ECOLOGICAL 69 



of life. If energy is used up in work, there must follow a period of 

 recuperation. Perhaps there are deeper reasons still, involved in the 

 colloidal structure of the living matter ; and here it should be noticed 

 that in some simple and apparently straightforward chemical reac- 

 tions, such as that between an acid and a metal, there is distinct 

 evidence of a rhythmic, not a continuous progress. Perhaps we may 

 go deeper still to the fact that energy is emitted or absorbed by an 

 atom not continuously, but as little bundles, parcels or "quanta"! 



MIGRATING AS A PARTICULAR ILLUSTRATION 



Migration is a seasonal mass-movement between a breeding and 

 nesting place and a feeding and resting place. It is best illustrated 

 among birds; but it also occurs among fishes, such as the salmon; 

 among reptiles, such as the marine turtles; and among mammals, 

 such as reindeer. It is to be distinguished from occasional mass 

 movements whose spur is over-population, as in the well-known 

 case of the Scandinavian lemming; and from the great movements 

 which some fishes, like herring, illustrate, when they follow their 

 food from one area in the sea to another, the shifting of the food- 

 organisms being due to changes in currents, temperature, and the 

 like. To apply the word migration to such movements is to blunt a 

 good term. Nor should it be applied to cases where animals find a 

 congenial area unoccupied and proceed to colonise it, as rats have 

 done in Britain. Migration is like a tide between winter quarters, 

 where there is recuperation and rest, and summer quarters, where 

 there is breeding and brooding; and for birds the rule is that they 

 nest in the colder part of their migrational range. Thus swallows 

 and storks which winter in Africa may nest in North Europe ; while 

 penguins which spend the summer on southern seas usually brood 

 on the shores of the Antarctic Continent. Typical migration implies 

 a double journey — ^to and from the breeding- place, but we may 

 legitimately speak of the migration of eels, although the adults die 

 after spawning and the return journey is confined to their offspring. 

 Since bird-migration is emphatically a seasonal phenomenon it 

 is interesting to contrast a tropical country, where the seasons are 

 but slightly marked, with a north temperate country, where there 

 is sharp punctuation. Mr. Beebe notes that a square mile of forest 

 in British Guiana may shelter as many different kinds of birds as 

 there are on the British list, that is to say, rather over four hundred. 

 But whereas far over three hundred of the British birds are emphati- 

 cally migratory, this cannot be said of more than forty or so in the 

 Guiana square mile. In other words, our summer visitors are birds 

 whose constitutions exclude the possibility of breeding in a tropical 

 country and of wintering in a northern one. 



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