!■ 



NATL'liM. IIISTOUY OF lilllUS. 



A 



li.-lii'il than that n warm-ttinpcrati' or siil>-tr<>|iical tlimatp jirivailod, down to the close 

 uf the tertiary e|HJcli, nearly to tlie nortiiern Jiole, ami that climate was jirevioiisly 

 everywhere so far equahle that the necessity of migration can hardly be supjiosed to 

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

 been crowded thence toward the tro|iics, and the strugjile for life, therefore, greatly 

 iutensitied. Tlie k-xs yielding forms may have become extinct ; those less sensitive to 

 climatic chaniic 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, incijiient 

 and trrndual, extending and strengthening as the cold wave receded and opened uj) a 

 wicler area within which existence in summer became ])ossibIe. What was at first 



a_/ 1^ 1 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 aji|)licable 

 to North America, similar evidence 

 lan be had from the OM \\'oil<l, 

 I'hly that the ])hcnoinen()U iiere is 

 somewhat different, and more con- 

 t'Tuiable to the second su)>position 

 mentioned above. It is j)robably 

 s:ife to say that northern and central 

 l!iiro]ie during the glacial ]ierio<l 

 vvere inhabited by few if any binls, 

 \\ Idle most of those which now live 

 I here were crowded tt)gether in the 

 u armer regions to the .south of the 

 \lps. They have consequently im- 

 migrated to their jiresent home from 

 the south, gr;i(lually, as the ice re- 

 iittunii iiiignuita, r.y. iiio ceded and the sunnners 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 Bcleetion' caused by the forced immigration or emigration accorditi" to 

 change of climate during earlier geological periods. 



lliTc is an appropriate jil.nce to consider for a few moments a jiain.staking work, 

 which started a new era in this bnmch of ornithology, viz., the book "On the IHi<n-at- 

 iiig Houtes of Birds," by Dr. J. A. Palmen, the genial Finnish zoologist. Earlier 

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

 paths while tniveling to or from their summer homes, and Professor Sundevall had 

 already in detail laid down the route of the common Eurojican crane (dn/s ortis); 

 but not before 1S74, when I'.ilmen jiublished his book, was it made evident that most 

 migrating birds travel along geograi)hicalIy defined routes which do not follow one 



Kiu. K 



>la<Emin oli.iwHiK till* iiiiiiii iiiiurntiiiK routes uf tlu* lit- 

 toral (pxcopt rluvl<>-ltl!<irnlU»iriU In FliiroiK'. .. liiiirliM'- 



•iifl ■ubmarliii 

 (JWvrm. 

 common eiilcr 



.'/. tbi* razor-bin anil tlio 



