306 PRINCIPLES OF ANIMAL BIOLOGY 



Continents have been broken apart and united, islands have appeared 

 and disappeared or been connected with and severed from the mainland, 

 and climates have changed. Even at the present time it is evident that 

 mountains are being destroyed, lakes filled in by debris, and the chemical 

 nature of the water is being changed in many places. Moreover, with 

 the constant modification w'hich the surface of the earth has undergone 

 and is undergoing the vegetation also has changed and is changing, 

 as is evidenced either by a displacement of one association by another, 

 such as a forest by a prairie, or by an alteration in the composition of 

 the plant associations. 



From what has been said concerning migration and dependence 

 upon environments it will be readily perceived that the earth provides 

 a multitude of environments, each of which may serve in turn as a barrier 

 to a form in a neighboring region, A region which it has been impossible 

 for an animal to enter may undergo such a change in some one or more 

 of its physical or biological features that it no longer serves as a barrier. 

 Thus the range of the potato beetle as stated above was, previously to 

 1850, apparently restricted on the east by the absence of a suitable food 

 plant in the adjoining territory, but when a food plant was introduced 

 migration in this direction began at once. Another instance, on a small 

 scale, has recently been noted. Previously to 1907, the fox squirrel, 

 a forest animal, was in northwestern Iowa confined to the timber along the 

 larger streams. Since that date, owing to the number and size of the 

 trees planted on the prairies by the residents, the squirrel has spread over 

 large parts of the prairie areas. That changes on a much greater scale 

 have in the past resulted in extensive changes of range seems to be shown 

 by paleontology. Thus there is reason to believe from the relations 

 of the faunas that Madagascar was at one time connected with Africa, 

 South America has been joined to Australia and New Zealand, and 

 North America and Asia have been united. Extensive migrations of 

 animals have apparently occurred between these countries and then 

 been halted by a breaking of the connections. Similarly the great 

 changes in climate which occurred in North America and Europe during 

 the Pleistocene epoch, from somewhat more equable and milder condi- 

 tions than prevail at present, to polar conditions, brought the musk-ox, 

 caribou, reindeer and similar forms into the United States and Europe 

 far south of their present ranges. 



Extinction. — Complete annihilation of a species in any region is also 

 to be regarded as a factor of distribution, though in a sense somewhat 

 different from that in which the preceding agencies are factors of dis- 

 tribution. Extinction does not affect the process of distribution, but 

 it does help det(Tmine the range occupied. Extinction of itself neither 

 helps nor hinders the spread of animals to new regions; but the destruction 

 of the forni in som.e part of the area occupied by it changes the range. 



