282 TROPICAL NATURE, AND OTHER ESSAYS. 



mined by the successively earlier and earlier stages of 

 civilization through which we can trace them, and by the 

 changes in physical geography and of animal and vege- 

 table life that have since occurred. As we go back 

 metals soon disappear, and we find only tools and 

 weapons of stone and of bone. The stone weapons get 

 ruder and ruder ; pottery, and then the bone imple- 

 ments, cease to occur ; and in the earliest stage we 

 find only chipped flints of rude design, though still of 

 unmistakably human workmanship. In like manner 

 domestic animals disappear as we go backward ; and 

 though the dog seems to have been the earliest, it is 

 doubtful whether the makers of the ruder flint imple- 

 ments of the gravels possessed even this. Still more 

 important as a measure of time are the changes in 

 the distribution of animals, indicating changes of 

 climate, which have occurred during the human period. 

 At a comparatively recent epoch in the record of pre- 

 historic times we find that the Baltic was far salter than 

 it is now and produced abundance of oysters, and that 

 Denmark was covered with pine forests inhabited by Caper- 

 cailzies, such as now only occur further north in Norway. 

 A little earlier we find that reindeer were common even 

 in the south of France ; and still earlier this animal was 

 accompanied by Ihe mammoth and woolly rhinoceros, 

 by the arctic glutton, and by huge bears and lions of 

 extinct species. The presence of such animals implies 

 a change of climate ; and both in the caves and gravels 

 we find proofs of a much colder climate than now 

 prevails in Western Europe. Even more remarkable are 

 the changes of the earth's surface which have been 

 effected during man's occupation of it. Many extensive 



