﻿4 2 6 riMiTHSONIAN MISCELLANEOUS COLLECTIONS [VOL. 4/ 



distance down into the lower boreal zone, provided the short summer 

 is not too intensely hot. In winter he does not migrate in the regular 

 sense of the word, no matter how cold it is, provided the streams 

 do not freeze over completely. Where they do that he is obliged 

 to go south or down some little distance until he strikes open water, 

 and when once on the move some individual may go astray possibly 

 a few hundred miles, but rarely far away from snow and ice if he 

 can help it. But it is the mountain torrent with its teeming life he 

 loves, and he abhors the sluggish flow of the water courses of the 

 boggy or heavily timbered plains. In this peculiarity we must seek 

 the explanation of the fact that the dipper has never crossed our 

 continent and that the streams of Labrador and the Appalachian 

 mountains do not know him. 



It is then evident that in order to emigrate from the plateau of 

 Asia to Alaska there must have been a continuous boreo-arctic life 

 zone occupying a portion, at least, of the hypothetical land bridge 

 across Bering Sea, and this life zone must have furnished in addition 

 the special conditions suited to the dippers' requirements. This 

 life zone stretches across to-day, but the question is whether the 

 climate in Eocene or early Miocene times would not be an insur- 

 mountable obstacle in the dipper's way. We have all been told 

 that during that period of the earth's development the climate was 

 much milder than to-day, and moreover, this same land bridge is 

 supposed to have been used by many other animals, inhabitants of 

 a warmer zone than the one to which the dipper is restricted. If 

 therefore this was his route of emigration we must suppose that 

 the land bridge in question possessed a sufficient elevation to provide 

 a suitably cool climate in its higher altitudes. This is a question for 

 the geologists to decide. 



That the dipper once on Alaskan soil found mountains sufficiently 

 elevated to suit his tastes is less problematical, and when the uplift 

 of the various systems which go to make up the long chain from 

 Alaska to Patagonia reached its highest limit a continuous boreal 

 life zone doubtless facilitated his march southwards. 



In its new home the dipper apparently flourished. Probably the 

 new conditions stimulated development in the direction which seems 

 inherent in nearly all birds, viz., a tendency to assume a more uni- 

 formly colored plumage which in time leads to the obliteration of 

 the originally striped or spotted plumage of the young. So the 

 American dippers lost the peculiarly sqnamated appearance of the 

 Old World cousins and the young became essentially like the adults. 1 



1 So far as I know, the fledgling plumage of the various South American 

 species is unknown. There are indications, however, that it is not sqnamated 



