THE VEGETABLE WORLD. 2 79 



earth's age ; according to which, therefore, I shall, in the 

 following pages, briefly sketch the gradual development of 

 the vegetable kingdom. 



But before I pass to this subject, I must once more 

 recur to the original condition of the atmosphere of our 

 earth, to its climatal condition and gradual alterations. 

 The temperature of our globe has two sources namely, 

 the proper internal heat existing within, and that which 

 it receives from the rays of the sun. Of the heat, how- 

 ever, which it possesses and obtains, it continually gives off 

 a certain quantity to the cold space of the Universe. 

 Cooling, and warming by the sun, now bear such a relation 

 to each other, that they maintain a perfect equilibrium, and 

 that at least for almost 3,000 years, the temperature 

 of the earth cannot have altered the tenth part of a 

 degree. We have two proofs of this ; one astronomical, 

 which is based upon the observations of the moon's 

 eclipses by Hipparchus, which I pass by here, and the 

 other botanical, which was first discovered by the ingenious 

 Arago. The Vine will no longer ripen its fruit where the 

 mean temperature of the year is higher than 84, and, on 

 the contrary, the Date will not flourish where the tem- 

 perature sinks below 84. These conditions exactly meet 

 in Palestine ; and the Jews, when they took possession of 

 this country, found the Date and the Grape together. 

 Now, had the temperature of the earth either risen or fallen 

 in the least since that time, one of those plants must 

 either have disappeared from Palestine, or have become 

 unfruitful there, which, however, is not the case. 



If the earth at present derives just so much heat from 

 the sun as it again loses by cooling into the space of the 

 universe, this is in other words to say, that the sun is now 

 the only source of heat, and, therefore, the heat must be 



