THE ORGANIC FACTOR 1053 



By organic conditions 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 regions 

 rich in plant life are, in general, well adapted for 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, however, is derived from tlie illuminated regions where 

 plants grow. 



Closely related to the food supply is the struggle for a living 

 among species and individuals. It is a well-known fact that most 

 of the lower animals have such an enormous offspring that, sup- 

 posing none were destroyed, in a short time all the space in a given 

 region would be occupied by the progeny of a single pair ; and the 

 number would be such as to exceed enormously that permitted by 

 the food supply. Migration 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 

 intraspecific struggle for existence would be witnessed in every 

 locality settled by these migrants — members of the same species 

 fighting among themselves for a living. Such a struggle would, 

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

 emigration of others. But, when the newly opened area is entered 

 simultaneously by several species, or if the area is already occu- 

 pied by other species, an interspecific struggle occurs, the outcome 

 of which depends on the relative ability of the contending species 

 to hold their own. 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 accommodate themselves side by side and jointly occupy the 

 disputed area. 



An example of a struggle between resident and invading spe- 

 cies is found in the faunas of the Portage beds of New York State. 

 The resident fauna was the Ithaca faima, derived by modification 

 from the preceding Hamilton fauna. The invading fauna came 

 from Eurasia, invading the New York area from the northwest. 

 The interspecific, or interfaunal. struggle continued throughout 

 Portage time, the invading species gradually acquiring the mastery. 



BlOGEOGRAPHICAL PrOVINCE.S. 



At each period of the earth's history, zoogeographic and phyto- 

 geographic provinces existed which were more or less distinctly sep- 



