50 Causes of Emigrations 



levels. If environment or species change and disturb the existing 

 balance, then numbers may increase or decrease. No matter how 

 specialized an animal may become in its relation to environment, it 

 can never lose its ability to spread. Johnstone (1908) says, "Yet 

 the limitation of habitat is partially compensated for by the evolu- 

 tion of larval stages in the life history of an organism. We nearly 

 always find that a sessile benthic animal has evolved a free-swim- 

 ming larval stage; or the primitive pelagic form has evolved a 

 sessile habitat during the latter period of its life history." Even 

 parasites which are securely nestled within favorable hosts at inter- 

 vals spread into the outside world. Some of them have very special 

 and elaborate means for doing so. Certain parasites are always 

 spread by bloodsuckers such as mosquitoes, ticks, or leeches. 



To maintain even a small place in the world, a species must have 

 a chance to spread. If this continual pressure to invade new situa- 

 tions is inherent in all organisms, it may at times, when circum- 

 stances are especially favorable, move certain animals out of the 

 ocean into littoral, estuarine, or paludal habitats. Most of the avail- 

 able niches are already filled, but when a new area is available it is 

 quickly populated. In this connection Hubbs cites "the bizarre 

 evolution of cichlids in the great lakes of Africa; the multiplication 

 of cyprinids from a single species in Lake Lanao, Mindanao 

 (Herre) ; and the very rich fauna of the Rio Lerma system in 

 Mexico, largely made up of 2 species groups." Price & Gunter 

 (1943) describe the effects of climatic changes in Texas. In 1870 

 some streams dried up and have been intermittent since. Then 

 live oaks died and there were other changes in vegetation. Tall 

 grass was replaced by brush jungles and animals of arid regions, 

 such as the armadillo and coati mundi, spread north and east. 



Succession 

 Organisms invade habitats in an orderly manner. If a new area 

 is opened for colonization, it is generally occupied at first by a few 



