178 THE ORIGIN OF SPECIES 



been time since the commencement of the last Glacial 

 period for their migration and subsequent modification 

 to the necessary degree. The facts seem to indicate 

 that distinct species belonging to the same genera have 

 migrated in radiating lines from a common centre; and 

 I am inclined to look in the southern, as in the northern 

 hemisphere, to a former and warmer period, before the 

 commencement of the last Glacial period, when the Ant- 

 arctic lands, now covered with ice, supported a highly 

 peculiar and isolated flora. It may be suspected that 

 before this flora was exterminated, during the last Glacial 

 epoch, a few forms had been already widely dispersed to 

 various points of the southern hemisphere by occasional 

 means of transport, and by the aid, as halting-places, 

 of now sunken islands. Thus the southern shores of 

 America, Australia, and New Zealand may have become 

 sliglitly tinted by the same peculiar forms of life. 



Sir 0. Lyell in a striking passage has speculated, 

 in language almost identical with mine, on the effects 

 of great alterations of climate throughout the world on 

 geographical distribution. And we have now seen that 

 Mr. Croll's conclusion that successive Glacial periods in 

 the one nemisphere coincide with warmer periods in the 

 opposite hemisphere, together with the admission of thej 

 slow modification of species, explains a multitude of 

 facts in the distribution of the same and of the allied 

 forms of life in all parts of the globe. The living waters! 

 have flowed during one period from the north and during] 

 another from the south, and in both cases have reaehec 

 the equator; but the stream of life has flowed withl 

 greater force from the north than in the opposite di^ 

 rection, and has consequently more freely inundated th< 



