TfilNGA. 

 Our first step must therefore be to ascertain these post-glacial species. 



417 



Palcearctic Species. 

 T. subarquata. 

 T. maritima. 



T. acuminata. 

 T. platyrhyncha. 

 T. subminuta. 



Nearctic Analogue. 



T. bonaparti. 

 T. couesi. 

 . T. ptilocnemis. 

 T. pectoralis. 

 T. bairdi. 

 T. minutilla. 



Post-glacial 

 species. 



Our subgeueric group A contains four species, which probably represent the descendants 

 of the party of Tringas which emigrated from the Polar Basin down Baffin's Bay, and were 

 during the Post-Pliocene Glacial Epoch isolated and differentiated on the American continent 

 into the Knot {T. canutus) and Bonaparte's Sandpiper {T. honaparti). Each of these 

 species appears to have extended its range across Behring Straits during the succeeding 

 warm period, the western emigrants of the Knot afterwards becoming T. crassirostris, 

 which breeds in East Siberia, and those of Bonaparte's Sandpiper afterwards becoming 

 1\ subarquata, which breeds somewhere north of the Lena. 



To the three species which form group B may unhesitatingly be assigned the Atlantic 

 coast of Europe as the route which their ancestors took when they escaped from the Polar 

 ice. Not only do they all breed in Iceland, but they are the only species of the genus 

 Tringa which do. Judging from their present winter-quarters, we may assume that 

 T. arenaria was differentiated in South Africa, T. alpina in the basin of the Mediterranean 

 (which probably accounts for the remarkable fact that it has been known to breed in Spain), 

 whilst T. maritima probably never left the outskirts of the glaciers. In post-glacial times 

 all three species became circumpolar. T. arenaria appears to have maintained its homo- 

 geneous characters ; T. alpina shows some signs of differentiation ; whilst T. maritima has 

 become more differentiated, into T. ptilocnemis in the Behring Sea, and into T. couesi in 

 Alaska. 



Subgeneric group C, consisting of only one species, is easily disposed of. The fact 

 that T. rvfescens is very abundant in Alaska, but " its presence on the Atlantic coast is 

 regarded as an infrequent event," suggests at once the Pacific coast of America as the 

 route of the emigrations of its ancestors. 



Subgeneric group D contains by far the greatest number of species, and consequently 

 requires for their differentiation the widest space and the greatest number of opportunities 

 of isolation. The conformation of the land in Southern Asia is precisely what is required, 

 and we may consequently accept the Pacific coast of Asia as the route of the emigrations of 

 their ancestors. Again, assuming the locality where they are most abundant in winter as 

 the most probable area of their isolation and differentiation, we arrive at the following 



3h 



Emigrants 

 along Atlan- 

 tic coast of 

 America. 



Atlantic 

 coast of 

 Europe. 



Pacific coast 

 of America. 



Pacific coast 

 of Asia. 



