THE EVOLUTION OF ANIMAL LIFE 



known only from their fossil remains until a living representative of one of 

 the lobe fin groups, the Coelacanths, was taken from the ocean off South 

 Africa in the late 1930's; since World War II, several additional specimens 

 have been caught and are being intensively studied. Even entire groups, 

 such as the sharks or the amphibians, may be regarded as survivals of 

 ancient types that have descended with modifications but without changes in 

 certain primitive features. 



Geographic Distribution. Consideration of the geographic distribution 

 of animals and its bearing on the problem of evolution requires that two 

 fundamental concepts be borne in mind. First, the ancestors of related 

 genera, for example, originate in a single locality, known as the common 

 center of origin. Second, migration from this center occurs, for reasons pre- 

 viously discussed (see pp. 610 611). Geographic barriers, such as mountain 

 ranges, large bodies of water, and the like, determine the direction of 

 migration by interposing conditions which the animals cannot overcome or 

 tolerate. Migration from common centers of origin, as directed by physical 

 barriers, is believed to account for the observed distribution of animals on 

 the surface of the earth. The natural distribution of animal types, occur- 

 ring through countless centuries, has been increasingly disturbed by human 

 migrations and the attendant transportation of animals and plants to new 

 habitats. 



As pointed out earlier, the problems of paleontology, which involve vertical 

 or temporal distribution, are inseparable from those of zoogeography, which 

 involve spatial or horizontal distribution. The present distribution of animals 

 can be understood only in relation to the distribution of their ancestors during 

 preceding millennia. This interrelationship is well illustrated by the camel 

 family. As it is now distributed, this family consists of two widely separated 

 groups. The genus Camelus includes two species, the Arabian camel, or 

 dromedary, and the Bactrian camel of Central Asia. On almost the opposite 

 side of the world, in the Andes of South America, occur the only other camel- 

 like animals now in existence. These are species of the genus Auchema, the 

 guanaco and the vicuiia, with their domesticated descendants, the llama and 

 the alpaca. The surprising separation of these two groups of related modern 

 forms can be logically explained only by the geologic distribution of the 

 ancestral stock of camel-like animals. As shown by the fossil record, camels 

 originated in North America as a family of hoofed mammals. Here they 

 flourished from the Eocene to the close of the Pliocene epoch. Migrations 

 occurred during the Pliocene, one group reaching South America, the other 

 attaining Asia by way of a former land connection in the region of the Bering 

 Sea. The extinction of the North American camels during the Pleistocene left 

 representatives in South America and Asia, and it is from these widely 

 scattered ancestors that the modern species have descended. Numerous other 

 examples might be cited from the fossil record, all showing that the theory 

 of organic evolution gives meaning to the present geographic distribution of 

 many types of existing animals. 



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