176 NOMOS. 



of the focal convergence within the earth of tuc 

 rays which impinge upon the earth. But if we had 

 no other explanation to offer, we must object both to 

 the nebulous and chemical theory. "We must object 

 to the nebulous theory because we cannot look 

 upon heat as a separable and independent agency 

 any more than we can look upon light as a separable 

 and independent agency; for, so far as we know, 

 both heat and light are signs of a definite action in a 

 definite machinery, such as suns and stars. We 

 must also object to this theory because we have no 

 evidence that the earth is gradually becoming cooler, 

 as it ought to do according to the theory. On the 

 contrary, we may argue from the fact that the date 

 and vine still continue to flourish in Palestine as 

 they did when the Jews took possession of the 

 country, that the earth has undergone no perceptible 

 change of temperature during the last 3000 yeHrs ; 

 for, as Arago has pointed out, the date will not 

 ripen its fruit if the mean temperature be lower 

 than 84, and the grape will not ripen at a higher 

 temperature than 84. Nor is there any good reason 

 for supposing that the earth has ever become cooler 

 by the giving up of inherent heat. The fossil re- 

 mains which are dug up in temperate latitudes be- 

 long, for the most part, to plants and animals which 

 could only have lived in a tropical climate; and 

 these remains have therefore been thought to show 

 that a tropical heat prevailed in these latitudes 

 during the life of these plants and animals. But 

 Sir Charles Lyell has shown that the necessary dif- 

 ference of temperature may be accounted for by 



