FLIGHT OF PLANTS AND ANIMALS. 425 



vegetation of Greenland and British America was transferred 

 far down toward the torrid zone on both the Eastern and the 

 Western Continent. Being once thus transferred, the forest 

 would be compelled to remain there until the retreat of the 

 ice began again to modify the conditions so as to compel a 

 corresponding retreat of plants toward their original north- 

 ern habitat. Thus it is that these descendants of the pre- 

 giacial plants of Greenland, arrested in their northward 

 march, have remained the characteristic flora of the latitudes 

 near the glacial boundary. On the other hand, the arctic 

 species, which can not endure even a temperate climate, and 

 which must have accompanied the advancing glacier south- 

 ward, found their natural conditions again in two ways : 1. 

 By following closely upon the steps of the retreating ice to 

 extreme northern latitudes ; and, 2. To use Professor Gray's 

 happy expression, by " taking to the mountains," and finding 

 near their summits the necessary arctic conditions. It is 

 thus that the mountains of New England and Labrador con- 

 tain many species of plants nearly identical with those on the 

 Alps in Europe. 



No better presentation of this subject can be given than 

 that of Professor Gray himself, made in an address delivered 

 in 1878, and which by the kindness of Mrs. Gray I am per- 

 mitted in large part to reproduce in this connection : * 



The forests of the whole northern hemisphere in the tem- 

 perate zone (those that we are concerned with) are mainly 

 made up of the same or similar kinds. Not of the same spe- 

 cies, for rarely do identical trees occur in any two or more 

 widely separated regions ; but all round the world in our zone 

 the woods contain pines and firs and larches, cypresses and 

 junipers, oaks and birches, willows and poplars, maples and 

 ashes, and the like. Yet, with all these family likenesses 

 throughout, each region has some peculiar features, some trees 

 by which the country may at once be distinguished. 



* " Forest Geography and Archaeology," a lecture delivered before the Har- 

 vard University Natural History Society, April 18, 1878, by Asa Gray. Printed 

 in the "American Journal of Science," vol. cxvi, pp. 85-94, 183-196. 



