37 o POPULAR SCIENCE MONTHLY. 



vast territory to the south known as the austral zone. It is 

 evident that the southern boundary of the boreal and the north- 

 ern 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 over- 

 lapping 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 sum- 

 mer 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 Fahren- 

 heit. It is a significant fact that this subdivision of the austral 

 zone into two temperature belts conforms exactly with its sub- 

 division into two characteristic life regions. The boundary sepa- 

 rating 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 geolo- 

 gists 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 vegeta- 

 tion 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. Be- 

 yond 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 



