ZOOLOGICAL ZONES 1527 



trich has not been found in any part of Asia, and 

 no other species of wingless birds has ever been 

 met with on that continent; the evidence in regard 

 to such large and conspicuous birds is conclusive to 

 that fact. Again, in order that the rhea, or three- 

 toed ostrich, should reach South America by travel- 

 ing along that element on which alone it is or- 

 ganized and adapted to make progress, it must, on 

 the hypothesis of dispersion from a single Asiatic 

 centre, have traveled northward into the inhospi- 

 table wilds of Siberia; it must have braved and over- 

 come the severer regions of the Arctic zone; it must 

 have maintained its life with strength adequate to 

 the extraordinary power of walking and running 

 more than a thousand miles of land or frozen ocean 

 utterly devoid of the vegetables that now constitute 

 its food before it could gain the northern division 

 of America, to the southern division of which it is 

 at present and seems ever to have been confined. 

 The migration in this case could not ihave been 

 gradual and accomplished by successive generations. 

 No individual of the large vegetable-feeding wing- 

 less bird that now subsists in South America could 

 have maintained its existence, much less hatched its 

 eggs, in Arctic latitudes, where the food of the spe- 

 cies is wholly absent. If we are still to apply the 

 current hypothesis to this problem in natural his- 

 tory, we must suppose that the pair or pairs of the 

 rhea that started from the highest temperate zone 

 in Asia capable of maintaining their life must have 

 also been the same individuals which began to prop- 

 agate their kind when they reached the correspond- 



