DISTRIBUTION AS AFFECTED BY TEMPERATURE. 659 



and, at the same time, marked phanges in the nature of the vegetation. 

 The scale of atmospherical temperature serves as a scale for the pro- 

 gress of vegetation. As regards the latitudinal distribution of heat, 

 the globe has been divided into eight regions, four northern and four 

 southern — viz., a tropical region, from the equator to the limits of the 

 tropics in each hemisphere ; subtropical, between this and 40° of lati- 

 tude; temperate, between 40° and 60° of latitude; arctic and antarctic, 

 beyond 60° of latitude. 



Each species of plant is adapted to thrive best between certain 

 limits of temperature. These limits do not necessarily coincide with 

 any definite parallels of latitude ; for it is well known that the climate 

 of different places in the same latitude is very different. It is of im- 

 portance, therefore, to ascertain the mean temperature of the year, but 

 particularly of different seasons. By drawing lines through different 

 places where the mean annual temperature is the same, Humboldt 

 established a series of isothermal ('I'oog, equal, and ^%/i»), heat) lines in- 

 tersecting the parallels of latitude. These lines run in curves, which 

 rise in their course from the eastern coast of America towards western 

 Europe, and sink towards the south in the interior of the continent — 

 and that so quickly, that Scotland lies in the same isothermal line as 

 Poland, and England as Hungary. It is clear, therefore, that the 

 isothermal lines in the higher latitudes do not, by any means, corre- 

 spond with the parallels of latitude. At the equator, however, these 

 lines coincide more nearly. Much depends upon the temperature of 

 the different seasons. Thus, a place which has a very cold winter 

 and a very warm summer, may be in the same isothermal line with 

 one in which the temperature of both these seasons is moderate, and 

 plants which succeed well in the one may not grow in the other. 

 Cherry-laurels and other Evergreens, which grow well in the open 

 air in England, wUl not stand the winter of places on the continent in 

 the same isothermal line. It is necessary, in determining the geogra- 

 phical distribution of plants, to take into account the mean summer 

 and the mean winter heat, and, better still, the mean monthly tem- 

 peratures. The distribution of temperature among the different months 

 of the year is of importance, especially in reference to the heat and 

 duration of the summer months ; for many plants protected by a 

 covering of snow are enabled to bear rigorous winters, provided the 

 summer be hot enough, and of sufficient duration. Lines passing 

 through places having the same mean summer temperature are called 

 isotheral (/Vos, equal, and Sigos, summer) ; those passing through places 

 with an equal mean winter temperature, are isocheimal {yiiiha, winter- 

 cold). The isocheimal lines in the interior of continents bend con- 

 siderably towards the south. In the interior of continents, the 

 isotheral lines, though doubtless bending considerably to the north, 

 follow more closely the parallels of latitude. Many circumstances 



