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other is prevented. Thus the marine faunas on opposite 

 sides of North or South America differ widely. 



The organic conditions of the sea likewise exert an impor- 

 tant influence in the distribution of marine life. By organic 

 conditions of the sea is meant, the nature and abundance of 

 food supply, and the relative importance of competing 

 organisms. Since plants primarily furnish the food supply 

 of animals, those portions of the sea rich in plant life are, 

 in general, well adapted to the existence of animal life. 

 Yet even in regions where plant life is wholly absent, 

 as in the deep sea, an abundant fauna exists, the food 

 supply of which is however derived from regions where 

 plants grow. 



Closely related to the food supply, is the struggle for a liv- 

 ing among species and individuals. It is a well known fact, 

 that most animals have such an enormous offspring that, 

 supposing none were destroyed, in a short time all the space 

 in a given region would be occupied by the progeny of a sin- 

 gle pair, and that the number would be such as to enor- 

 mously exceed that determined by the food supply. Migra- 

 tion to new regions is, therefore, a necessity, and emigrants 

 are continually sent out in all directions from the mother 

 country. If no other occupants were in the region, an intra- 

 specific struggle for existence would characterize every local- 

 ity settled by these migrants, members of the same species 

 fighting among themselves for a living. Such struggle 

 would, of course, result in the destruction of vast numbers 

 and the emigration of others. When, however, the newly 

 opened area is entered simultaneously by several species, or 

 if the area is already occupied by other species, an inter- 

 specific struggle will occur, the outcome of which will depend 

 on the relative ability of the contending species to hold their 

 ground. The resident species may be driven out by the new- 

 comer, or it may hold its own and prevent the intruder from 

 settling; or, again, what is perhaps more common, the two 

 species may enter into a compromise and jointly occupy the 

 area. 



