18 



NATURAL HISTORY OF BIRDS. 



lished than that a warm-temperate or sub-tropical climate prevailed, down to the close 

 of the tertiary epoch, nearly to the northern pole, and that climate was previously 

 everywhere so far equable that the necessity of migration can hardly be supposed to 

 have existed. With the later refrigeration of the northern regions, bird life must have 

 been crowded thence toward the tropics, and the struggle for life, therefore, greatly 

 intensified. The less yielding forms may have become extinct ; those less sensitive to 

 climatic change would seek to extend the boundaries of their range by a slight removal 

 northward during the milder intervals of summer, only, however, to be forced back 

 again by the recurrence of winter. Such migration must have been, at first, incipient 

 and gradual, extending and strengthening as the cold wave receded and opened up a 

 wider area within which existence in summer became possible. What was at first 



a forced migration would become 

 habitual, and, through the heredity 

 of habit, give rise to that wonder- 

 ful faculty we term the instinct of 

 migration." 



While we thus feel justified in 

 accepting the theory as applicable 

 to North America, similar evidence 

 can be had from the Old World, 

 only that the phenomenon here is 

 somewhat different, and more con- 

 formable to the second supposition 

 mentioned above. It is probably 

 safe to say that northern and central 

 Europe during the glacial period 

 were inhabited by few if any birds, 

 while most of those which now live 

 there were crowded together in the 

 warmer regions to the south of the 

 Alps. They have consequently im- 

 migrated to their present home from 

 marine- the south, gradually, as the ice re- 

 ceded and the summers made the 

 countries inhabitable, but were 

 driven back every winter when the cold reduced the insect-life, and covered the fields 

 with snow and the waters with ice. 



We are now prepared to accept the theory that the regular habit is due to 

 ' natural selection ' caused by the forced immigration or emigration according to 

 change of climate during earlier geological periods. 



Here is an appropriate place to consider for a few moments a painstaking work, 

 which started a new era in this branch of ornithology, viz., the book " On the Migrat- 

 ing Routes of Birds," by Dr. J. A. Palm.cn, the genial Finnish zoologist. Earlier 

 authors had been aware that some birds followed well-defined and rather narrow 

 paths while traveling to or from their summer homes, and Professor Suudevall had 

 already in detail laid down the route of the common European crane (Grus grus); 

 but not before 1874, when Palmen published his book, was it made evident that most 

 migrating birds travel along geographically defined routes which do not follow one 



FlG. C. Diagram showing the main migrating routes of the lit- 

 toral (except fluvio-littoral) birds in Europe. _ 

 and submarine-littoral migrants, e.g. the razor-bill and the 



divers pelago- and glacial-littoral migrants, e.g. the 



common eider and the king eider. 



