PROPOSED EXPLORATIONS AND INVESTIGATIONS 253 



sent extinct species ; but it frequently happens that the remains are 

 referred to members of the fauna now existing in other regions. 

 In the majority of such cases the animals are now found far to the 

 northward, the unavoidable conclusion being that when the cave 

 deposits were formed the climate was such as to permit their exist- 

 ence in the neighborhood, yet in few instances has sufficient critical 

 comparison been made between the cave remains and the bones of 

 the existing species they are supposed to represent. Hence it is 

 impossible to determine whether the time which has elapsed since 

 the climate of central Europe was such as to allow the existence of 

 arctic animals is sufficiently great to have permitted the survivors 

 to become in any way differentiated, or whether it is so short that 

 no changes have taken place. In one instance — the lemming of the 

 cave in Portugal — in which a comparison of this kind was made, 

 the result showed that the remains differ materially from those 

 of the animal now living in Norway. To continue such compari- 

 sons would be an important branch of the investigations here pro- 

 posed. 



6. Migratio7i of Birds in Northern Europe and Asia. — Palmen has 

 clearly demonstrated that the routes of certain migrating birds fur- 

 nish good indications of former land connections, or former shore 

 lines, as the case may be, as well as of the road by which the species 

 have extended their range from the original center of distribution. 

 For the solution of the problems before us it is therefore of the 

 utmost importance that these migration routes be determined and 

 analyzed in detail for as many species as possible. Palmen himself 

 was able to trace several such routes in Europe and western Asia 

 which have shed considerable light on former distribution of land 

 and water. I need only refer to the route from Novaya Zemlya to 

 the White Sea, to the Gulf of Finland along the Baltic, across Hol- 

 stein, to the North Sea and southern England and Ireland, thus 

 plainly outlining the shore of the Arctic marine transgression at a 

 certain period in glacial times. Similar routes across the Mediterra- 

 nean have been shown to follow the old land bridges between Europe 

 and Africa. In Europe these routes have been indicated for a number 

 of species, though many birds are simply referred to them on gen- 

 eral principles. In Asia, however, the work of determining these 

 routes is still in its hypothetic stage. One of the present writers as 

 far back as 1885 broadly outlined the migration route of at least two 

 species from Bering Strait to India, showing it to lie in an entirely 



