Geographical Distribution 157 



as others the salt eontent of tlie water does limit their distri- 

 bution. The extent to Avhieh fresh water fisli can with- 

 stand salt water and vice versa is still a moot question, but 

 the salt content certainly does i^\n.j a determining role in the 

 distribution of all fish. 



The inhabitants of inland w^aters also find the chemical 

 environment one of the determining factors in their distri- 

 bution. Here one may find all degrees of salinity from fresh 

 or slightly saline waters, to those such as the Dead Sea in 

 Palestine or the Great Salt Lake of Utah, containing much 

 greater amounts of salt than does the ocean. Correspond- 

 ing to the differences in the saltiness of these inland waters 

 there are marked diiferences in the kinds of life inhabiting 

 them. 



The barriers to the spread of land animals are more nu- 

 merous than are those which affect aquatic forms. As with 

 the latter so too with these does temperature play an impor- 

 tant part. Every one is familiar with the great differences 

 between the animal life of the tropics and the poles, and 

 equally marked are those which strike the observer as he as- 

 cends some lofty mountain, and the more so the farther south 

 the mountain lies. Mountain ranges have a notable influ- 

 ence in separating one fauna from another. The animal life 

 of the Great Plains of the east is distinctly different from that 

 of the Great Basin to the west of the Rocky Mountains. 

 Wide stretches of desert present an impassable obstacle to 

 most forms of life, while bodies of water may with equal em- 

 phasis say to the animal wanderer "Thus far shalt thou go, 

 and no farther." 



Plant journeys are wholly passive ones, and affected mainly 

 by the carriage of fruit or seed by wind, water and animals. 

 The tumble-weed driven across the prairie by the wind and 

 heaping itself up in great piles along the fences, the down of 

 the dandelion as it floats idly through the air on a summer 

 afternoon to settle softly in some protected corner of your 

 front lawn, and the long-awned heads of the fox-tail grass 

 tumbling merrily over field and roadside all bear emphatic 

 testimony to the part played by wind in the spread of plants, 

 especially those which are an unmitigated nuisance ; while 

 any one who has watched a dog disentangling himself from a 

 coat full of burs will realize the important role played by 

 animals in plant distribution. One of the most important 

 factors in plant distribution is man himself. To prove this 

 one need only follow a railway track or a highway and note 

 the never-ending succession of Avecds, which are distributed 

 thereon. ]\Iany of the most common and pestiferous mem- 

 bers of plant as well as of animal and human society are im- 



