THE GENESIS AND MIGRATIONS OF PLANTS 219 



their houses most of our garden flowers, as roses, fuchsias, and 

 geraniums, showing that it is merely warmth, and not light 

 that is required to enable a subtropical flora to thrive in Green- 

 land. Even in Canada, which has a flora richer in some re- 

 spects than that of temperate Europe, growth is effectually 

 arrested by cold for nearly six months, and though there is 

 ample sunlight there is no vegetation. It is indeed not im- 

 possible that in the plans of the Creator the continuous 

 summer sun of the Arctic regions may have been made the 

 means for the introduction, or at least for the rapid growth and 

 multiplication, of new and more varied types of plants. It is a 

 matter of familiar observation in Canada that our hardy garden 

 flowers attain to a greater luxuriance and intensity of colour 

 in those more northern latitudes where they have the advan- 

 tage of long and sunny summer days. 



Much, of course, remains to be known of the history of the 

 old floras whose fortunes I have endeavoured to sketch, and 

 which seem to have been driven like shuttlecocks from north 

 to south, and from south to north, especially on the American 

 continent, whose meridional extension seems to have given a 

 field specially suited for such operations. 



This great stretch of the western continent from north to 

 south is also connected with the interesting fact that, when 

 new floras are entering from the Arctic regions, they appear 

 earlier in America than in Europe ; and that in times when the 

 old floras are retreating from the south, old genera and species 

 linger longer in America. Thus, in the Devonian and Cre- 

 taceous new forms of those periods appear in America long 

 before they are recognised in Europe, and in the modern 

 epoch forms that would be regarded in Europe as Miocene 

 still exist. Much confusion in reasoning as to the geological 

 ages of the fossil flora has arisen from want of attention to 

 this circumstance. 



What we have learned respecting this wonderful history has 



