( 10 ) 



lu the case of a migrant, the faculty of orieutation is not only advantageous to 

 the individuals (as in the other cases mentioned), bnt absolutely indispensable 

 to the existence of tlie species, and leaves little room for adventitious elements. 



I do not doubt that birds possess a sense of direction — indeed, this is evinced 

 in the well-known wanderings of Albatrosses in the Southern Ocean. While these 

 birds are extremely local during the breeding season, at other times they wander 

 great distances in any direction, although seldom beyond definite north and south 

 limits. The faculty whereby they direct their fliglit back to tlie breeding stations, 

 over hundreds of miles of open water, is doubtless akin to that exhibited by 

 savages and Pigeons. 



ORIGIN OF BIRD-MIGRATION. 



Several attempts have been made to trace the origin of the impulse nf migration 

 in the northern hemisphere to tliose secular changes of climate which resulted in 

 the Glacial Period. The theory is admirably stated by Dr. Allen {op. cit. pp. 100- 

 102), and I may here attemjjt a summary. 



During the southward progress of the " ice-cap," the area occupied by many 

 species of birds would be gradually encroached upon, bnt the effect produced would 

 vary greatly in different cases. A species having a restricted northern habitat might 

 become extinct ; another species with an extensive latitudinal range, especially if 

 the northern limits of the range did not previously extend much beyond the southern 

 boundary of the ice, might be unaffected save for a lessening of area. " Opportunity 

 was given for the gradual adaptation of many forms to a lower temperature 

 than that to which they had been accustomed, and to an enforced change of food," 

 thus leading to the evolution of new types. Dr. Allen thinks there was " a great 

 crowding together of exiles from the north into the more favoured regions to the 

 southward." This may be doubted. The process was so gradual that it is more 

 likely there resulted extinction or modification of the northern forms, and at the 

 culmination of the period of glaciation we may supjjose that a state approaching 

 equilibrium was reached. " Finally the ice receded to its present limits, and the 

 whole north, under radically altered climatic conditions, became again available for 

 occupation by the more or less modified descendants of the pre-glacial exiles." It 

 was at the time of the recession of the ice tliat the impulse of migration is sui)posed 

 to have originated and become established. During the milder i)eriod of the 3-ear 

 some species would seek to extend the bounds of their range — only, however, to be 

 driven back uj)on the approach of winter. This incipient migration would become 

 more orderly and also more extended as habitable land became available. 



All that this h3'pothesis claims is that we must look to the changes of climate 

 induced in the northern hemisphere by the decline of the Glacial Period as the 

 ultimate cause of migration in this part of the globe. Indications are not wanting, 

 however, that, under conditions obtaining at the present time, the migratory impulse 

 tends to strengthen in some forms and to weaken in others. 



An excellent illustration of this tendency is afforded by the American forms of 

 (Hocor'm alpt'stris. In Mr. Harry C Oberholser's careful and elaborate treatise 

 ('' A Review of tlie Larks of the Genus Otocorin," I'lvc. U.S. yaf. Mks. xxiv. 

 pp. 801-883, pll. xliii-xlix) twenty-two New World forms are recognized. Of these 

 eleven are migratory, ten apparently resident, and one from lack of material 

 doubtful— namely, Otocoris alpcstrig pallida. The migratory forms mostly fall into 



