INVESTIGATION OF TEMPERATURE CHANGES. 231 



the human race any longer to make headway against the prevailing unfavor- 

 able influences, we might be furnished with evidence which, at first sight, 

 would seem to indicate that a very rapid change had taken place, but which 

 would perhaps, in reality, only mean that, after a long period of deteriora- 

 tion, a point had finally been reached where resistance was no longer pos- 

 sible, the extreme limit of adaptation having been passed. 



A change of climate by which certain portions of the earth's surface should 

 be rendered too hot for inhabitation by man is something of which we have, 

 up to the present time, no experience, for there is no country in this condi- 

 tion. The hot uninhabited region.^, like the Sahara, are deserted not because 

 the temperature is high, but on account of the absence of sufficient moisture. 

 So, too, if there are parts of the earth which w'ere formerly too hot to be 

 inhabited by man, but which have now become sufficiently cooled for that 

 purpose, it would be extremely difficult to have any proof of such change 

 having taken place. The question whether there is any geological evidence 

 that the equatorial regions were once too hot for the development of vege- 

 table life will be taken up farther on. 



With these introductory remarks, we may turn to the facts collected by 

 Arago and others bearing on the question before us. It will, liowever, be 

 well to take up first a statement most frequently quoted, and generally con- 

 sidered the most brilliant of all the results of Arago's investigations in this 

 line of inquiry, namely, his supposed demonsti-ation that the climate of 

 Palestine has not undergone axiy sensible alteration during the past 3,300 

 years. ["Tout nous porte done a reconnaitre que 3,300 ans n'ont pas altere 

 d'une maniere appreciable le cliniat de la Palestine."*] 



This statement is based on the following evidence. At the most remote 

 historical period the vine and the palm-tree flourished together in Palestine. 

 The date-palm requires a temperature of 21° (centigrade), or a little more, 

 for its successful cultivation ; the vine will not bear a higher one than 22°. 

 Consequently, where the palm and the vine grew and throve, there the 

 mean temperature must have been 21°.5, which is stated by Arago to be that 

 of Palestine at the present time, as inferred from the mean temperature 

 of Cairo, the nearest point for which any trustworthy determination was 

 to be iuid. 



It needs but little consideration to show how very slender is the thread 

 of fact on which this result hangs. In the first place, the climate of 



« 1. c, p. 218 



