CHAPTER XX 



GEOGRAPHICAL DISTRIBUTION 



It has long been known that the life on two islands separated 

 by a narrow strip of water might differ more than the plants 

 and animals of northern Em'ope and Canada. Geographical dis- 

 tribution attempts to explain these similarities and differences, 

 and depends upon geography and geology for the solution. The 

 general term distribution is divided into time distribution, or 

 palaeontology, which studies the relative periods in history when 

 certain groups have lived; and geographical distribution which is 

 limited to living organisms and their relative position on the 

 earth's surface. The latter is frequently divided into vertical 

 distribution, life from the depths of the ocean to the tops of the 

 mountains; and horizontal distribution, the spatial relationships 

 on the surface of the earth. The present discussion will confine 

 itself largely to the horizontal distribution of vertebrates and 

 its causes. 



Geographical distribution depends largely upon two factors: 

 (1) the migration of animal groups, and (2) the paths which 

 they have followed. Migration would tend to bring animals into 

 new environmental conditions, with the elimination of those least 

 adapted to the particular conditions ; and in time, with the slow 

 shifting of the earth's surface, new species would evolve. Nat- 

 urally the greatest similarit}^ in animals would be found under 

 conditions where free migration is possible. 



There are several causes for the migration of animals. Passive 

 migrations (when animals are carried by winds, water currents, 

 or floating masses of debris) are of slight importance except 

 among the fish and birds. A too swift current which sweeps fish 

 from their original location would start a new center for their 

 multiplication and spread; and a storm may carry birds miles 

 away from their natural habitat. Some sea birds have in this 



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