368 POPULAR SCIENCE MONTHLY. 



gently sloping lowland between them and the ocean the Atlan- 

 tic 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 bound- 

 ary, 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 Penn- 

 sylvania, 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 be- 

 tween "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 dif- 

 ferent 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 white- 

 washed 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 knowl- 

 edge of just where the subtle change takes place. For it is by 

 the range of country that a bird inhabits or where some particu- 

 lar 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- 



