202 THE CHAIN OF LIFE. 



connected with Greenland in this or in earlier times.* Such a 

 junction, cutting off access of the Gulf Stream to the Polar Sea, 

 would, as some think, other things remaining as they are, 

 almost of itself give glaciation to Europe. Greenland may be 

 referred to, by way of comparison, as a country which, having 

 undergone extreme glaciation, bears the marks of it in the 

 extreme poverty of its flora, and in the absence of the plants 

 to which its southern portion, extending six degrees below the 

 Arctic Circle, might be entitled. It ought to have trees, and 

 might support them. But since destruction by glaciation no 

 way has been open for their return. Europe fared much better, 

 but suffered in its degree in a similar way. 



"Turning for a moment to the American continent for a con- 

 trast, we find the land unbroken and open down to the tropic, 

 and the mountains running north and south. The trees, when 

 touched on the north by the on coming refrigeration, had only 

 to move their southern border southward, along an open way, 

 as far as the exigency required ; and there was no impediment 

 to their due return. Then the more southern latitude of the 

 United States gave great advantage over Europe. On the 

 Atlantic border, proper glaciation was felt only in the northern 

 part, down to about latitude 40°. In the interior of the country, 

 owing doubtless to greater dryness and summer heat, the limit 

 receded greatly northward in the Mississippi Valley, and gave 

 only local glaciers to the Rocky Mountains ; and no volcanic 

 outbreaks or violent changes of any kind have here occurred 

 since the types of our present vegetation came to the land. So 

 our lines have been cast in pleasant places, and the goodly 

 heritage of forest-trees is one of the consequences. 



"The still greater richness of North-east Asia in arboreal vege- 

 tation may find explanation in the prevalence of particularly 

 favourable conditions, both ante-glacial and recent. The trees 



* Gray's reasoning is based on the extreme view of the Glacial period 

 now prevalent in America, contrary, as it appears to me, to the actual facts ; 

 but with limitations it holds good on more moderate views as well. 



