38 OKIGIN OF LIFE IN AMEEICA 



weight," it once more permitted life from the south to move 

 into the newly opened territory. The first wave of life, he 

 thinks, which became dispersed over the glaciated region, as I 

 have already mentioned above, was the barren-ground fauna 

 and flora, the types of the 'far north. " Hardly a trace," 

 he continues, " now remains in the eastern United States of 

 this pioneer class, and this class is now restricted to very 

 limited areas or mountain top ' islands/ as in th.e White 

 Mountains. The present distribution of these arctic-alpine 

 e islands ' brings up a number of very interesting points. How 

 is it that these alpine plants and animals are now found so far 

 south and only upon high mountain peaks ? This question 

 could not be rationally explained until the influence of the Ice 

 Age upon life became recognised, and would, indeed, be a 

 difficult one to answer if we did not take into consideration 

 past conditions of climate and topography. We must recall 

 that the arctic conditions which now occur outside of the. arctic 

 regions only on high mountain tops at that time extended to 

 the very base of the mountains, and as the ice retreated to the 

 north this cold zone gradually moved up the sides of the 

 mountains carrying with it a characteristic flora and fauna. 

 Thus with the retreat of the ice the first wave of life had 

 two evident possibilities before it: first, to follow the ice 

 north, or to follow the cold zone up the mountains. By this 

 means small colonies of arctic plants and animals became 

 separated from the main body of forms, and thus became 

 alpine. To be sure, this segregation could only occur where 

 the mountain peaks are isolated. If the mountains had been 

 of sufficient height and had extended far to the north, a high- 

 way would have been retained to the original stock, and thus 

 the distribution would not have become discontinuous. From 

 the above interpretation it seems fair to conclude that the 

 arctic forms which now occur at alpine heights are relicts of 

 the former widespread glacial fauna and flora, whose dis- 

 tribution has become discontinuous by a change of climate." 

 Very similar ideas were held by Mr. Grote.* He thought 

 the White Mountain butterfly (Oeneis semidea) was pushed 

 southwards by the advance of the great northern ice-sheet. 



* Grote, A. E., "Effect of Glacial Epoch," p. 441. 



