264 MAN AND NATURE. 



While speculating on the climate and conditions of 

 the earth's surface at the time most nearly coeval with 

 the advent of Man, we are bound to admit the difficulty 

 of the problem which the glacial period brings before 

 us. Our eminent geologist, Sir C. Lyell, has bestowed 

 all his ability and zeal in seeking to decipher the 

 probable causes of this great catastrophe the inter- 

 position, between two periods of higher temperature, 

 of a long period of such cold as to cover much of our 

 Northern hemisphere (and proofs to the same effect have 

 lately come to us from the Southern) with glaciers, the 

 magnitude of which is very feebly pictured by those 

 we now look upon in the Alps and Greenland seas. He 

 has sought to connect this enquiry with his larger re- 

 searches into changes of climate as affected by altered 

 proportions of land and sea in different geological eras. 

 But the line of discovery here has not yet fairly touched 

 the ground. The astronomical relations of our planet 

 give no aid towards a solution. Its internal condition, 

 as a molten mass crusted over, and losing heat, as we 

 presume it to have been lost through prior ages by 

 radiation into space, while plausibly explaining some 

 phenomena, leaves others in the same darkness as be- 

 fore. The total question, including its relation to the 

 human race, is one that science has not hitherto solved, 

 but to which many avenues are open, and a crowd of 

 naturalists pressing forwards upon them. 



We have thus far been but upon the threshold of 

 the subject which forms the material of Mr. Marsh's 

 volume. Yet these preliminary views are necessary to 



