gently sloping lowland between them and the ocean—the Atlantic coast plain.
The coast plain is first found as a narrow strip in New Jersey. A line drawn from about the locality of Long Branch diagonally across the State to the Delaware River, at a point some distance below Philadelphia, serves roughly to indicate its inland boundary, marking it off from the upland terraces that form the foot of the highland slope. The line of demarcation then runs more and more inland, cutting off a small section of southeastern Pennsylvania, and, proceeding across the upper streams and estuaries of Chesapeake Bay, passes along the edge of the mountainous regions of Maryland and Virginia and the upland slopes of the Piedmont lands in North Carolina.
The traveler who journeys southward through William Penn's "low counties" finds himself on this line of demarcation between "the North" and "the South." Philadelphia, the last of the "Northern cities," lies behind him, and when Baltimore is reached the traveler begins to feel that he has passed into a different atmosphere. A certain unmistakable difference in voice and speech and a softer manner are, more than anything else, the first Southern characteristics to strike the stranger. The colored folk become more plentiful, and pickaninnies at the doors of whitewashed cabins form a not unfamiliar foreground touch in the landscape south of the city of Penn. From a car window one sees little of the change that comes over the face of Nature in passing from one region to another. But to him who fares by the way, with a keen instinct for things afield, comes the knowledge of just where the subtle change takes place. For it is by the range of country that a bird inhabits or where some particular tree or wild flower grows that Nature maps out the boundary lines of regions.
Naturalists have long recognized the fact that certain kinds of animals and plants were characteristic of certain regions of country, and that the boundaries of these regions coincided with lines of temperature or isotherms. Every species of animal and plant is definitely related to a certain fixed quantity of heat which is required for the full development of its reproductive activities. It is a habit fixed by purely physiological conditions. Various species of animals and plants, for some occult causes dating back to a remote period in their history, require a greater amount of heat throughout the period of reproductive activity than do other species, even though they be closely related. The species of animal or plant that requires the greater sum total of heat will find the northward limits of its range farther south than the species that requires a less amount. The breeding range of many birds, the dispersal of various species of mam-
