THE EVOLUTION OF THE PLANT-WORLD 1 1 



over the equator, or great circle midway between the 

 poles, twice in the year, viz. at the spring and autumn 

 equinoxes. At those seasons the " circle of illumina- 

 tion," or great circle dividing day from night, will pass 

 through the poles, so that day and night will be equal, 

 i.e. twelve hours each, everywhere. At other seasons 

 they will be equal only on the equator, which is, 

 therefore, termed the equinoctial line, their inequality 

 increasing with the latitude, i.e. towards the poles, 

 where we have practically a day and night each of six 

 months' duration. Not only does the difference between 

 day and night increase with the latitude, owing to this 

 inclination of the earth's axis, but the sun's altitude, 

 or the angle which its noon position makes with the 

 horizon, diminishes, from the same cause, towards the 

 poles. Only within 23 of the equator is it ever 

 vertical. Only within 23% of the poles does it ever 

 remain for twenty-four hours or more above or below 

 the horizon. Thus this inclination of the earth's axis 

 gives us the difference between the length of day and 

 night at different times of the year, and the consequent 

 seasonal variations in the heat and light received during 

 any part of the year by any latitudinal zone of the 

 earth's surface. Incidentally, too, it may be remarked 

 that upon these variations in the heat received from 

 the sun depends, in the main, the rate of evaporation 

 of water and, as a consequence, to a great extent, the 

 amount of its precipitation, i.e. of the rainfall. 



Presuming this inclination of the earth's axis to have 

 been the same, or nearly the same, in past ages as it now 

 is, and the equat6r and the poles to have occupied the 

 same position on the surface of the globe as they do 

 at present, there will always have been an area of cold 

 round about each pole, and a zone of great heat midway 

 between them. Such an equatorial belt of extreme 

 heat will form a serious barrier against the migration 

 of plants adapted to a milder climate from the Northern 

 to the Southern Hemisphere, or vice versd, at the sea- 

 level. It must, however, be borne in mind that as we 

 ascend from sea-level to higher altitudes no matter in 

 what part of the world though the duration of day 

 and night remain the same, the temperature, or sensible 

 heat of the air, which is mainly derived from radiation 

 from the earth, gradually diminishes. At the present 



