214 Canon Tristram on the Polar Origin of Life. 



all retained their hereditary attachment to the north, and 

 the extent of their migration from their winter in the tropics 

 to the shores of the Arctic Ocean is not surpassed by any 

 land-bird. 



A far more important body of colonists was that which 

 worked down Western America to Panama and then divided 

 eastward and southward. From these spring the bulk of the 

 neotropical Thrushes, which, with the exception of T. migra- 

 ' torius, seem to have lost, from isolation, the habit of any save 

 vertical migration ; while others, as T.falklandicus, are, from 

 circumstances, strictly sedentary. But those which followed 

 the eastward route from Panama afford a remarkable instance 

 of the effects of isolation. We have first the continental 

 ! group of Mexico and Central America, forming the recognized 

 i genus Catharus; all the species of which are, so far as we 

 'know, sedentary. For this exception I can suggest no ex- 

 planation, unless that their ancestors, baffled by the sub- 

 merged area of the now Mississippi valley, abandoned the 

 eflFort to revisit the north. In the case of the colonists of 

 South America, we must remember that, travelling south by 

 the lofty Andean ranges, they would escape the then intolerable 

 heat of the lower equatorial regions, and that when once 

 settled on the side of the mountains or in the temperate 

 southern regions, the heat of the equatorial belt might repel 

 their efforts to revisit the north. Perhaps the most inter- 

 Zesting development of all in this family is that of the Mimo- 

 cichlce in the Antilles. These certainly have a common 

 origin, and seem to come most naturally as among the results 

 of this great migration. Each species is confined to a single 

 island, and they certainly must have been difi^erentiated 

 where they now are. I am not at all certain that we ought 

 not also to include among these results the Cichlherminia 

 group of the Lesser Antilles. It is certainly remarkable that, 

 whether in the Pacific or West Indies, a species which has 

 once bred in an island, even the smallest, seems always to 

 lose its migratory instinct, however strongly that habit may 

 be impressed on the family generally. 



A large party of emigrants must have chosen the East 



