1892 



THE MIGRATION OF BIRDS. 775 



not been distributed from the South Pole. If, in some remote period, 

 life existed there — and there is no reason to suppose the contrary — 

 it has long since been extinguished and crushed out by the super- 

 incumbent ice-cap, from which no retreat was left open, A 

 comparatively narrow strait or channel of ancient sea is known to be 

 sufficient to separate and keep apart very distinct faunas, and to have 

 proved an all-sufficient barrier to the passage and inter-breeding of 

 birds ; and, so far as our knowledge goes, from the very earliest period 

 of this earth's history the great Antarctic continent has been isolated 

 by the oldest and deepest oceans — a belt 700 miles across in the 

 narrowest part — reposing on a foundation of primitive rocks, and 

 without any sedimentary deposits suggestive of ancient land-lines. 3 



The only probable representatives now surviving of an Antarctic 

 avifauna are the Penguins and, perhaps, some of those Petrels 

 which are so numerous in the Southern ocean. 



It is to the North Pole then that we look for the origin of present 

 life on the globe, both animal and vegetable, and it is in this polar 

 origin of life that we must in the future expect to find the key to the 

 riddle of migration in Post-pliocene ages. In the great North Polar 

 continent of the Eocene period, with its tropical warmth and abun- 

 dance of space and food, and the facilities of unbroken land routes 

 extending to the boundaries of the Antarctic Circle, and the subsequent 

 vast climatic changes of the Pliocene age, which were slowly and 

 gradually evolved, we find the primary factors in bringing about the 

 present distribution of bird life on the globe, and the segregation 

 and differentiation of species. The subject, however, is much too 

 vast to enter upon in a short review. + The autumn migration, then, 

 of birds to south of the equator cannot be explained by heredity or 

 any inherited love of home, at a time when the Antarctic world was 

 the favourite ancestral breeding ground, and the Northern Hemisphere 

 a winter home. 



Mr. Dixon gives in a tabulated form, under the headings of 

 " Incipient," " Short," "Moderate," "Long," " Extended," illustra- 

 tions of the longitudinal range of various species ; and as an example 

 of " incipient " migration, with a normal range of 1,000 miles down- 

 wards, cites the Grey Phalarope [Phalaropiis fulicarms), a species circum- 

 polar in its breeding quarters, and migrating southward for winter to 

 Scinde, Northern Africa, and Central America. Again, the knot is 

 quoted as an instance of" extended " migration, covering from 7,000 to 

 10,000 miles. The fact is, however, that the winter range of those 



" " The Polar Origin of Life considered in its b.^aring on the Distribution and 

 Migration of Birds." .By H, B. Tristram. The Ibis. Part I., 1887, p. 236 ; part II., 



1888, p. 148. 



^ Those interested in the subject of the North Polar distribution of life should 

 read Mr. Seebohm's "Evolution of Birds," "Differentiation of Species," "The 

 Glacial Epoch," " Migration," being chaps. II. to V. of " The Geographical Distri- 

 bution of the Charadriidas." — H. Sotheran & Co., London. 1888. 



