146 RELATIVE GEOLOGICAL ANTIQUITY OF TEEES. 



slowly assumed its present appearance, and the evidence would 

 seem to lead us irresistibly to the conclusion, that changes 

 have taken place in the organization of plants, by which their 

 forms have been gradually and contemporaneously adapted to 

 the ever changing landscape. Hence the history of the de- 

 velopment of plants is intimately associated with the history 

 of those physical changes which the earth's surface has under- 

 gone. Just as the present form of a grand and venerable tree, 

 which appears to us to be fixed, but in reality is as fleeting 

 as all the other forms through which that tree has passed from 

 its first life-movement in the seed, is the final result of a long 

 series of antecedent changes, so with the globe which we in- 

 habit. The present appearance or, more truthfully speaking, 

 phase of creation, is the necessary result of a long succession 

 of antecedent changes of which the earth's crust has preserved 

 the memorial. This world, what is it but a great and ancient 

 theatre, where the scenery of life is ever changing ? And who 

 dare say that the present arrangements of land and water, the 

 forms of our modern forest trees, and of the entire animal 

 creation, are now any more fixed .and unalterable than at any 

 previous epoch ? Nothing on earth is permanent, if there is 

 any truth in the teachings of the past, and any constancy in 

 Nature. 



When a traveler at the foot of a mountain in the tropics 

 ascends toward its summit, he remarks the same change in 

 the features of fcie landscape as in advancing from the equator 

 to the poles. In both instances he finds that the temperature 

 declines, that water, whether in the form of vapor or fluid, 

 changes to eternal ice, and with this loss of heat the vegetable 

 kingdom diminishes and becomes substantially altered. The 

 plant-covering of the earth, from this point of observation, 

 may therefore be regarded as a living geographical ther- 

 mometer. The condition of vegetation at the equator and 

 the poles form in this respect a striking contrast. At the 

 equator the quicksilver in the tube of the thermometer reaches 

 its maximum elevation ; at the poles it is depressed to a mini- 

 mum. So with the vegetable kingdom. At the equator, 

 its types reach their highest expansion ; the stem, leaves, and 



