80 



of the species. Thus the common Oak which in England reaches its best development 

 in Sussex and in Hampshire, dwindles down to little more than a shrub in North Africa 

 or on the mountains of Scotland ; the decline being due in the one case to excessive heat 

 and in the other to extreme cold. In Natal Pinus insignis is of comparatively slow 

 growth at low elevations ; and P. Austriaca and other trees of cold climates grow stunted 

 even so high up as Howick. Some Gums which thrive exceedingly well in the Midlands 

 cannot gnm in the Drakensberg. Again, Cupressus macrocarpa thrives better, at high 

 elevations, than C. Lusitanica, while the reverse is the case at low altitudes. 



The distribution of mean temperature over a country is influenced by latitude and 

 by altitude. Temperature decreases rapidly as we ascend from sea-level, so that if we 

 wish to eliminate the effect of altitude in comparing temperatures observed at stations 

 differing in height we must reduce the observations to sea-level. The rate of decrease 

 varies considerably in different countries and localities. Ferrel (*) gives values of this 

 rate ranging from 0'450C in Mexico to O720C in Hong Kong, but these are extreme 

 cases, and a mean of 0'552 Q C per 100 metres, or 3'03 Fahr. per 1,000 feet, represents, 

 accurately enough for practical purposes, the average decrease of temperature with 

 altitude over the globe. 



Map III., taken, in part, from Scott's Meteorology, shows the course of the isothermal 

 lines for mean temperatures of 50, 60, 70 and 80. Now, adopting a normal rate of 

 decrease of 3 Fahr, per 1,000 feet, we may find what the mean temperature would be 

 at any altitude along each of these lines, and the following table gives some of the 

 values : 



Thus, along the isotherm for 70, which crosses Natal, the mean temperature is 60 

 at 3,333 feet, and 50 at 6,667 feet. If we know the geographical and altitudinal range 

 of any tree, we may see, by inspection, which is the mean temperature which it affects . 

 And if we know also the distribution of temperature over Natal, we can tell what zone is 

 the most suitable for the growth of the species. The Table forming Appendix VIII. 

 gives, for some exotic species, the altitudinal range which would correspond, in Natal, to 

 their natural habitat. 



The distribution of temperature over Natal cannot be derived from the few direct 

 observations that have been hitherto taken. But the heights of a great number of points 

 are known ; and using a decrease of 1*5 Fahr. for an increase of 1 in latitude, and a 



* Recent Advances in Meteorology, Washington, 1886, p. 180. 



