or THE PEKUYIAIS^ A1!^DES. 25 



brief discussion of the reasons whicli go to prove fhe remoteness 

 of the period at which the original colonization of the Antarctic 

 region must have occurred. The undoubted fact of the diffusion 

 throughout the world of those natural groups of plants which I 

 have called cosmopolitan leads directly to the conclusion tliat 

 their diffusion must have been effected at a period when either 

 the relative distribution of land and sea must have been widely 

 different, or when the physical conditions of the earth's surface 

 were quite other than those now j)i*Gvailiug. Accepting, within 

 moderately wide limitSj the doctrine of the permanence of con- 

 tinental areas, I cannot resort to the first alternative, and I see 

 in tlie second the more probable explanation. The skill and 

 energy with which tlie doctrine of the uniformity of physical 

 conditions of the earth's surface during past geological periods 

 has been advanced by eminent modern authorities has gone far 

 towards obtaining general assent ; but I cannot believe that it 

 will maintain its ground against the stringent arguments with 

 which it has been met by physicists and astronomers. Allowing 

 a considerable margin for the action of modifying causes, it seems 

 impossible to doubt that, at a period not very remote in a geolo- 

 gical sense, the temperature of the earth's surface was sensibly 

 higher, the difference between polar and equatorial temperature 

 greater, and in consequence the currents both of the air and 

 ocean more rapid. Along with this the action of the tides was 

 far more violent, and the inequalities of the earth's surface more 

 considerable. The relief of the surface at any given period 

 represents the sum of the results through previous ages of two 

 sets of opposing forces — those of upheaval, and those which lower 

 eminences and fill up depressions. Chief amongst the former is 

 contraction of the earth's crust through gradual cooling ; and 

 this must necessarily have been more efficient in the earlier stages 

 of the earth's progress, while at a later period the levelling forces 

 have gradually assumed a supremacy which they obviously possess 

 at the present time. 



Various considerations tend to the conclusion that the dispersal 

 of the chief cosmopolitan genera of plants may have coincided 

 Avith the period of the deposition of the older Secondary rocks, 

 and at that period physical agencies far transcending those of 

 our experience ^^revailed throughout the earth. If the ancestors 

 of the Antarctic types of vegetation were then established in a 

 south-polar continental area, and were developed from them by 



