3 



CAUSES OF EMIGRATIONS FROM THE SEA 



I "HOUGH there is little doubt that most groups of animals 

 had their origin in the ocean and gradually invaded fresh-water 

 and land habitats, there is perhaps some uncertainty as to why 

 animals should leave the stable, dependable ocean and migrate into 

 highly variable situations where life is perhaps more exciting but at 

 the same time more precarious. Some who have considered this 

 question believe that "lures" of various sorts have enticed animals 

 from their ancestral homes (Hesse, 1920), and there appears to be 

 evidence to support such a view. But it is also evident that some 

 animals have been forced to leave the ocean to escape from dangers 

 which threatened racial extinction. Perhaps some also left because 

 of factors which were neither attractive nor repellant. Heape 

 (1931) lists three incentives to emigration: (1) food, (2) climate, 

 and (3) overpopulation. 



Spreading 



Probably all species of animals and plants tend to extend their 

 ranges by spreading into all available habitats. Multiplication of 

 individuals leads to overpopulation. In nature various factors op- 

 erate as checks which keep the numbers of individuals at certain 



Fig. 8. A coral reef on the north coast of Barbados. 

 The reef and rock cliffs shelter many animals that 

 live alternately in the water and the air as the tide 

 changes. (Photo by R. E. Blackwelder.) 



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