Geographic Distribution of Animals and Plants 593 



mountains, in valleys, in caves, and in the various depths of the sea. 

 Undoubtedly there are regions of animal distribution as we ascend from 

 the lowest depths of the ocean to the top of the highest mountains. 



Fig. 311. — Diagram of the parallel distribution of organisms in longitude, .(4, 

 and altitude, such as mountains, B, 1, 2, Tropical and subtropical organisms; 

 3, deciduous trees; 4, evergreen trees; 5, limited varieties of such plants as mosses, 

 lichens, etc. ; a similar transition exists from the equator to the South Pole. 



III. PRINCIPLES OF GEOGRAPHIC DISTRIBUTION 



There are probably many principles of zoogeography, but the more im- 

 portant will be discussed only rather briefly in such a work as this. The 

 principle of dispersion illustrates the fact that animals naturally tend to 

 migrate or disperse from their birthplace. This is necessary because more 

 offspring are usually produced than ^normally can be accommodated in 

 that particular habitat. This so-called reproductive pressure tends to 

 overpopulation. The latter leads to dispersal in an attempt to remedy 

 these conditions. It is also known that offspring and parents cannot har- 

 moniously occupy the same area because they all possess inherently the 

 attitudes of survival of the fittest and the struggle for existence. Parents 

 frequently destroy, or at least actively compete with, the offspring which 

 they have produced. 



The problem of overpopulation may be overcome in one of the follow- 

 ing ways, or in a combination of several of them : ( 1 ) There can be a 



