256 THE GEOLOGICAL HISTORY OF PLANTS. 



winter snow not only protects evergreen plants from those 

 sudden alternations of temperature which are more de- 

 structive than intense frost, and prevents the frost from 

 penetrating to their roots, but, by the ammonia which it 

 absorbs, preserves their greenness. According to Dr. 

 Brown, the Danish ladies of Disco long ago solved this 

 problem.* He informs us that they cultivate in their 

 houses most of our garden flowers — as roses, fuclisias, and 

 geraniums — showing that it is merely warmth and not 

 light that is required to enable a subtropical flora to 

 thrive in Greenland. Even in Canada, which has a flora 

 richer in some respects than that of temperate Europe, 

 growth is eiiectually arrested by cold for nearly six 

 months, and though there is ample sunlight there is no 

 vegetation. It is, indeed, not impossible that in the 

 plans of the Creator the continuous summer sun of the 

 arctic regions may have been made the means for the in- 

 troduction, or at least for the rapid growth and multipli- 

 cation, of new and more varied types of plants. 



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 shuttle- 

 cocks 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 re- 

 gions, they appear earlier in America than in Europe, 

 and that in times when- old floras are retreating from the 

 south old genera and species linger longer in America. 

 Thus, in the Devonian and Cretaceous new forms of those 

 periods appear in America long before they are recognized 



* " Florula Dlscoana," Botanical Society of Edinburgh, 1868. 



