania and central Ohio, marks the 
limits of ten species. A line extending from central Massa- 
chusetts through the centre of Lake Ontario, touching the 
southern extremity of Lake Huron and thence into southern 
Wisconsin, marks the northern limit of eight species. Four 
species extend as far north as Montreal, and two to Quebec, 
while only one species extends a few miles further, and thus 
reaches the extreme northern limits of distribution. 
The tulip tree has its center of distribution in Kentucky, 
Western Virginia and the eastern half of Tennessee. Its 
extreme limits reach southward, almost to the Gulf of 
Mexico, westward to the Mississippi, eastward to the 
Atlantic, and northward to the Great Lakes, finding their 
termination just within the Dominion, along the northern 
shores of Lake Erie. 
This law of distribution was fully recognized by the elder 
Michaux who, 102 years ago, undertook to determine the 
centres of distribution for all our North American trees, a 
task which led him over the greater part of the United 
States and Canada, and resulted in one of the most impor- 
tant contributions to American botany prior to this cen- 
tury. 
Distribution of different species over a comraon area and 
therefore under similar conditions, constitutes a flora. 
Between one floura and another, there are no sharply 
dividing lines—each merges more or less into the other by 
insensible degrees, yet each is distinguished by certain pre- 
vailing forms. It therefore follows from what has thus far 
been stated, that any division, in point of distribution, of 
the vegetation which covers the surface of the earth, must 
be based upon purely arbitrary considerations. 
Recognising these laws, Grisebach divides the surface of 
the earth into twenty-four great regions, each of which is 
distinguished by the characteristic or most prevalent forms 
of plant life, together with the part of the world in which 
it lies. 
