454 PLANT DISTRIBUTION 



tutes a barrier difficult but not always impossible to pass. 

 To such, a river may be a very efficient check to migra- 

 tion provided that it is not practicable to go round its 

 head-waters. An ocean constitutes a practically impassa- 

 ble barrier around an island or a continent to such animals 

 or to plants whose seeds cannot be carried by wind or 

 water. Fishes and strong-flying birds on the contrary 

 as well as plants with thistle-down-like seeds readily 

 cross very large bodies of water. Large land masses of 

 course constitute efficient barriers against fishes. In fact 

 in the Inter-Mountain Region of the United States there 

 are streams which now lose their waters in the desert 

 sands within a few miles of each other and yet have 

 different species of fish in them, although they were both 

 connected in the not distant geological past by the waters 

 of the ancient and now dried-up lake into which they 

 emptied. Aside from plants with small seeds easily dis- 

 persed by wind, plants in general are less rapid in ex- 

 tending their range than the higher animals. They too 

 find oceans and high snow-clad mountains and such- 

 like physical forms serious and sometimes impassable 

 barriers. It is not intended that this paragraph should 

 attempt to list all the sorts of things which may consti- 

 tute tangible barriers, but merely to indicate the fact by 

 illustration that there are many sorts of physical elements 

 which constitute efficient barriers for particular species 

 and that what is a barrier for one may not be for another. 

 Intangible Barriers. — It is readily seen that plants 

 often fail to spread into adjacent areas where there are 

 no tangible barriers to stop them. For illustration the 

 pine trees of Michigan would have neither mountain, 

 rlosort. nor ocean to check their spread south through the 

 prairie region to the Gulf of Mexico but, nevertheless, 

 they do not do so. Neither do the prairies nor the indi- 

 vidual species which grow in them spread north into the 

 forest country to any great extent or west into the drier 

 plains and mountains, although many species have seeds 



