Page:Popular Science Monthly Volume 51.djvu/382

370 vast territory to the south known as the austral zone. It is evident that the southern boundary of the boreal and the northern boundary of the austral zones do not coincide, but leave more or less of an intermediate territory where the peculiar types of the northern and the southern life mingle. This overlapping area is known as the transition zone.

Dr. Merriam has shown that these three great life zones—boreal, transition, and austral—are also temperature zones, each one of which is characterized by a definite sum total of heat throughout the reproductive period. Thus the mean daily summer temperature of the boreal zone never aggregates above ten thousand degrees Fahrenheit, while the daily temperatures of the transition zone always aggregate above this. The austral zone is marked by two temperature belts—the upper austral, aggregating above eleven thousand five hundred, and the lower austral, aggregating above eighteen thousand degrees Fahrenheit. It is a significant fact that this subdivision of the austral zone into two temperature belts conforms exactly with its subdivision into two characteristic life regions. The boundary separating these is indicated by a line that, starting at the mouth of Chesapeake Bay, runs southwestward to the borders of Georgia and Alabama, and, then turning northward, reaches the mouth of the Ohio. This line of demarcation coincides with what geologists know as the "fall line," where the various rivers, in their course from the highland region to the sea, break into a series of rapids as they flow from the higher and older formations of the Piedmont lands to the lower and more recent Tertiary deposits of the alluvial plains.

Each of these great zones is characterized by the presence of certain animals and plants that do not range beyond its limits. A traveler journeying northward from the tropical shores of the Gulf States, with their flocks of pelicans and flamingoes and their characteristic palms and mangrove swamps, marks the change from one region to another in the different species of plants and animals which he encounters. The change in vegetation alone is striking. The persimmons, tulip trees, magnolias, sweet gums, sassafras, papaws, and other forms that characterize the landscape of the Southern States give place to the oaks, hickories, and chestnuts, and, farther north, to the maple and beechwoods and the birches and aspens of the highland and mountain regions in the Middle States and New England. Beyond these deciduous woods of the transition zone the traveler enters the vast domain of coniferous forests that mark the boreal region of North America. Days of journeying through the wilderness of evergreens bring the wayfarer at length out into the "tree-line zone," scattered clumps of spruces and firs that