78 LIFE : OUTLINES OF GENERAL BIOLOGY 



which the slow (lisp<.rsal of animals may be brought about by 

 "migration", meaning the spreading of the en\-ironment, as in the 

 extension of the tundra zone or the forest belt. Hut this application 

 of the term is unnecessary, and it can be avoided by speaking of the 

 extension or shrinkage of a particular type of environment, or by 

 using some other phrase, like "ecological succession". 



The distinctive features of true migration are the following. 

 {a) It is a racial custom, enregistered in the constitution, but 

 actiN-ated by particular stimuli, both internal and external. (6) It 

 is of regular recurrence, either sea.sonal and jx'riodic, or involved 

 in the animal's attainment of a particular age or state, (c) It is a 

 geogra])hical or to]X)graphical change from one haunt to another, 

 one Ixing the ])reeding-placc. And in t>T)ical cases there is a return 

 journey, as is familiar in the ebb and flow of the feathered tides of 

 migrant birds. In some cases, such as the Common Eel, the adults 

 die after spawning, and the return journey is thus confined to the 

 larval stages. 



Here we would (juote a few clear sentences from A. L. Thomson's 

 Problems of Bird-Mii^ration (1926). "To deserve the description 

 'migratory', in its strict sense, movements need not necessarily 

 have a very great geographical amplitude, but at the least they 

 must involve a definite change of locality. They must be purposive 

 in that the change of scene is associated with some definite advantage 

 which serves as its raison d'etre, and there must be return move- 

 ments to the original area. They must be periodic in that they 

 corres]x)n(i to .some recurrent change cither in the environmental 

 conditions themselves or in the animal's reaction thereto. True 

 migrations are changes of habitat, periodically recurring and alter- 

 nating in direction, which tend to secure optimum conditions at 

 all times." 



Mamm.m.s. - IVrhaps the best instances of true migration in 

 mammals are to b<' found among seals, though the Common Seal 

 [Phoca vitulina) in Britain is practically a resident species. More 

 tj^iiral is the Alaska Fur Seal [Callorhinns alascanus). which winters 

 as far .south as California, and returns in spring across the North 

 Pacific for 2.000 miles to its remote breeding-place on the fog- 

 hidden Prihilof Islands. Cetaceans, being more thoroughly adapted 

 to marine life than the Pinniped Carnivores, do not need to come 

 to the shore to breed, yet there is some evidence of their migratory 

 movements. The same may be said in regard to reindeer, but the 

 facts are not very easy to understand. The Newfoundland Caribou 

 {Rattiii/cr tcrranov<r) moves in autumn from the stormy uplands, 

 where the "reindeer moss" and the hke are apt to be buried deep 

 in snow, to the less strenuous conditions towards the south coast 

 of the island. The .southward movement takes place after the 

 mating season, usually late in October, and is somewhat leisurely. 



