60 TBOPICAL CLIMATE OF THE ANCIENT EABTII. 



organic remains they contain consist of forms of zoophytes 

 allied to such as are indigenous to tropical seas ; the shells, 

 to genera and species that live in the seas of the torrid zone. 

 The Crustacea are analogous to those of India, and the 

 fishes indicate a tropical clime. The carboniferous period 

 affords evidence derived from the land, which confirms that 

 obtained from the ocean. The flora of this period is remark- 

 able for the great proportion of monocotyledonous over the 

 dicotyledonous plants of trees allied to the fern, the palm, 

 the cane, and the bamboo ; the luxuriant growth of vegetable 

 life the great development of types of structure, as ferns and 

 club-mosses, into organisms of the magnitude of forest trees 

 the enormous extent of this vegetation, reaching, as it does, 

 from Europe to Australia, with other characters, derived 

 from comparison with allied genera of existing tropical 

 plants, prove that the earth, during this period of its 

 history, possessed a universal climate, the heat of which was 

 not merely as great as that of tropical regions in our day, 

 but which Adolphe Brogniart conjectures as far surpassed 

 that of our tropics, as these exceed that of the temperate 

 zone of our time. As we advance onward, we meet with 

 accumulating proofs of the same fact. On arriving at the 

 new red sandstone, we reach the dawn of that age of reptiles 

 which extended to the close of the secondary rocks, the 

 discovery of which is due to the comparative anatomists of 

 the nineteenth century. 



Various hypotheses have been proposed to account for this 

 change of climate. Sir C. Lyell has suggested that a reversal 

 of the present distribution of the land and sea would afford 

 a sufficient cause ; and has imagined that, if a greater de- 

 velopment of land now prevailed in the southern hemisphere, 

 and of water in the northern, as is presumed to have been 

 the case in the early conditions of our globe, its ancient 

 climate would be restored. Mr. Babbage, with considerable 

 appearance of probability, has sought the cause in the radi- 

 ation of internal heat, suggesting that the accumulation of 

 fresh deposits, and the substitution of sedimentary sub- 

 stances, which are bad conductors, for water, which is a good 

 conductor of heat, may have been adequate to the result. 

 Other inquirers have referred the change to astronomical 

 causes, and Sir John Herschel has recently suggested an 



