68 Causes of Emigrations 



instance where an animal has migrated from the .ocean to fresh 

 water or land to escape parasites. Most plants and animals which 

 attack animals as parasites associate themselves with their hosts in- 

 sidiously. There are some exceptions to this rule. Parts of Africa 

 have been depopulated by tsetse flies; the caribou migrate north in 

 summer in attempting to escape swarms of mosquitoes and warble 



Fig. 12. Two gobies resting on the roots of mangroves (Rhizophora) 

 in a Nigerian estuary. When disturbed these fish do not dive but 

 skip on the surface of the water to another root. 



flies. Parasites may at times be a factor in exterminating hosts in 

 particular habitats, but they have apparently not played an impor- 

 tant role along seashores or the borders of bodies of freshwater in 

 causing hosts to change habitats. 



Toleration Ranges; Resistance 



Animals that live along the transition zones between land and 

 water or between salt water and fresh water are often subject to 



